| Leaf senescence is a genetically regulated and orderly series of events signifying the final stage of leaf development prior to organ death. During senescence, the loss of photosynthesis is associated with the breakdown of chlorophyll and the disassembly of the photosynthetic apparatus. Cell structure changes, organelles are disassembled and proteins, lipids, nucleic acids and carbohydrates are hydrolyzed while the nutrients released by hydrolysis are transported and recycled to growing tissues or are stored in reserve for latter use. The factors regulating leaf senescence are complex and involve both external and internal stimuli, such as darkness, pathogens, developmental signals and hormones. These factors initiate signal transduction pathways that repress photosynthesis and activate the expression of genes involved in cellular disassembly, recycling and integral defense-related processes. The genes for mRNAs that increase markedly in abundance during senescence have been designated as “senescence-associated genes” or SAGS.; This work characterizes senescence in Anthurium andraeanum through expression analysis of the Arabidopsis senescence-upregulated promoter SAG12, isolation of a new senescence-associated gene (anth17 ), and expression profiling under dark induced and age dependent senescence. Analysis of Pr-SAG12 revealed a 20 fold increase in expression when transiently bombarded into senescent leaves vs. healthy leaves. This shows the senescence-dependent activation of the Arabidopsis Pr-SAG12 in Anthurium implying the existence of an orthologous senescence-regulated system with associated genes, signal transduction pathways, and transcription factors in this plant. Two cysteine protease cDNAs from Anthurium, anth16 and anth17, were sequenced and found to be expressed differently throughout development and differ structurally in comparison with known cysteine protease genes. These cDNAs are the first to be cloned and characterized from this plant. Anth16 is expressed in immature leaves and is structurally similar to a thiol protease sequenced from Matricaria chamomilla, possibly defining an alternate role in development for these proteases. Anth17 is characterized as a senescence-associated gene based on Northern blot analysis of developmental leaf stages in comparison with psbA, and cab genes as indicators of photosynthetic gene expression. Furthermore, cytokinin treatments decreased significantly anth17 mRNA levels during dark-induced senescence of dark detached whole leaves, and leaf discs. |
