Plants: Diversity, Structure, Transport and Reproduction

Almost every breath you take and almost every calorie you eat began as light captured by a plant. Plants built the oxygen-rich atmosphere, the soils, the forests and the fields that the rest of terrestrial life depends on, and they did it while rooted in place, unable to run from drought, heat or hungry animals. This chapter follows plants from their algal ancestors to flowering plants, then opens them up to see how they are built, how they move water and sugar without a heart, how they reproduce, and how they sense and respond to a changing world.

Section 1Plant evolution: from algae to flowering plants

What makes a plant a plant

Land plants (the embryophytes) are multicellular, photosynthetic eukaryotes that protect their embryos inside the tissues of the parent. They share with green algae chlorophylls a and b, starch stored in plastids, and cellulose cell walls. Their closest living relatives are the charophytes, freshwater green algae that share rosette-shaped cellulose-synthesizing complexes, a phragmoplast that guides cell plate formation, and flagellated sperm. Ancestors living at drying pond edges were selected for surviving brief desiccation, setting the stage for the move onto land roughly 470 to 500 million years ago.

Land offered more light, faster-diffusing carbon dioxide and mineral-rich soils, but also drying, no buoyant support and no water for sperm to swim through. Each major plant innovation solved one of these problems.

Derived traits of land plants

Five features appear in land plants but not in charophytes: alternation of generations between a multicellular haploid gametophyte and a multicellular diploid sporophyte; multicellular dependent embryos nourished by the parent through placental transfer cells; walled spores coated with drought-resistant sporopollenin and made in sporangia; multicellular gametangia, egg-holding archegonia and sperm-making antheridia; and apical meristems at shoot and root tips. Most land plants also have a waxy cuticle, stomata and fungal partners (mycorrhizae).

Alternation of generations

All land plants run the same basic cycle. The diploid sporophyte makes spores by meiosis in sporangia. A spore is haploid and divides by mitosis to grow into a multicellular haploid gametophyte. The gametophyte makes gametes by mitosis (not meiosis, because it is already haploid). Sperm fertilizes egg to form a diploid zygote, which grows by mitosis into a new sporophyte. Students often expect gametes to come from meiosis, as in animals; in plants, meiosis makes spores and gametes come later by mitosis.

The key evolutionary trend is the shift in which generation dominates. In bryophytes, the green leafy plant is the gametophyte, and the sporophyte is a small stalked capsule that stays dependent on it. In ferns, the large leafy plant is the sporophyte and the gametophyte is a tiny, free-living, heart-shaped prothallus. In seed plants, the gametophytes shrink to a few cells inside the sporophyte: the pollen grain (male) and the embryo sac in the ovule (female). A dominant diploid generation can mask harmful recessive mutations and can grow tall, vascular and long-lived.

Bryophytes: the nonvascular plants

The three bryophyte lineages, liverworts, mosses and hornworts, lack lignified vascular tissue, so they rarely grow more than a few centimeters tall. They absorb water over their surfaces, are anchored by filamentous rhizoids rather than true roots, and have flagellated sperm that must swim through a film of water to the archegonium. Mosses can still be ecologically huge: peat mosses (Sphagnum) store roughly 30 percent of the world's soil carbon in peatlands.

Seedless vascular plants

Vascular tissue was the next breakthrough. Xylem conducts water and minerals, and its conducting cells, called tracheids, are reinforced with lignin, a rigid polymer that also supports the plant against gravity. Phloem distributes sugars and other organic molecules. With pipes and support, plants could grow tall and the sporophyte could become dominant. Two lineages survive. Lycophytes (club mosses, spike mosses and quillworts) have microphylls, small leaves with a single unbranched vein. Monilophytes (ferns, horsetails and whisk ferns) have megaphylls, leaves with a branched network of veins that capture far more light. True roots also appear in vascular plants. Carboniferous swamp forests of giant lycophytes and horsetails became today's coal deposits.

Seedless vascular plants still need water for sperm to swim, and they disperse by spores. Most are homosporous (one spore type, bisexual gametophyte); some are heterosporous, making small microspores that develop into male gametophytes and large megaspores that develop into female gametophytes. Heterospory is the condition from which seeds evolved.

