Growth and Development: Hormonal Control + Tropisms Photoperiodicity

Phloem Transport and Water Movement

• Pressure-flow model: sucrose moves from source cells into companion cells and then sieve-tube elements (Fig. 30.37).
  • Active loading of sucrose into phloem reduces water potential in phloem.
  • Water enters phloem from xylem by osmosis, generating positive turgor pressure
  • The sap (sucrose–water) is pushed down toward roots; sucrose unloaded at roots.
  • Water is recycled back to leaves via the xylem by transpiration, maintaining circulation.

Plant Sensory Systems and Responses (Overview)

• Plants respond to light, gravity, temperature, and touch using receptors that relay information to effector systems via chemical messengers.
• Receptors are photoreceptors or other sensors linked to signaling networks that regulate growth and development.

Plant Responses to Light

• Photomorphogenesis: growth and development in response to light to optimize light capture and space.
• Photoperiodism: plants use day length to track time of day and year; controls flowering, winter-bud setting, vegetative growth.
• Phototropism: directional growth toward or away from light; mediated by photoreceptors.
• Photoreceptors: chromoproteins (protein + chromophore) that sense light; chromoproteins=chromophore-bound photoreceptors.

Photoreceptors and Light Quality

• Visible light regions of interest:
  • Red and far-red (phytochrome system) and violet-blue (blue/UV-A; cryptochromes and phototropins) trigger developmental changes.
  • In terrestrial habitats, chlorophyll absorbs blue/red mostly; canopy shifts spectrum toward far-red light as blue/red are absorbed.
  • Blue light helps algae and aquatic plants detect light, while red light is absorbed by chlorophyll in water.

Phytochrome System and Red/Far-Red Response

• Phytochromes are chromoproteins with a linear tetrapyrrole chromophore; two interconvertible forms: Pr and Pfr.
  • Pr absorbs red light at ≈ 667 nm and converts to Pfr.
  • Pfr absorbs far-red light at ≈ 730 nm and converts back to Pr.
  • Absorption causes conformational changes; Pfr is the physiologically active form; red light yields activity, far-red inhibits.
  • The active Pfr form can activate cytoplasmic targets or be trafficked to the nucleus to regulate gene expression.
• Phytochrome as a light switch:
  • It monitors level, intensity, duration, and color of environmental light.
  • Red light immediately activates phytochrome (to Pfr); far-red light can reverse activity by converting Pfr to Pr.
  • In darkness, Pfr gradually reverts to Pr; this reversibility allows dynamic responses to changing light.
• Ecological and developmental implications:
  • Full-spectrum sunlight contains more red than far-red; chlorophyll absorbs red; shade (far-red enriched) leaves accumulate Pr, slowing growth.
  • In shade, plants sense far-red and grow toward light (escape shade).
  • In seeds, phytochrome is used to detect light availability rather than direction/quality; seeds with little reserves germinate only when light is detected at the soil surface (Pr → Pfr triggers germination).
  • Phytochrome also senses seasonal changes via photoperiodism, helping plants time flowering and dormancy.
• Photoperiodism details:
  • Short-day (long-night) plants flower when nights exceed a critical length (often ≈ 8 hours or fewer).
  • Long-day (short-night) plants flower when nights are short (often ≈ 8–15 hours).
• Figure references:
  • 30.38: Pr and Pfr dynamics; red/far-red reversal and nuclear signaling.

Horticulture and Practical Applications

• Etymology: horticulturist from hortus (garden) + cultura (cultivation).
• Greenhouse management uses light control and environment manipulation to optimize growth and development:
  • Blackout shade cloth lengthens nights to promote flowering in long-night crops.
  • Red-light irradiation in winter promotes flowering in long-day plants.
  • Light quality for seedlings: fluorescent light high in blue wavelengths promotes leafy growth.
  • Incandescent lamps rich in red light promote flowering for some crops.
• Hormones and ripening: hormones can be applied to adjust fruit ripening timings.
• Breeding and climate adaptation: ongoing development of crop varieties with improved yields, pest resistance, and transportability.
• Career pathways: greenhouse management, private/government labs, botanical gardens; integration with genetics, plant physiology, pathology; interdisciplinary studies with economics and computer science.

