The single-gene male-sterile 7B-1 mutant in tomato (Lycopersicon esculentum Mill.) shows reduced de-etiolation of hypocotyl growth, has an elevated level of endogenous abscisic acid (ABA) and reduced amounts of growth-active gibberellins (GAs), is supersensitive to exogenous ABA, and is resistant to abiotic stresses in light but not in the dark. The existence of crosstalk between light signaling and plant hormones, and the interaction of ABA and GA biosynthetic pathways with ethylene, led us to investigate the possible role of ethylene in the 7B-1 mutant. In the dark, 7B-1 seedlings exhibited the normal triple response to 1-aminocyclopropane-l-carboxylic acid (ACC), an ethylene precursor, similar to the wild type. In light, however, hypocotyl growth of mutant seedlings was more sensitive to ACC but less sensitive to the inhibitors of ethylene’s action, such as silver thiosulfate, than the wild type. The 7B-1 mutant seedlings produced reduced levels of ethylene, which could account for the increased sensitivity to exogenous ACC. The mutant leaf discs also contained higher chlorophyll content and showed greater chlorophyll degradation in response to ACC than the wild type, and these could also be related to low ethylene production by the mutant. Fluridone, an inhibitor of ABA biosynthesis, countered the ACC-induced inhibition of hypocotyl elongation, and it also restored the wild-type phenotype in 7B-1 plants. The results suggest that the reduced de-etiolation of hypocotyl growth and the increased sensitivity of the mutant to ethylene in light are due to reduced ethylene production, which in turn may be related to high endogenous ABA. The data presented support our earlier findings that the 7B-1 mutant has a defect in light perception which affects both hormonal sensitivity and endogenous levels, thereby affecting hypocotyl and shoot growth.
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