Seed plants: gymnosperms

Seed plants added four linked innovations: reduced gametophytes kept inside the sporophyte, heterospory in all species, ovules, and pollen. An ovule is a megasporangium wrapped in protective layers called integuments; the megaspore inside develops into the female gametophyte, which makes the egg. A pollen grain is a male gametophyte enclosed in a sporopollenin wall. Because pollen travels by wind or animals, seed plants freed fertilization from water. After fertilization, the ovule becomes a seed: an embryo plus a food supply wrapped in a seed coat. A seed can remain dormant for years and travel far, a major advantage over a single-celled spore.

Gymnosperms ("naked seeds") bear seeds exposed on the scales of cones rather than enclosed in an ovary. The four living groups are the conifers (pines, firs, spruces, redwoods), cycads, Ginkgo (a single living species) and the gnetophytes. Conifers dominate cold and dry forests partly because their needle leaves have thick cuticles and sunken stomata. In a pine, an ovulate cone takes two to three years from pollination to mature seed.

Angiosperms: flowers and fruit

Angiosperms are the flowering plants, with roughly 300,000 described species, about 90 percent of all living plant species. Two traits define them. The flower is a specialized shoot for sexual reproduction that often attracts animal pollinators, which carry pollen precisely between plants of the same species. The fruit is a mature ovary that protects seeds and aids their dispersal by wind, water or animals. The ovules are enclosed in a carpel, which is why seeds are "covered" (the meaning of angiosperm). Angiosperms also have vessel elements, wide water-conducting cells that move water faster than tracheids, and double fertilization, which produces a nutritive endosperm only when an egg is actually fertilized.

Angiosperms include a few basal lineages (such as Amborella and water lilies), the magnoliids, the monocots (grasses, orchids, palms) and the eudicots (roses, beans, oaks); monocots and eudicots hold about 97 percent of flowering plant species.

Section 2Plant tissues, organs and growth

Three organs, two systems

A vascular plant body is built from just three organs: roots, stems and leaves. They form two systems. The root system anchors the plant, absorbs water and minerals, and often stores carbohydrates. The shoot system (stems plus leaves, and flowers in angiosperms) captures light and carbon dioxide. Each system depends on the other: roots cannot photosynthesize and depend on sugar from the shoot, while the shoot depends on the water and minerals the roots deliver.

Most eudicots have a taproot system: one main vertical root with lateral roots, good for reaching deep water and for storage (carrots). Most monocots have a fibrous root system, a mat of thin roots that holds soil against erosion (grasses). Roots that grow from stems or leaves are adventitious. Most absorption happens near root tips, where root hairs, outgrowths of single epidermal cells, vastly increase surface area.

A stem has nodes, where leaves attach, and internodes between them. An axillary bud in the angle between leaf and stem can grow into a branch, while the apical bud at the shoot tip drives upward growth. Modified stems include rhizomes (horizontal underground stems), stolons (surface runners of strawberries) and tubers such as potatoes, whose "eyes" are axillary buds, proof that a potato is a stem.

A typical leaf has a flat blade and a stalk, the petiole. Leaves are modified as pea tendrils, cactus spines, onion storage leaves and the traps of carnivorous plants.

Three tissue systems

Each organ is made of three tissue systems that run continuously through the whole plant. The dermal tissue system is the outer covering. In young, nonwoody parts it is the epidermis, a layer of tightly packed cells covered by the waxy cuticle; it includes guard cells, root hairs and trichomes (hairlike outgrowths that reduce water loss and deter insects). In woody parts the epidermis is replaced by periderm. The vascular tissue system consists of xylem and phloem. The ground tissue system is everything else: tissue inside the vascular tissue is pith, and tissue outside it is cortex. Ground tissue does photosynthesis, storage and support.

Plant cell types

Three basic cell types make up ground tissue, and specialized cells make up xylem and phloem. Mature plant cells differ mostly in their cell walls. All have a primary cell wall made while the cell grows; some add a thick, often lignified secondary cell wall inside the primary wall once growth stops.

Meristems and primary growth

Animals mostly stop growing at maturity, but plants show indeterminate growth: they keep growing throughout life because they keep meristems, populations of undifferentiated, dividing cells. Apical meristems at root and shoot tips produce primary growth, which lengthens roots and shoots. Lateral meristems produce secondary growth, which thickens stems and roots in woody plants. Annuals complete their life cycle in one year, biennials in two, and perennials live many years.