Blue Light Responses and Phototropism

• Phototropism: bending toward (positive phototropism) or away from light (negative phototropism or skototropism).
• Phototropins: blue-light receptors; protein-based receptors with a flavin chromophore; part of a class called flavoproteins.
• Other blue-light responses controlled by phototropins: leaf opening/closing, chloroplast movement, stomatal opening.
• Historical milestones:
  • Darwin and his son Francis (1880) described phototropism and localization of sensing at the tip with a response at the base.
  • Peter Boysen-Jensen (1913) showed that a chemical signal from the tip mediates bending, traveling on the shaded side; signal is a growth stimulant because it causes faster elongation on the shaded side.
• Mechanism details:
  • Light passing through the stem diffracts and activates phototropin across the stem; activation occurs mainly on the lit side, causing accumulation of the hormone auxin (IAA) on the shaded side.
  • IAA promotes cell elongation on the shaded side, producing bending toward the light.
• Cryptochromes: blue-light receptors with flavin chromophores; help set circadian rhythms and may interact with phototropins to mediate phototropic responses.

Plant Responses to Gravity (Gravitropism)

• Gravitropism ensures roots grow downward and shoots grow upward, regardless of initial orientation.
• Growth directions:
  • Shoot apical tip growth upward is negative gravitropism.
  • Root growth downward is positive gravitropism.
• Amyloplasts (statoliths): starch-containing plastids that settle to the bottom of gravity-sensing cells (root cap and other specialized cells).
  • Settling amyloplasts contact the endoplasmic reticulum, triggering calcium ion (Ca2+) signaling.
  • Ca2+ signaling causes polar transport of the hormone IAA toward the bottom of the cell.
  • In roots: high IAA concentration at the bottom inhibits elongation, slowing growth on the lower side; upper-side cells elongate relatively more.
  • In shoots: higher IAA concentration on the lower side promotes cell expansion, causing the shoot to bend upward.
• Post-orientation adjustment: amyloplasts return to their standard positions after reorientation.
• Additional hypotheses exist to explain gravitropism in mutants lacking amyloplasts; some show weak gravitropic responses, suggesting other cellular components may contribute.

Growth Responses and Plant Hormones (Overview)

• Plant hormones act as chemical messengers influencing growth, development, and responses to environmental cues.
• Hormones can be produced by many cells and act locally or be transported to distant tissues; responses often reflect synergistic or antagonistic interactions among hormones.
• Five major traditional plant hormones: auxins (notably IAA), cytokinins, gibberellins, ethylene, and abscisic acid (ABA).
• Some growth factors and nutrients also influence plant growth.

Auxins (IAA)

• Etymology: auxin from Greek auxein, meaning to grow.
• Roles:
  • Primary drivers of cell elongation in phototropism and gravitropism.
  • Influence differentiation of meristematic tissue into vascular tissue; promote leaf development and arrangement.
  • Apical dominance: auxins from the apical meristem suppress lateral buds.
  • Involved in flowering, fruit set, ripening, and abscission inhibition.
  • Act as a relay for blue light and red/far-red light signaling.
• IAA is the only naturally occurring auxin with strong physiological activity.
• Commercial uses:
  • Rooting hormone to promote adventitious root formation on cuttings.
  • Greenhouse applications: synchronize fruit setting and drop; promote normal fruit development in tomatoes; induce seedless fruit in some crops.

Cytokinins

• History: first found when coconut endosperm stimulated growth in plant embryos; cytokinin promotes cytokinesis (cell division).
• Diversity: nearly 200 naturally occurring or synthetic cytokinins identified.
• Where they are abundant: growing tissues (roots, embryos, fruits) where cell division occurs.
• Roles:
  • Delay senescence in leaves; promote mitosis and meristem differentiation in shoots and roots.
  • Often act in concert with auxin or other hormones to influence development.
  • Balance with auxin affects apical dominance and bushier growth (auxin–cytokinin balance).

