The Biological Role of Abscisic Acid in Precocious Seed Germination Present by Collin Mecke April 1st 61.325

Introduction to ABA -Synthesis of ABA occurs in the chloroplasts and other plastids using the terpenoid pathway (fig 23.2) Detected in all parts of the plant Transported by both phloem and xylem Inhibits stomata opening Regulates seed and bud dormancy

Discovery of ABA Unknown inhibitory compounds that effect the dormancy of seeds and buds were known about for a long period Inhibitory compounds that differed from auxin were discovered (1953) A substance that promotes abscission in cotton fruit was found ten years later and called Abscission II (1963) At the same time another compound that promoted bud dormancy was found and call Dormin These two substance (Abscission II and Dormin) were found to be identical and called Abscisic Acid.

How is it Measured? Bioassay-Measuring coleoptile growth inhibition. Minimum detection is 10-7 M -Closure of stomata (10-9M) Physical Methods- Gas Chromatography (10-13g) -High Performance Liquid Chromatography Immunoassay-Recognition of ABA by antibodies (10-13g)

How ABA Causes Dormancy At high levels of ABA (just before dormancy occurs) late-embyrogenesis abundant proteins (LEA) are produced LEA’s are thought to be involved in desiccation tolerance ABA has been shown to affect the amount and composition of storage proteins in a double mutant of Arabidopis (aba/abi3-1) and ABA deficient mutants of maize. This process is called embryo induced dormancy

Precocious Germination -Immature embryos were removed from seeds and cultured midway through development (before the onset of dormancy) -These embryos germinated precociously (without passing through normal dormancy) - When ABA was added to the medium precocious germination did not occur.

Further Evidence Zea maize has several mutant species; vp2, vp5 and vp7 that are ABA deficient (fig 23.2) The embryos of these plants germinate precociously on the cob. This process is called viviparous. The addition of exogenous ABA prevented precocious germination. Also, there is a vp1 mutant which is insensitive to ABA. It will germinate precociously even with the addition of exogenous ABA.

Current Research It has been well documented that the absence of ABA causes precocious germination. Is a certain amount of GA required? Fluridone was used to cause ABA deficiency and paclobrutrazol or ancymidol to reduce GA in corn Measured the amount of GA in each seed and whether precocious germination occurred. It was found that a minimum amount of GA was required for precocious germination.

Current Research vp5 maize mutant was cross with a dwarf maize (d1-GA deficient) The resulting double mutants (vp5/d1) did not undergo precocious germination Addition of exogenous GA to double mutant did produce precocious germination

Current Research Low levels of GA did not diminish vivipary in vp1 mutants. This suggests that the GA and ABA signaling converges upstream from the site of vp1 action. It was proposed that vp1 controls seed maturation and germination through 2 processes; 1. Transcriptional activator of ABA-inducible gene expression 2. ABA-independent repressor of germination specific gene expression

Uses in Bio-Tech Causing seeds to germinate faster (skipping the dormancy stage of development) will results in plants growing to maturity faster. This could result higher crop production.