A root tip shows primary growth as overlapping zones. The root cap protects the meristem and senses gravity. Behind it, the zone of cell division holds the apical meristem; in the zone of elongation, cells lengthen up to tenfold by filling their vacuoles with water, pushing the tip through soil; in the zone of differentiation, cells specialize and root hairs appear.

Apical meristems give rise to three primary meristems: protoderm (becomes epidermis), ground meristem (ground tissue) and procambium (primary xylem and phloem). In a eudicot root, vascular tissue forms a central core, often an X of xylem with phloem between its arms, surrounded by the pericycle (origin of lateral roots) and the endodermis, the innermost cortex layer. In stems, vascular tissue forms vascular bundles, in a ring in eudicots and scattered in monocots.

Leaf anatomy

A leaf must exchange gases while losing little water. Cuticle-covered upper and lower epidermis enclose the mesophyll; stomata, each a pore between two guard cells, are usually most numerous on the lower surface. In eudicots, columnar palisade mesophyll under the upper epidermis does most photosynthesis, and loose spongy mesophyll below has air spaces connected to the stomata. Veins are wrapped in a bundle sheath, the site of the Calvin cycle in C4 plants.

Secondary growth

Secondary growth adds girth in gymnosperms and many eudicots, rarely in monocots. The vascular cambium, a cylinder of dividing cells between primary xylem and phloem, makes secondary xylem inward and secondary phloem outward, far more xylem than phloem. Accumulated secondary xylem is wood. Wide spring cells and narrow late-summer cells produce annual growth rings. Older central heartwood no longer conducts water; outer sapwood does.

As the stem widens, the cork cambium forms in the outer cortex and makes suberin-coated cork cells; together they form the periderm, which replaces the epidermis, with lenticels for gas exchange. Bark is everything outside the vascular cambium, including secondary phloem. Girdling a trunk therefore cuts off sugar to the roots, and the tree slowly dies.

Section 3Transport in plants and plant nutrition

Water potential

To predict where water will go, plant biologists use water potential (Ψ), the potential energy of water per unit volume relative to pure water, measured in megapascals (MPa). The rule is simple: water moves from higher water potential to lower water potential. Pure water in an open container has Ψ = 0 MPa.

Two components matter most:

Ψ = Ψ_S + Ψ_P

• Solute potential (Ψ_S, also called osmotic potential) is always zero or negative. Dissolved solutes bind water molecules and lower their freedom to move, so adding solute makes Ψ_S more negative. A 0.1 M sucrose solution has a solute potential of about −0.23 MPa.
• Pressure potential (Ψ_P) is the physical pressure on the water. It can be positive, as in the turgor pressure of a living cell pushing its membrane against the wall, or negative, as in the tension that pulls water up through dead xylem cells.

Consider a flaccid cell (Ψ_P = 0) with Ψ_S = −0.7 MPa placed in pure water (Ψ = 0). Water enters because 0 is higher than −0.7. As the cell swells, the wall pushes back and Ψ_P rises until it reaches +0.7 MPa, at which point Ψ inside equals 0 and net movement stops: the cell is turgid. Placed instead in a 0.4 M sucrose solution (about −0.9 MPa), the cell loses water, the protoplast shrinks away from the wall, and the cell undergoes plasmolysis. Wilting is the loss of turgor across many cells. Water crosses membranes faster through aquaporins, channel proteins that change the rate, not the direction, of water movement.

Water potential drops steadily from soil to air, with the largest drop between leaf and atmosphere. The sun and dry air supply the energy; the plant spends no ATP lifting water.

Getting water and minerals into the root

Water and dissolved minerals cross the root from the soil to the xylem by three routes. In the apoplastic route, they move through the continuum of cell walls and extracellular spaces without crossing a membrane. In the symplastic route, they cross a plasma membrane once, then move from cell to cell through the cytoplasm via plasmodesmata, cytoplasmic channels that connect neighboring cells. In the transmembrane route, they repeatedly cross membranes, in and out of cell after cell.

The apoplastic route is blocked at the endodermis, the innermost layer of the cortex. Each endodermal cell is ringed by the Casparian strip, a belt of waxy suberin in its radial and transverse walls, which water and dissolved ions cannot pass. Anything traveling in the walls must cross a plasma membrane into an endodermal cell before reaching the vascular cylinder. That gives the plant a checkpoint: membrane transporters decide which ions enter, keeping out many toxins and pathogens and preventing minerals already in the xylem from leaking back out. The Casparian strip works like the tight junctions between epithelial cells in animals.

