EFB530 Plant Physiology

Abscisic acid, stress, and dormancy

Abscisic acid (ABA) was first discovered as an active compound involved in initiating bud dormancy in sycamore (originally called dormin) and in initiating cotton boll abscission (originally called abscisin II)

• abscisic acid is not a very good name, since ethylene is definitely the principle hormone involved in abscission
• dormin would have been a better name, but its too late now

ABA is a 15-carbon compound (a sesquiterpene), which is synthesized in a pathway that involves carotenoid intermediates

• carotenoids are synthesized by assembly of isoprene units
• violaxanthin (a carotenoid) is cleaved to yield the precursor to abscisic acid (ABA)
• reactions occur primarily in plastids
• ABA can be synthesized throughout the plant, where there are plastids, but is primarily made in roots

Structure and activity

• ABA can be inactivated by chemical modification or by conjugation to glucose
• isomers (cis,trans) and enantiomers (S,R) of ABA are occur in different proportions in plants, can have different activities
• the S-cis form is the most abundant naturally occurring form, and is the active form

ABA can be transported through the xylem or the phloem, and can also be conjugated to glucose

ABA can be redistributed in the leaf, based on its chemistry and pH (see chapter 23, Fig. 23.4)

• in a well-watered plant, the xylem sap is more acidic (around pH 6.3)
• under drought stress, the sap becomes more basic (higher pH, around 7.2; lower H⁺ conc.)
• ABA tends to become deprotonated to ABA^-, then can't cross the membrane, so it tends not to enter the mesophyll cells, instead more ABA goes to the guard cells
• as more ABA flows to the guard cells, it induces stomatal closure, even though the absolute levels of ABA have not changed, more ABA has been redistributed to the guard cells

ABA can also be sequestered within the mesophyll cell (see chapter 25; Fig. 25.3)

• when photosynthesis is active, the pH of the chloroplast stroma rises (lower [H+] as H+ are pumped into the lumen), therefore more of the ABA is in the form ABA- (deprotonated carboxylate group)
• ABA- doesn't cross the membrane well and starts to accumulate in the stroma
• under drought stress, photosynthetic activity declines; therefore stromal pH drops and more of the ABA is in the protonated form (ABA-H); which can pass across the membrane and be released out of the cell to eventually move to the guard cells

1) ABA is a signal of plant stress, including drought, salt, and cold stress

• ABA levels go way up under drought stress
• ABA induces stomatal closure by modifying the activity of ion channels and the H⁺-ATPase
• drought can be sensed by roots, even when there is no change in leaf water potential
• when the leaf begins to dehydrate, can get stomatal closure before rise in whole leaf ABA levels-due to redistribution of ABA in leaves
• mutants that are insensitive to ABA or synthesize low levels of ABA are droopy/wilty because they can't trigger stomatal closure

2) ABA inhibits auxin-induced growth, has been called a growth inhibitor

tends to stimulate root growth though (more roots, less shoots under drought stress) - this encourages water conservation and root growth to seek out new water sources

• ABA may inhibit the PM H⁺-ATPase, preventing acidification of cell wall
• ABA inhibits overproduction of ethylene, thus allowing for improved root growth
• ABA may induce genes that strengthen the cell wall

3) ABA promotes seed maturation and dormancy, inhibits seed germination

• seed maturation not only includes growth and development of the embryo (embryogenesis); but also involves accumulation of storage reserves and preparation for desiccation - which occurs in the last stages of seed maturation
• protein storage reserves accumulate through the expression of a small set of abundantly expressed proteins (seed storage proteins=zein in corn, conglycinin in soybean; also lectins in bean)
• the expression of these genes is regulated by ABA, whose levels rise, then fall through the process of seed development
• the seed prepares for desiccation by producing proteins that allow the cell to survive very low water levels - these proteins act to stabilize other proteins when the cell is dehydrated
• these proteins are grouped together as late-embryogenesis abundant (LEA) proteins, also include dehydrins and RAB (responsive to ABA) proteins = all of these genes are induced by ABA
• ABA prepares the seed for desiccation and imposes dormancy on the embryo to prevent premature germination

Germination is regulated by "competition" or antagonism between ABA (dormancy) and GA (promotes growth & mobilization of storage reserves

• applied ABA prevents germination of seeds of many species
• GA induces expression of amylase and proteinase in germinating seeds; ABA represses expression of amylase
• mutants lacking ABA display vivipary = precocious germination ( a lack of seed dormancy) - this is a bad thing when we try to grow grain crops and harvest the seeds for long-term storage

ABA may also be involved in imposing dormancy on tree leaf and flower buds in the fall

4) ABA promotes senescence (antagonist of cytokinin)

• may be involved in inducing ethylene - for example in inducing abscission under drought stress

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