Reduced de-etiolation of hypocotyl growth in a tomato mutant is associatedwith hypersensitivity to, and high endogenous levels of, abscisic acid

A recessive single gene mutant, 7B-1, in tomato was originally selected for its photoperiod-dependent male sterility. The 7B-1 mutant also has some pl eiotropic effects including reduced light-induced inhibition, i.e. de-etiol ation, of the hypocotyl in long days (LD), increased seed size and weight, and reduced transpiration rate. These traits led us to investigate the sens itivity of 7B-1 to exogenous hormones and the interaction of these response s with daylength. In LD, but not in short days (SD), 7B-1 was more sensitiv e than wild-type (Wi) to exogenous abscisic acid (ABA) for inhibition of se ed germination, root elongation and transpiration rate. 7B-1 mutant also ex hibited reduced responses to exogenous gibberellin (GA(3)) for hypocotyl el ongation, and to inhibitors of GA biosynthesis for seed germination and roo t and hypocotyl elongation. 7B-1 hypocotyls contained a higher level of end ogenous ABA than WT in both photoperiods, although ABA levels were higher i n LD than in SD. In contrast, growth-active GAs, i.e. GA(1), GA(3) and GA(4 ), and IAA were low in the mutant hypocotyls. The 7B-1 mutant appears to be an ABA-overproducer, and the photoperiod-regulated ABA levels may be respo nsible for the hypersensitivity of the mutant to exogenous ABA.