DELAYED LEAF SENESCENCE IN ETHYLENE-DEFICIENT ACC-OXIDASE ANTISENSE TOMATO PLANTS - MOLECULAR AND PHYSIOLOGICAL ANALYSIS

To determine the role of ethylene during tomato (Lycopersicon esculent um Mill. cv. Ailsa Craig) leaf senescence, transgenic ACC oxidase anti sense plants were analysed. Northern analysis of wild-type plants indi cated that ACC oxidase mRNA accumulation normally begins in pre-senesc ent green leaves but was severely reduced in the antisense plants. Alt hough the levels of ethylene evolved by wild-type and transgenic leave s increased during the progression of senescence, levels were extremel y low in transgenic leaves. Leaf senescence, as assessed by colour cha nge from green to yellow, was clearly delayed by 10-14 days in the ant isense plants when compared with wild-type plants. Northern analysis o f the photosynthesis-associated genes, cab and rbcS, indicated that le vels of the corresponding mRNAs were higher in transgenic leaves which were not yet senescing compared with senescing wild-type leaves of ex actly the same age. Northern analysis using probes for tomato fruit ri pening-related genes expressed during leaf senescence indicated that o nce senescence was initiated the expression pattern of these mRNAs was similar in transgenic and wild-type leaves. In the antisense plants c hlorophyll levels, photosynthetic capacity and chlorophyll fluorescenc e were higher when compared with senescing wild-type plants of the sam e age. Photosynthetic capacity and the quantum efficiency of photosyst em II were maintained for longer in the transformed plants at values c lose to those observed in wild-type leaves prior to the visible onset of senescence. These results indicate that inhibiting ACC oxidase expr ession and ethylene synthesis results in delayed leaf senescence, rath er than inducing a stay-green phenotype. Once senescence begins, it pr ogresses normally. Onset of senescence is not, therefore, related to a critical level of ethylene. The correlation between higher levels pri or to senescence and early onset, however, suggests that ethylene expe rienced by the plant may be a significant contributing factor in the t iming of senescence.