THE TIMING OF MAIZE LEAF SENESCENCE AND CHARACTERIZATION OF SENESCENCE-RELATED CDNAS

Leaf senescence was characterised in two Zea mays lines, earlier senes cence (ES) and later senescence (LS). Loss of chlorophyll was delayed in LS compared with ES, but the decline in photosynthesis occurred sim ultaneously in the two lines. Western analysis detected transition poi nts during senescence of both lines when major quantitative and qualit ative changes occurred in a number of leaf proteins. Differences in th e pattern of translatable mRNAs were apparent earlier than alterations in pigment or protein levels. A cDNA library was constructed using mR NA from ES leaves early in senescence and differential screening was e mployed to isolate senescence-related clones. Two senescence-enhanced cDNAs showed sequence homology with cDNAs for seed proteins - a cystei ne protease and a protein-processing enzyme. These findings suggest th at there are similarities between gene expression during seed maturati on, germination and leaf senescence. Other senescence-enhanced cDNAs w ere related to genes implicated in gluconeogenesis and chlorophyll bre akdown.