EFFECTS OF ABSCISIC-ACID METABOLITES AND ANALOGS ON FREEZING TOLERANCE AND GENE-EXPRESSION IN BROMEGRASS (BROMUS-INERMIS LEYSS) CELL-CULTURES

Optical isomers and racemic mixtures of abscisic acid (ABA) and the AB A metabolites abscisyl alcohol (ABA ale), abscisyl aldehyde (ABA ald), phaseic acid (PA), and 7'hydroxyABA (7'0HABA) were studied to determi ne their effects on freezing tolerance and gene expression in bromegra ss (Bromus inermis Leyss) cell-suspension cultures. A dihydroABA analo g (DHABA) series that cannot be converted to PA was also investigated. Racemic ABA, (+)-ABA, (+/-)-DHABA, and (+)-DHABA were the most active in inducing freezing tolerance, (-)-ABA, (+/-)-7'0HABA, (-)-DHABA, (/-)-ABA ald, and (+/-)-ABA ale had a moderate effect, and PA was inact ive. If the relative cellular water content decreased below 82%, dehyd rin gene expression increased. Except for (-)-ABA, increased expressio n of dehydrin genes and increased accumulation of responsive to ABA (R AB) proteins were linked to increased levels of frost tolerance. PA ha d no effect on the induction of RAB proteins; however, (+/-)- and (+)- DHABA were both active, which suggests that PA is not involved in free zing tolerance. Both (+)-ABA and (-)-ABA Induced dehydrin genes and th e accumulation of RAB proteins to similar levels, but (-)-ABA was less effective than (+)ABA at increasing freezing tolerance. The (-)-DHABA analog was inactive, implying that the ring double bond is necessary in the (-) isomers for activating an ABA response.