Plant growth regulators

1. Plant growth regulators Biol 3470 Chapter 16 March 7, 2006

2. Biochemistry of synthesis and metabolism of PGRs • Skipped: in chapter 15 if interested • Good background on history of discovery and their classification • We will restrict our discussion to the effects of PGRs in vivo on controlling growth and development

3. PGRs control the organism’s development, just like animal hormones PGRs, plant hormones, phytohormones are all the same molecules • All play key signaling roles in development: the progression of cells from undifferentiated to having very specific roles in plant function • Rather than walk through the functions of each, let’s approach function through their effects on key plant developmental events • Brief overview: 5 major “classic” classes of PGRs • Auxins (IAA, NAA, 2,4-D, 2,4,5-T) • Cytokinins (kinetin) • Abscisic acid (ABA) • Gibberellins (GAx) • Ethylene (C2H4) • Also: New emerging PGRs include brassinosteroids and jasmonic acid • Key concept: PGRs do not act independently! They are part of a web of control of development • Concept that some PGRs are inhibitory for metabolism (e.g. ethylene) while others promote growth (e.g., GAs, cytokinins) is similarly outdated

4. PGR role #1: Cell division, enlargement and differentiation • Occurs mostly at shoot and root apical meristems--actively dividing areas at tips • Cytokinins play a significant role • Discovered in plant tissue culture: required with auxin to induce cell division and growth in all cells • Good demonstration is uncontrolled cell proliferation in neoplastic (cancerous!) crown gall growth Fig 16.1 • Galls produce auxin and cytokinin that results in neoplastic cell growth • Cytokinins and auxins regulate progression of plant cells through the cell cycle • Absence of regulation of PGR levels does not stall cells in resting phases between division/mitosis and DNA replication (S) (G phases)

5. Auxins stimulate cell enlargement Oat coleoptile • Shown when coleoptile (oat seedling stem) segments are floated on IAA • Does not work on intact seedlings (they contain enough endogenous IAA) • Auxin is required to start cell growth in tissue culture (with cytokinins) Auxin mechanism of action: acid growth increases cell extensibility • activates ATPases that excrete H+ into apoplasm (cell wall) via signal transduction chain • Activates expansin activity (loosens X-links) • Turgor pressure allows cellulose fibrils to slide past each other and the cell to grow (different concentrations of IAA) Fig 16.4 Response curve Critical conc Cell wall / apoplasm Looks a lot like a nutrient response curve! Fig 16.5

6. Auxin action over the long term involves gene activation • Auxin causes acid growth in the short term • Need to activate transcription of IAA sensitive genes in the long term • Fastest response: short auxin upregulated RNAs • Slightly later: Aux/IAA genes whose transcription is normally prevented by regulatory protein:DNA interactions • Auxin may induce ubiquitin binding and proteasome mediated degradation of these proteins • All of these genes are thought to induce transcription of genes necessary for growth and development

7. Polar transport of auxin important in plant development Fig 16.7 Fig 16.9 Unidirectional: moves basipetally (apex down) Mechanism: distinct import and export proteins at opposite ends of cell • e.g., in the gravitropic orientation of roots and shoots! • Thought to be important in other developmental responses as well • Polar transport different than vascular transport (which also occurs) • Polar transport is via a chemiosmotic mechanism: pH gradient between apoplast and cytosol that permits establishment of an auxin gradient (apoplast) Influx carrier imports protonated IAA only (all around cell) Efflux carrier exports deprotonated IAA only (at basipetal end of cell only)

8. Auxins and cytokinins are required for vascular differentiation APEX • i.e., the development of xylem and phloem from undifferentiated p_______ cells • Need auxin (IAA) to induce xylem differentiation • e.g., if a developing vascular bundle is wounded, it can regenerate as long as auxin provided (from shoot apex or exogenously applied) • Providing cytokinins allows the simultaneous regeneration of phloem sieve tube and xylem vessel elements Vascular regeneration initiation site wound Fig 16.10

9. PGR role #2: seed development and germination Fig 16.11 • Assay concentrations of PGRs during seed set, dormancy/quiescence and germination • Supports “growth” role for cytokinins, GAs, auxins and “dormancy” role for ABA • We know: • Cytokinins promote cell division; GA elongates stem tissue • Auxin: cell enlargement • ABA: dormancy and prevention of premature germination (vivipary) Cell division, endosperm and embryo formation Cell enlargement (endosperm chromosome replication) Develop dessication tolerance Fig 16.12 • Upon germination, need GAs to mobilize food reserves in seed endosperm (starch!) • GA signals pass from embryo to aleurone layer surrounding seed • Aleurone secretes protease which activates amylases, resulting in starch breakdown • GA activates synthesis of amylase mRNA and protein • Regulates both gene transcription and translation in imbibing seeds Dormant seeds - added GA promotes little starch hydrolysis Dormant seeds + added GA results in production of enzymes that break down starch