Mineral uptake is mostly active. Root cells use proton pumps (H⁺-ATPases) to create a membrane potential; cations such as K⁺ enter through channels, and anions such as NO₃⁻ enter by cotransport with H⁺. Ions accumulated in the vascular cylinder draw water in by osmosis. At night this can build root pressure, which forces droplets out of leaf margins (guttation) but lifts water only a few meters.

Long-distance transport: the cohesion tension mechanism

Water rises in xylem by bulk flow, the movement of a fluid driven by a pressure difference, here a negative pressure (tension) generated at the leaves. The cohesion tension hypothesis explains it in steps:

1. Transpiration, the evaporation of water from leaves, occurs as water vapor diffuses out of the air spaces through stomata into drier air.
2. Evaporation from the thin film of water coating mesophyll cell walls makes that film retreat into tiny pores in the wall. Surface tension at the curved air-water interfaces creates a negative pressure (tension) that pulls water from the xylem.
3. Cohesion, hydrogen bonding between water molecules, transmits this pull down through an unbroken column of water in the xylem all the way to the roots.
4. Adhesion, attraction of water to the hydrophilic walls of tracheids and vessels, helps the column resist gravity and stay attached.
5. Tension in root xylem lowers its water potential below that of the soil, so water enters the roots.

An air bubble can break the column, a cavitation event that blocks a vessel; drought and freezing increase the risk. Water detours through pits into neighboring cells, and narrow conifer tracheids resist cavitation better than wide vessels.

Stomata: the compromise between photosynthesis and water loss

A plant must open its stomata to take in carbon dioxide, but every open stoma also loses water. A single corn plant can transpire over 200 liters during a growing season, and more than 90 percent of the water a plant takes up is lost by transpiration. Transpiration also cools leaves by evaporation and drives mineral delivery, so it is not pure waste.

Guard cells take up potassium ions (K⁺), driven by proton pumps; water follows by osmosis, and radially wrapped cellulose microfibrils make the swelling cells bow apart, opening the pore. When K⁺ leaves, the cells go limp and the pore closes. Stomata open at dawn in response to blue light, low internal CO₂ and an internal circadian clock; abscisic acid (ABA), made during water stress, closes them.

Xerophytes, plants of dry habitats, add thick cuticles, small or rolled leaves, stomata sunk in hairy pits and water-storing stems. CAM plants open stomata only at night, and C4 plants concentrate CO₂ so stomata can stay partly closed (Chapter 6).

Phloem transport: the pressure flow model

Translocation is the transport of organic molecules in phloem. Phloem sap is mostly water with up to 30 percent sucrose, plus amino acids, hormones and RNA signals. It flows from a sugar source (mature leaves, or a sprouting tuber breaking down starch) to a sugar sink (roots, buds, growing leaves, fruits, seeds). Phloem can flow up or down, and one organ can be a sink in one season and a source in another.

The pressure flow hypothesis explains phloem movement as bulk flow driven by a positive pressure gradient:

1. At the source, sucrose is loaded into sieve-tube elements, often actively by H⁺/sucrose cotransporters in companion cells, using the proton gradient made by proton pumps.
2. High sucrose lowers the water potential in the sieve tube, so water enters from nearby xylem by osmosis, raising the pressure.
3. At the sink, sucrose is unloaded, and the water potential rises, so water leaves and the pressure falls.
4. The pressure difference between source end and sink end pushes sap along the sieve tube by bulk flow. Water leaving at the sink returns to the xylem and recycles.

Aphid stylets inserted into single sieve tubes keep oozing sap under pressure after the aphid is cut away, supporting the model.

Plant nutrition and soil

A plant builds its body mostly from air and water: about 96 percent of dry mass is carbon, oxygen and hydrogen taken in as CO₂ and H₂O. Minerals from soil make up the remaining few percent but are just as essential. An element is essential if the plant cannot complete its life cycle without it, which is tested by hydroponic culture, growing plants in solutions that leave out one element at a time. Seventeen elements are essential for most plants. Macronutrients are needed in large amounts; micronutrients in tiny amounts, usually as enzyme cofactors.

Where deficiency symptoms appear first is diagnostic. Mobile nutrients such as N, P, K and Mg can be moved out of older leaves to supply young growth, so older leaves show symptoms first. Immobile nutrients such as Ca and Fe are locked in place, so the youngest leaves suffer first. Fertilizer bags list three numbers, the N-P-K ratio, because nitrogen, phosphorus and potassium are the nutrients most often in short supply.

