ISOLATION AND ANALYSIS OF CDNAS ENCODING TOMATO CYSTEINE PROTEASES EXPRESSED DURING LEAF SENESCENCE

Several cDNAs for mRNAs that change in abundance during tomato leaf se nescence were isolated. In this paper we report molecular cloning and expression analysis of two cysteine proteases. SENU2 is identical to t he cDNA C14 which encodes a cysteine protease previously shown to be e xpressed in response to extremes of temperature in tomato fruit [43]. SENU3 cDNA clone was 1.2 kb in length and hybridized to a transcript o f 1.4 kb which suggested that the clone was not full-length. The missi ng 5' end was isolated using rapid amplification of cDNA ends (RACE). Southern blot analysis of tomato genomic DNA indicates that SENU3 is e ncoded by a single or low copy gene. SENU3 was also shown to have sign ificant homology with known cysteine proteases. These two senescence-a ssociated cysteine proteases are also expressed during other developme ntal processes, including seed germination, consistent with a role in protein turnover. SENU2 and SENU3 mRNAs were detectable in young fully expanded leaves and increased in abundance with leaf age, reaching a maximum during the later stages of visible leaf senescence. Such a pat tern of expression suggests that the onset of leaf senescence is a gra dual event. Analysis of senescence in transgenic plants deficient in e thylene biosynthesis, in which leaf senescence is delayed, indicated t hat enhanced accumulation of SENU2 and SENU3 mRNA was similarly delaye d but not prevented.