Hormone intro & HW assignment

Auxins

Giberellins

Abscisic acid

Ethylene

Cytokinins

HORMONES

Although we tend to look at plants as inanimate objects, we know they not only grow and reproduce with ease, but also respond to any environmental fluctuations occurring around them, including:

• abiotic signals such as changes in temperature or light as well as to,
• biotic presence's including pathogens, herbivores and competitors or more benign pollinators.

In some cases plants may alter their growth rate accordingly, either increasing or decreasing cell division and production of storage products. In other cases changes in light or temperature may indicate the wisdom of sexual reproduction and the investment of new tissue for flower and seed production. Shading may induce individuals to shoot up rapidly, while the force of wind may cause others to thicken their cell walls or add additional supportive cell layers.

Unlike animal systems where stimulated nerves moderate a cascade of appropriate responses, plants use hormones and associated biochemicals to both perceive and initiate appropriate reactions. Today in class, we'll go over specific effects of hormone action and next period review a few examples of how plants perceive the environment about them.

• The word hormone is derived from the Greek verb meaning to excite.
• By definition, hormones are organic substances synthesized in one tissue and transported out where their presence results in physiological responses ( not always true; may act at or close to synthesis site). They are required in minute amounts.... 10 to the -6 power to as low as 10 ^{to the -8 power (.00000001)}
• Each hormone may result in multiple effects -- the particular effect depending on a number of factors:
  • the presence of other hormones and the presence of activator molecules ( calcium, sugars)
  • the amount of the hormone.. auxin may cause leaf drop or prevention of abscission dependent on concentration... more graphically it can cause death or continued growth in a plant
  • the sensitivity of that tissue to the hormone... hormones dock on receptors in cell membranes; the appropriate receptor must exist in the membrane for the cell to react.
  • the condition of the plant itself is critical: what is the condition of the plant? its age?

   

• Moderation of impact:

  Site of action x developmental stage x concentration of hormone - thus the same concentration of hormones but in a earlier or later stage will have little or maximum impact, or in one tissue or another show no or induce major changes.

   

  In general hormones control plant development by: division of cells elongation differentiation of cells physiological response to environmental for short term periods ( stomatal opening)

  They do this by regulating: gene expression activity of enzymes changing membrane properties

• 

  Traditionally five major classes of plant hormones are recognized:

  • Auxin
  • Gibberellin
  • Cytokinin
  • Ethylene
  • Abscisic Acid

   

• Additional substances gaining recognition as hormones include : information comes from http://www.plant-hormones.bbsrc.ac.uk/education/Keno.htm and your text.

  Brassinosteroids There are approximately 60 steroidal compounds known as brassinosteroids named after the first one identified, brassinolide, which was found in mustard pollen. Effects include: 1. Stimulation of stem elongation. 2. Inhibition of root growth and development. 3. Promotion of ethylene biosynthesis and epinasty.

  Salicylates Salicylates have been known to be present in willow bark for quite some time. Salicylic acid is synthesized from the amino acid phenylalanine. Effects include 1. Thermogenisis in Arum flowers. 2. Plant pathogen resistance-stimulates plant pathogenesis protein production. 3. Reported to enhance longevity of flower ( practice of adding a bit to cut flowers). 4. Reported to inhibit ethylene biosynthesis. 5. Reported to inhibit seed germination. 6. Blocks the wound response. 7. Reverses the effects of ABA.

  Jasmonates Jasmonates are represented by Jasmonate and its methyl ester. They were first isolated from the jasmine plant in which the methyl ester is an important product in the perfume industry. Jasmonic acid is synthesized from linolenic acid which is an important fatty acid. Jasmonates have a number of effects such as: 1. Inhibition of many processes such as growth and germination. 2. Promotion of senescence, abscission, tuber formation, fruit ripening, pigment formation, and tendril coiling. 3. They appear to have important roles in plant defense by inducing proteinase synthesis.:defence mechanism against fungi

  Polyamines There is some controversy as to whether these compounds should be classified with hormones. They are widespread in all cells and exert regulatory control over growth and development at very low levels. Development is affected by plants having low levels of polyamines. Polyamines have a wide range of effects on plants. They appear to be essential in growth and cell division.

  Other hormone like substances include the peptide systemin and batasins

  
  

  •