Soil is a mix of sand, silt and clay particles, humus (decomposing organic matter), water, air and organisms; loam, a roughly even mix, holds water and air well. Negatively charged clay and humus bind cations such as K⁺, Ca²⁺ and Mg²⁺. Roots release H⁺, which displaces those cations into soil water for uptake, a process called cation exchange. Anions such as nitrate are not held and leach away easily.

Help from other organisms

Plants cannot use atmospheric N₂. Nitrogen fixation, the reduction of N₂ to ammonia by the enzyme nitrogenase (about 16 ATP per N₂), is done only by certain prokaryotes. Legumes form root nodules housing Rhizobium bacteria: the plant supplies sugar, the bacteria supply fixed nitrogen. Nodule leghemoglobin binds O₂ and keeps free oxygen low, protecting the oxygen-sensitive nitrogenase. Farmers rotate legumes with other crops to restore soil nitrogen.

Mycorrhizae are mutualisms between fungi and the roots of about 90 percent of plant species. Hyphae extend the absorbing surface, especially for phosphate, in exchange for sugar. Ectomycorrhizae sheath the root and grow between cortex cells (conifers, oaks); arbuscular mycorrhizae penetrate root cell walls to form branched arbuscules and are the most common type. Parasitic plants such as dodder tap host vascular tissue, and carnivorous plants of nitrogen-poor bogs digest insects.

Section 4Flowers, fertilization, seeds and fruit

The structure of a flower

A flower is a compressed shoot whose modified leaves, called floral organs, are attached in four whorls to a stem tip called the receptacle. From the outside in, the whorls are the sepals, the petals, the stamens and the carpels. Sepals and petals are sterile; stamens and carpels make spores and so are the reproductive organs. Each stamen has a stalk, the filament, topped by an anther containing microsporangia (pollen sacs). Each carpel has a sticky stigma that receives pollen, a style leading down to the ovary, and one or more ovules inside the ovary.

Botanists use a few precise terms to describe flowers. A complete flower has all four whorls; an incomplete flower lacks one or more (many wind-pollinated grasses lack petals). A perfect flower (bisexual) has both stamens and carpels; an imperfect flower (unisexual) has only one. A species with separate staminate and carpellate flowers on the same individual, such as corn, is monoecious ("one house"); a species with them on separate individuals, such as holly, willow and date palm, is dioecious ("two houses"). Only female holly plants make berries.

Making the gametophytes

In each anther, diploid microsporocytes divide by meiosis to form four haploid microspores. Each microspore divides once by mitosis to form a pollen grain, an immature male gametophyte of just two cells: a large tube cell and a small generative cell inside it, all wrapped in a sculptured sporopollenin wall. The generative cell later divides by mitosis to form two sperm cells, either before pollen is released or as the pollen tube grows.

In each ovule, a diploid megasporocyte divides by meiosis into four megaspores, and usually three die. The survivor's nucleus divides by mitosis three times, giving eight haploid nuclei in a seven-celled embryo sac, the female gametophyte. Near the micropyle (the opening in the integuments) lie the egg and two synergids; at the far end are three antipodal cells; in the middle, a large central cell holds two polar nuclei.

Pollination

Pollination is the transfer of pollen from anther to stigma. About 80 percent of angiosperm pollination depends on animals and most of the rest on wind. Wind-pollinated flowers (grasses, oaks, ragweed) are small and drab and release vast amounts of light pollen. Animal-pollinated flowers match their pollinators: bee flowers are blue or yellow with ultraviolet nectar guides, hummingbird flowers are red, tubular and unscented, and bat and moth flowers are pale, night-opening and fragrant. Such reciprocal fitting is coevolution.

Many plants avoid self-fertilization, which reduces genetic variation. Some separate male and female flowers or have stamens and carpels mature at different times. Many have self-incompatibility: a plant rejects its own pollen by a self-recognition system controlled by S-genes. If a pollen grain carries an S-allele matching one in the stigma, the pollen fails to germinate or its tube stops growing. Other species, such as peas and wheat, routinely self-pollinate, which guarantees seed even without a pollinator.

