Plant Hormones 1

• Plant Hormones


  Signal molecules that individually or cooperatively direct the development of individual cells or carry information between cells and thus coordinate growth and development
• Plants are multicellular organisms and as such require coordination between cells
• In order to coordinate their activities cells must be able to communicate with each other
• The principle means of communication within plants are the hormones
• Although molecules that influence growth and differentiation in plant cells have been studied for nearly 100 years the concept of hormones in plants is still steeped in controversy
• Charles Darwin first describes the outcomes of light on motion of canary grass (Phalaris canariensis) coleoptiles


  1880
• It was Darwin's and others observations and experiments that led to F.W. Went, almost 50 years later, to describe a hormone like substance as the causative agent when plants grew towards the light
• Hormones (according to current usage)

  Naturally occurring organic substances that, at low concentrations, exert a profound influence on physiological processes
• Differences between plant and animal hormones

  • The site of synthesis of plant hormones is not are clearly localized as in animal systems, synthesis of plant hormones appears to be much more diffuse and cannot always be localized to discrete tissues
  • Whether plant hormones act in a concentration dependent manner is still a subject of some debate
  • Plant hormones have a multiplicity of effects, each group of plant hormones is known to influence a wide variety of developmental events, and each of these events can be influenced by more than one hormone group
• Traditionally recognized groups of plant hormones

  • Auxins
  • Gibberellins
  • Cytokinins
  • Abscisic acid
  • Ethylene
  • Brassinosteroids
• Jasmonic acid and salicylic acid are elicitor molecules that appear to be active in regulating some aspects of plant growth and may qualify for hormone status
• Auxin


  The first plant hormone to be discovered, generally ubiquitous in the plant, with the highest concentrations detected in meristematic regions and actively growing regions
• Auxins


  • Stimulate cell elongation, influence root initiation, vascular differentiation, tropic responses and the development of axillary buds, flowers and fruits
• Naturally occurring auxin compounds

  • Indole-3-acetic acid (IAA)
  • Indole-3-ethanol
  • Indole-3-acetaldehyde
  • Indole-3-acetonitrile
• IAA is considered to be the principle natural auxin
• Other naturally occurring auxins

  • IBA
  • 4-chloroindole-3-acetic acid
  • Phenylacetic acid
• Synthetic auxins


  • 2,4-D
  • NAA
• The levels of IAA present depend on a number of factors including the type and age of tissue and its state of growth
• In vegetative tissue the amount of IAA generally falls in the range of between 1 mg and 100 mg kg-1 fresh weight (5.7 – 570 nanomoles)
• In seeds the levels of IAA appear to be much higher with levels reaching up to 300 picomoles
• Tryptophan independent IAA Biosynthesis


  Biological Importance Remains Unclear
• Auxin Deactivation


  1. Oxidative decarboxylation of IAA catalyzed by peroxidase

  2. Oxidation of the indole ring, forming an oxindole

  3. Decarboxylation of the acetate side chain without oxidizing the indole ring
• The decarboxylated indole pathway is the most likely normal catabolic pathway in plants
• Auxin Conjugates


  Conjugation of IAA with amino acids leads to irreversible deactivation, the fate of the IAA-amino acid conjugate varies depending on the species but usually involves oxidation of the indole ring and subsequent addition of one or more glucose residues
• Reversible Conjugations


  IAA also exists as chemical conjugates such as glycoysl esters, these conjugates are inactive but release free active IAA by enzymatic hydrolysis
• Gibberellins


  Members of a large and varied family of plant molecules known as the terpinoids, can be defined on the basis of their chemical structure as much as by their biological activity
• Types of Gibberellins


  • C20-gibberellins
  • C19-gibberellins
• GA3 (gibberellic acid) was one of the first to be isolated and characterized, and is the most common commercially available form
• GA1 and GA20 (C19-GAs) are probably the most active and therefore most important gibberellins in higher plants
• Gibberellins


  • A large proportion have little or no biological activity, they are either intermediates in the synthesis of active forms or metabolic products that still retain the basic structure but are no longer biologically active
  • The number of active forms is smaller than the total number of gibberellins found in any one species or organ
  • Evidence suggests that GA1 may be the principle if not only active gibberellin regulating stem elongation in higher plants
• Structural features of biologically active Gibberellins

  • A carboxyl group at C7 is required for biological activity
  • C19-GAs are more biologically active than C20-GA molecules
  • Those GAs with 3-β-hydroxylation, 3-β,13-dihydroxylation are generally more active
  • Those with both 3-β-OH and 1,2-unsaturation demonstrate the highest activity
• Gibberellin Biosynthesis

  1. The principle sites are developing seeds and fruits, the young leaves of developing apical buds, elongating shoots and the apical regions of roots

  2. Gibberellins are terpenes sharing a core pathway with several other hormones and a wide range of secondary products

  3. Gibberellins are synthesized from the precursor molecule isoprene (5-carbon) via mevalonic acid (C6)
• GA1 may be the principle if not only active gibberellin regulating stem elongation in higher plants
• Gibberellins


  • Carboxyl group at C7 is a feature and is required for biological activity
  • C19-GAs are more biologically active than C20-GA molecules
  • GAs with 3-β-hydroxylation, 3-β,13-dihydroxylation are generally more active
  • GAs with both 3-β-OH and 1,2-unsaturation demonstrate the highest activity
• Gibberellin Biosynthesis


  1. Developing seeds and fruits

  2. Young leaves of developing apical buds

  3. Elongating shoots

  4. Apical regions of roots
• Gibberellins


  Terpenes sharing a core pathway with several other hormones and a wide range of secondary products
• Gibberellin Synthesis


  1. From precursor molecule isoprene (5-carbon) via mevalonic acid (C6)

  2. D3iPP and D2iPP enter the terpene biosynthetic pathway

  3. Sequential addition of isopentyl pyrophosphate units form terpenes of increasing carbon number

  4. GA12-7-aldehyde is inactive but serves as the precursor to all other gibberellins
• Proposed pathway for GA biosynthesis in pea


  • Major pathway (bold arrows) occurs in seeds and shoots
  • Pathway shown in light arrows occurs only in shoots
  • Asterisk indicates known endogenous forms
• Antigibberellins


  • Reduce stem elongation and produce plants that are shorter and more compact with darker green foliage
  • Block the synthesis of gibberellins and their effects can be reversed by the application of gibberellins to the plant
• Gibberellins deactivation


  • 2β-hydroxylation (Best Characterized)
  • Other GA Inactivation Pathways