While it has been difficult to determine what molecular events are necessary for the control of senescence in leaves,Grbic and Bleecker (37) have recently shown that the hormone ethylene is a modulator of the senescence syndrome. Thus, mutants of Arabidopsis that are blocked in the perception of ethylene have greater leaf longevity probably because there is a delay in the onset of induction of the expression of SAG genes. A role for ethylene in modulating the rate of fruit ripening in tomato has also been suggested (38, 39).