Plant growth regulatorsBiol 3470

Chapter 16

March 7, 2006

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

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PGRs are inhibitory for metabolism (e.g. ethylene) while others promote growth (e.g., GAs, cytokinins) is similarly outdated

PGR role #1: Cell division, enlargement and differentiation

Fig 16.1

• 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

• 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)

Auxins stimulate cell enlargement

Fig 16.4

(different concentrations of IAA)

Response curve

Looks a lot like a nutrient response curve!

Critical conc

• 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

Cell wall / apoplasm

Fig 16.5

Oat coleoptile

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

Polar transport of auxin important in plant development

• 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

Unidirectional: moves basipetally (apex down)

Mechanism: distinct import and export proteins at opposite ends of cell

Influx carrier imports protonated IAA only (all around cell)

Efflux carrier exports deprotonated IAA only (at basipetal end of cell only)

(apoplast)

Fig 16.9Fig 16.7

Auxins and cytokinins are required for vascular differentiation

• 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

Fig 16.10

wound

Vascular regeneration initiation site

APEX

PGR role #2: seed development and germination

Cell division, endosperm and embryo formation

Develop dessication tolerance

• 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)

Fig 16.11

Dormant seeds - added GA promotes little starch hydrolysis

Dormant seeds + added GA results in production of enzymes that break down starch

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

Cell enlargement(endosperm chromosome

replication)

PGR role #3: shoot and root developmentStem elongation

• GAs elongate intact stems (internodes)

• Evident in mutants in GA synthesis pathway: dwarfs!

• Heritable, recessive mutations

Dwarf peaDwarf pea + GA

Fig 16.16

Rosette Arabidopsis

+ GA

• 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

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

Ctrl bean

Apexremoved

Apexremoved+ IAA

Fig 16.17

Fig 16.19

Causes growth of axillary buds

Auxin and ethylene control root growth• 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

+ low IAA

Controls (- IAA)

Fig 16.21

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

Fig 16.22+ cytokinin controls

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

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

achene

auxin

PGR role #6: ethylene• 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!

C2H4