Double fertilization

After a compatible pollen grain lands on a stigma, it absorbs water and germinates, and the tube cell grows a pollen tube down through the style. The tube is guided by chemical signals from the synergids, enters the ovule through the micropyle and discharges its two sperm into the embryo sac. Then comes the event unique to angiosperms, double fertilization:

• One sperm fuses with the egg, forming a diploid zygote (2n), which will become the embryo.
• The other sperm fuses with the two polar nuclei in the central cell, forming a triploid (3n) cell that becomes the endosperm, the food-storing tissue of the seed.

Double fertilization synchronizes food storage with embryo formation: the plant invests in endosperm only if an egg has been fertilized, so it does not waste resources on ovules that would never form seeds. The synergids, antipodals and other leftover cells degenerate. By contrast, in gymnosperms the female gametophyte itself stores food before fertilization, and it is haploid, not triploid.

From ovule to seed

After fertilization, each ovule becomes a seed and the ovary becomes a fruit. The endosperm develops first (coconut "milk" is liquid endosperm). The zygote divides to form the embryo and a suspensor that anchors it and feeds it from the parent.

A eudicot embryo, such as a bean, has two cotyledons (seed leaves) on an embryonic axis. Above their attachment is the epicotyl, ending in the shoot tip and first leaves (the plumule); below is the hypocotyl, ending in the embryonic root, the radicle. Bean cotyledons absorb most of the endosperm during development. In a monocot such as corn, the endosperm stays large and the single cotyledon, the scutellum, absorbs it during germination; sheaths called the coleoptile and coleorhiza protect the young shoot and root. The seed coat forms from the integuments.

Many seeds enter dormancy, a state of arrested growth broken by cues such as moisture, cold, light or fire. Germination begins with imbibition, the uptake of water, which ruptures the seed coat and restarts metabolism. In cereal grains, the embryo releases gibberellin, which signals the aleurone layer to secrete α-amylase that digests endosperm starch into sugar. The radicle emerges first; in beans a hypocotyl hook pushes through the soil, while in corn the coleoptile pushes straight up.

Fruits and seed dispersal

A fruit is a mature ovary, sometimes with other flower parts. As seeds develop, hormones from them cause the ovary wall to thicken into the pericarp, which may become fleshy (tomato, grape, peach) or dry (bean pod, nut, grain). In botanical terms, tomatoes, cucumbers, peppers, pea pods and grains of corn are all fruits. Fruits are classified by how many flowers and carpels they come from.

Fruits protect seeds and help them disperse. Wind carries maple and dandelion fruits, water carries coconuts, and animals eat fleshy fruits and pass the seeds or carry hooked fruits in their fur. Fleshy fruits ripen from green and sour to colorful and sweet when the seeds are ready.

Asexual reproduction

Plants also reproduce asexually. In vegetative reproduction, clones grow from stolons, tubers or roots; apomixis produces seeds without fertilization (dandelions); and growers use cuttings, grafting and tissue culture. Clones preserve a good genotype but are uniformly vulnerable to disease.

Section 5Plant hormones and responses to the environment

Signal transduction in plants

Plants respond to their surroundings by changing growth and chemistry, using a three-step pathway: reception by a receptor, transduction through second messengers such as Ca²⁺ and cyclic GMP, and a response, usually altered gene expression. A potato sprouting in the dark shows etiolation (pale, spindly shoots); light sensed by phytochrome triggers de-etiolation, or greening.

Plant hormones

Plant hormones are signaling molecules made in small amounts that coordinate growth and responses. Each has many effects, and outcomes usually depend on the balance between hormones and on the target tissue.

Auxin (indoleacetic acid, IAA) was discovered through phototropism, growth toward light. Charles and Francis Darwin found that grass coleoptiles bend toward light only if the tip is present, so the tip sends a signal downward. Frits Went later collected that chemical in agar blocks, which made decapitated coleoptiles bend in darkness. Auxin moves by polar transport from shoot tip toward base through PIN efflux carriers, and one-sided light shifts it to the shaded side.

Auxin stimulates elongation by the acid growth hypothesis: it activates proton pumps that acidify the cell wall, activating proteins called expansins that loosen cellulose fibers. The turgid cell then takes up water and stretches. Synthetic auxins have many uses: they promote rooting of cuttings, and the auxin mimic 2,4-D is a selective herbicide that kills broadleaf eudicot weeds by overstimulating their growth while sparing monocot lawns and grain crops.

