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

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Abstract
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 pleiotropic effects including reduced light‐induced inhibition, i.e. de‐etiolation, of the hypocotyl in long days (LD), increased seed size and weight, and reduced transpiration rate. These traits led us to investigate the sensitivity of 7B‐1 to exogenous hormones and the interaction of these responses with daylength. In LD, but not in short days (SD), 7B‐1 was more sensitive than wild‐type (WT) to exogenous abscisic acid (ABA) for inhibition of seed germination, root elongation and transpiration rate. 7B‐1 mutant also exhibited reduced responses to exogenous gibberellin (GA3) for hypocotyl elongation, and to inhibitors of GA biosynthesis for seed germination and root and hypocotyl elongation. 7B‐1 hypocotyls contained a higher level of endogenous ABA than WT in both photoperiods, although ABA levels were higher in LD than in SD. In contrast, growth‐active GAs, i.e. GA1, GA3 and GA4, 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 responsible for the hypersensitivity of the mutant to exogenous ABA.

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