Gibberellins (GAs)

• Family: about 125 related plant hormones.
• Roles:
  • Stimulate shoot elongation, seed germination, and maturation of fruits and flowers.
  • Promote breaking dormancy in seeds that require cold or light cues for germination.
  • Involves gender expression, seedless fruit development, and delaying senescence.
• Sources: synthesized in roots, shoot apical meristems, young leaves, and seed embryos.
• Practical examples:
  • GA antagonists used to control growth of trees near power lines.
  • Grapes treated with GA to increase fruit size and loosen clusters (reduced mildew; Fig. 30.40).

Abscisic Acid (ABA)

• History: discovered as the agent causing dropping of cotton bolls; later found to have broader roles.
• Stress response: ABA accumulates in response to dehydration, cold, or short days.
• Antagonist relationships: ABA counters growth-promoting effects of GA and auxins.
• Effects:
  • Inhibits stem elongation; induces dormancy in lateral buds.
  • Induces seed dormancy by inhibiting germination and promoting storage protein synthesis.
  • Promotes winter bud development and mediates apical meristem-to-dormant bud conversion.
  • Under low soil moisture, ABA levels rise, causing stomatal closure to reduce water loss.
  • ABA breakdown over winter helps seed germination when conditions improve.

Ethylene

• Nature: ethylene is a volatile gaseous hormone (C2H4).
• Sources: produced by aging tissues, senescing leaves, and plant organs (e.g., nodes).
• Roles:
  • Fruit ripening: promotes starch-to-sugar conversion, accelerating maturation.
  • Leaf and fruit abscission; flower fading/dropping; sprouting in some cereals and potato sprouting.
• Agricultural applications:
  • Used to regulate ripening in stored fruit.
  • Horticulturists remove ethylene from greenhouses via ventilation to delay senescence.

Nontraditional Hormones and Related Growth Factors

• Jasmonates:
  • Key role in defense against herbivory; levels rise upon wounding.
  • Promote synthesis of toxic secondary metabolites and volatile compounds that attract natural enemies of pests (e.g., caterpillar attack on tomato triggers jasmonic acid and volatile emissions).
• Oligosaccharins:
  • Involved in defense against bacterial and fungal infections; act locally and can be transported to other tissues.
• Strigolactones:
  • Promote seed germination in some species; inhibit lateral apical development in the absence of auxins.
  • Play a role in establishing mycorrhizal associations between roots and fungi.
• Brassinosteroids:
  • Important for many developmental and physiological processes; interact with auxin and gibberellins to amplify their effects.
  • Positively influence apical dominance, seed germination, gravitropism, and frost resistance.
  • Inhibit root growth and reduce fruit dropping in some contexts.

Plant Responses to Wind and Touch (Thigmotropy, Thigmonasty, Thigmomorphogenesis)

• Thigmotropism: directional growth in response to touch or contact (e.g., tendrils wrapping around supports).
  • Tendrils show rapid coiling when lightly touched; cells on contact side contract while opposite side expands.
  • Jasmonic acid can trigger tendril coiling even without mechanical stimulus.
• Thigmonasty: a touch response that is not directionally oriented with respect to the stimulus (non-directional movement/response).
• Thigmomorphogenesis: slow, developmental change in plant form due to continuous mechanical stress (e.g., wind pressure causes thicker trunks and stronger tissues, especially xylem).
  • Ethylene and jasmonate are likely involved in thigmomorphogenesis.

Defense Responses Against Herbivores and Pathogens

• Physical barriers: bark, waxy cuticle, thorns, and spines deter herbivory and protect against pathogens.
• Mechanical damage creates entry points for pathogens; plants deploy enzymatic and chemical defenses to limit spread.
• Secondary metabolites: not directly required for primary metabolism but deter predators and pathogens; many are toxic or deterrent.
  • Alkaloids: pungent or bitter compounds (e.g., caffeine) that can stimulate or paralyze herbivores or pathogens.
  • Cyanogenic glycosides (e.g., in cassava): release cyanide upon herbivore ingestion.
• Local and systemic defense signaling:
  • Wounding triggers local defenses and signals that travel to distant tissues (systemic response).
  • Jasmonates promote synthesis of toxic compounds and volatiles that attract natural enemies of herbivores (parasitoids).
  • Tissue damage can lead to abscission of injured tissues to prevent pathogen spread.