| Leaf senescence as an integral component of plant development plays an important role in plant life by recycling nutrients from dying tissues to actively growing regions. However, the underlying molecular mechanisms are poorly understood. Toward a molecular understanding of leaf senescence, we first analyzed the molecular changes during natural leaf senescence in Arabidopsis and found that its leaf senescence proceeds through defined phenotypic and biochemical changes with a high degree of uniformity. We then constructed and differentially screened a cDNA library representing mRNAs from senescing Arabidopsis leaves. Six cDNA clones (designated SAG12 through 17) corresponding to mRNAs that substantially increase in abundance during senescence were isolated. Nuclear run-on analyses showed that expression of four of them (SAG12 through 15) are highly senescence specific and upregulated primarily at the transcriptional level. SAG12 and SAG13 were further characterized. SAG12 is a unique gene. It consists of one intron and two exons and encodes a cysteine proteinase. Its nucleotide sequence appears conserved between Arabidopsis and Brassica. Reporter gene studies suggested that the regulatory mechanism of this gene expression is conserved between Arabidopsis and tobacco plants. SAG13 is recently duplicated, both copies have an identical nucleotide sequence except for 1-bp at 949bp upstream of the translation start codon. It consists of three introns and four exons, and encodes a short-chain alcohol dehydrogenases.;We also developed an autoregulatory senescence-inhibition system in which the senescence-specific SAG12 promoter was fused to the cytokinin synthesis gene IPT. At the onset of leaf senescence this promoter will direct IPT expression and subsequently increase the cytokinin level such that it prevents the leaf from senescing, which in turn results in down-regulation of the promoter. In the transgenic tobacco plants containing this construct, leaf and floral senescence is significantly retarded, and seed yield and biomass are markedly increased, but other aspects of plant growth and development are very normal. This system thus has a great potential in agricultural application. |