Apical dominance shows hormone balance at work. Auxin from the apical bud, together with strigolactones, inhibits axillary buds, while cytokinins from the roots promote their growth. Pinching off the shoot tip removes the auxin source, and the buds below grow into branches, which is why gardeners prune to make bushier plants. Gibberellins cause dramatic stem elongation: the tall height of many crops depends on them, and the dwarf wheat and rice varieties of the Green Revolution carry mutations in gibberellin signaling that put energy into grain instead of stalk. Growers spray gibberellins on seedless grapes to make larger fruit on longer stalks.

Ethylene is a gas, and ripening is driven by positive feedback (ethylene stimulates more ethylene), so one ripe fruit speeds ripening of its neighbors. A seedling blocked by a stone shows the triple response: slower elongation, a thicker stem and sideways growth. In autumn, a falling auxin to ethylene ratio activates the abscission layer and leaves drop. Abscisic acid is the stress hormone: it closes stomata in drought and maintains seed dormancy.

Responses to light

Plants detect the color, direction, intensity and duration of light. Blue light photoreceptors, including phototropins and cryptochromes, control phototropism, stomatal opening and the slowing of stem elongation as a seedling emerges into light. Phytochromes absorb mainly red and far-red light. A phytochrome exists in two interconvertible forms: Pr absorbs red light (about 660 nm) and is converted to Pfr, the active form; Pfr absorbs far-red light (about 730 nm) and is converted back to Pr. Pfr also slowly reverts to Pr in darkness. Sunlight contains both wavelengths, but leaves absorb red light, so the light under a canopy is enriched in far-red. A seedling in shade therefore has a low Pfr to Pr ratio and responds by elongating its stem, a strategy called shade avoidance. Small seeds such as lettuce germinate only after a flash of red light; if a far-red flash follows, they stay dormant. The last flash wins.

Many plant activities follow circadian rhythms, cycles of about 24 hours driven by an internal clock that persist even in constant conditions. Phytochromes and cryptochromes reset the clock to the actual day each morning. Photoperiodism is a response to the relative lengths of night and day, and it controls flowering in many species. Researchers named the categories by day length, but plants actually measure the length of uninterrupted darkness.

Red flashes interrupt the night effectively and a far-red flash afterward cancels the effect, which shows that phytochrome is the photoperiod sensor. Leaves detect the photoperiod, but flowers form at the shoot apex, so a signal must travel. This mobile signal, long called florigen, is a protein called FLOWERING LOCUS T (FT), made in leaves and carried through phloem to the apical meristem. Some plants also need a period of cold before they can flower, called vernalization; winter wheat planted in autumn flowers in spring for this reason.

Responses to gravity, touch and stress

A tropism is growth toward or away from a stimulus. In gravitropism, roots grow downward (positive) and shoots grow upward (negative). Plants sense gravity with statoliths, dense starch-filled plastids (amyloplasts) that settle to the low side of cells in the root cap and in stem tissue. Their settling redistributes auxin to the lower side. In shoots, more auxin on the lower side speeds elongation there and bends the shoot up; in roots, which are far more sensitive to auxin, the same high concentration inhibits elongation on the lower side and bends the root down.

In thigmotropism, growth responds to touch, as when a tendril coils around a support, and repeated wind produces short, stocky stems (thigmomorphogenesis). Touching the sensitive plant Mimosa sends an action potential through the leaf; motor cells lose K⁺ and water and the leaflets fold within seconds.

In drought, ABA closes stomata and roots grow deeper. In flooding, ethylene triggers cell death in the root cortex, forming air channels (aerenchyma). Under salt stress, cells accumulate solutes to keep water potential low. Heat induces heat shock proteins that help other proteins keep their shape, and cold acclimation adds unsaturated fatty acids that keep membranes fluid.

Defense against herbivores and pathogens

Against herbivores, plants use thorns and trichomes, toxins such as nicotine and tannins, and calls for help: caterpillar-damaged corn releases volatiles that attract parasitoid wasps, often via the jasmonate pathway.

Against pathogens, plants have two immune layers. In PAMP-triggered immunity, receptors recognize molecules common to classes of microbes, such as bacterial flagellin. Pathogens whose effectors suppress this meet effector-triggered immunity: plant R genes encode receptors for specific effectors (the gene-for-gene relationship). Recognition triggers the hypersensitive response, in which cells around the infection die and wall it off, and salicylic acid signaling then sets up systemic acquired resistance throughout the plant.

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