10. PGR role #3: shoot and root development Dwarf pea + GA Dwarf pea + GA Rosette Arabidopsis Stem elongation • GAs elongate intact stems (internodes) • Evident in mutants in GA synthesis pathway: dwarfs! • Heritable, recessive mutations Fig 16.16 Fig 16.17 • Other mutants do not respond to exogenous PGRs: defective in signal transduction chain mediating plant response • GA synthesis induced by cool temperatures and long days: causes bolting • prelude to flowering in spring • Inhibiting GA biosynthesis commercially valuable: small ornamental plants Ctrl bean • Apical dominance • controlled by auxin: cytokinin ratio • Axillary buds (at junction of new leaf and stem) normally grow much more slowly • Fed by basipetal IAA transport: axillary buds very sensitive • + Cytokinin releases buds from apical dominance: Cyt:Aux ratio most important! • Mechanism may also involve ABA: poorly understood Apex removed + IAA Apex removed Causes growth of axillary buds Fig 16.19

11. Auxin and ethylene control root growth + low IAA • Role of auxin confusing • Very low concs stimulate rooting • Higher concs inhibit it • Due to high auxin stimulating ethylene biosynthesis • Ethylene inhibits cell growth • Signal transduction pathways undefined • Auxin : cytokinin ratio determines shoot and root growth in callus culture • ~Equal: callus • High: roots • Low: shoots Controls (- IAA) Fig 16.21

12. PGR role #4: senescence and abscission • Senescence: rapid breakdown in metabolism in aging mature tissues (fruits, leaves) • Ethylene promotes senescence • leaf drop, climacteric fruit ripening (starch  sugar, pigment & flavour synthesis, cell wall degradation) • Cytokinins inhibit senescence • Exogenous cyt delays leaf senescence • Cyt levels fall when apoptosis begins • Constitutive expression of cyt delays tobacco leaf senescence • Abscission: leaf and fruit drop through development of abscission layer • ABA causes premature senescence of cells in organ to be shed • Accelerated by high [auxin] on stem side of layer • Aging leaves/fruits make less auxin, causing high stem:leaf ratio controls + cytokinin Fig 16.22

13. PGR role #5: flower and fruit development • How does the plant shift from vegetative to floral growth? • Must “reprogram” the meristem to make flowers instead of leaves • The elusive “florigen” was thought to control this: flowering hormone • Flowering is now thought to be under the control of phytochrome • LONG DAY vs SHORT DAY plants • There is more red light produced during longer days • Short day plants need short days to flower: lots of Pr • Long day plants need long days to flower: lots of Pfr • Flowering is moderated by PGRs • Auxin inhibits flowering • Gas (ethylene) stimulates stem bolting: a necessary precursor to flowering • Cytokinins increase cell division in meristem • . Fig 19.2

14. PGRs influence fruit set and development • Fruits normally require fertilization and pollination (fertilized ovary) • Can use exogenous (applied) auxin to • Speed up fruit set and growth (tomato) • Form fruit without pollination (parthenocarpy) in Solanaceae, Cucurbitae (cucumbers, watermelons), citrus • Seedless fruits! High economic value! • GAs have a similar role in pears & citrus • Developing seed is source for auxin that permits continuous fruit development • Remove seeds (achenes) from strawberry: stops development, restore by auxin spray • Auxin enhances fruit drop right after fruit set (fertilization): good for larger fruit • Applied later: prevents abscission (increases conc on fruit side) and prevents premature fruit drop • GAs applied to grapes: reduces fruit number per bunch and increases size of remaining grapes auxin achene

15. PGR role #6: ethylene C2H4 • Unique among PGRs: a gas! • Produced during fruit ripening • Also affects multitude of plant developmental events • Stimulates (aquatic plants, rice) or inhibits (pea) elongation of stems, petioles, internodes, floral structures • Stimulatory effects on stem length are similar to those of GA! • High [ethylene] stimulates a number of abnormal growth responses • e.g., epinasty: downward curvature, twisted, spindly growth of leaves when plant is waterlogged or severely stressed • Caused by imbalances in PGRs • Orients leaves to reduce photosynthetic capacity and transpiration • These are severely reduced in waterlogged plants because they possess reduced capacity for gas exchange PGRs are thus involved in every aspect of plant growth and development and their effects in vivo are intricately interrelated!