Chapter 23

 One molecule!  Named on assumption of role in abscission of leaves and other tissues. 2

 Primary functions  Prevention of precocious germination  Initiation and maintenance of seed dormancy  Stomatal control  Protection of cells in seeds from desiccation  Other functions may include:  Induction of storage proteins in seeds  Heterophylly  Initiation of secondary roots  Flowering and senescence  Notice what’s missing ….. 3

 ABA is synthesized from a  -carotene, typically in mature (water stressed) leaves.  Begins in the chloroplast  Completed in the cytosol  Highly mobile  Accumulate in sink tissues  Roots and seeds. 4

 Based upon the expression of genes encoding enzymes involved in ABA synthesis, there is also evidence that ABA is synthesized in:  Guard cells  Water stress, senescent leaves, and cotyledons  Phloem companion cells  Xylem parenchyma 5

 3 steps 1.Cell divisions and tissue differentiation – embryogenesis and endosperm proliferation 2.Cell divisions cease; storage compounds accumulate 3.Seeds dehydrate → quiescent/dormant  Quiescent → germination upon rehydration  Dormant → additional treatment required 6

 Regulates embryo maturation and seed germination  Peaks during embryo maturation  ABA imposes dormancy on the embryo  Prevents vivipary & precocious germination  ABA also induces seed desiccation 7

 Cultured embryos before dormancy will germinate (i.e., precocious germination) – ABA inhibits the process  Vivipary – preharvest sprouting  Happens in corn if it matures in wet weather  Lack of ABA triggers (unless deficient in GA!) 8

 ABA allows for dessication tolerance  Normally dessication damages membranes and cellular contents  Triggers synthesis of proteins and lipids  Only one of several signals controlling expression of these genes  Maintains mature embryo in dormant state 9

 Primary Dormancy – seeds released from the plant in a dormant state  Secondary Dormancy – seeds released from the plant in a nondormant state  become dormant if germination conditions unfavorable 10

 Temporal delay in germination process  Additional time for seed dispersal  Maximizes embryo survival by preventing germination in unfavorable circumstances  Coat-imposed dormancy (e.g., quiescent)  Embryo dormancy (e.g., dormant)  Role of cotyledons (peach, hazel, ash …)  See 11

 Prevention of water uptake.  Mechanical constraint.  Interference with gas exchange.  Retention of inhibitors.  Inhibitor production. 12

 Presence of inhibitors – ABA  Absence of promoters – GA  Maintenance – ABA biosynthesis  Loss – decrease in ABA/GA ratio  ABA also inhibits GA-induced synthesis of enzymes breaking down storage reserves 13

 External Triggers  After ripening – moisture content drops  Chilling (0-10°C)/ stratification  Single vs double dormancy  Light  Simple exposure  Specific photoperiod  All light-requiring seeds exhibit seed coat dormancy → removal of seed coat allows germination in absence of light. 14

 Promotes root growth; inhibits shoot growth  Wild type – normal ABA levels  Viviparous -- ABA deficient  Ample water  shoot growth greater in wild type  Root growth slightly greater  Low water potential  Shoot growth greater in viviparous mutant  Root growth much greater in wild type  Inhibits ethylene production!  Increase in root/shoot ratio at low water potential And ….. 16

 Mediates response to water stress  Accumulates in water-stressed leaves  Inhibits stomatal opening/triggers stomatal closure  Exact mechanism difficult to discern – ABA ubiquitous  Stomates close before ABA detected  Current line of thought …..  Inhibition of electron transport lowers stroma pH  Increase in apoplast pH  pH gradient triggers ABA release into apoplast 17

 Mediates response to water stress  Accumulates in water-stressed leaves  Stomates close before ABA detected  ABA concentration in apoplast triggers initial closure  Increased synthesis prolongs effect  Closure not always rely on deficit within leaves  Response to soil desiccation before loss of turgor pressure  Some sort of feed-forward loop – closes stomates before water potential changes 18

 Additional roles for ABA include:  Inhibition of lateral/secondary root development  A possible role in the control of flower formation.  Promotion of leaf senescence in absence of ethylene  Accumulates in dormant buds  Levels drop after exposure to cold temps 19