Flower senescence: some molecular aspects.

Some molecular aspects of flower senescence have been reviewed. The isolation, identification and characterization of different genes from various flowers (mainly from petals) associated with senescence have been discussed. The isolated genes were divided into different groups. A large proportion of genes have been found to be upregulated during flower senescence while some genes were also found to be downregulated indicating that there exists a complex interplay between the expression patterns of various genes. The genes involved in petal expansion are found to be upregulated during normal flower development from anthesis to open flower stage, but XTH (Xyloglucan endotransglucosylase hydrolase) is found to be involved in petal expansion as well as abscission. Cysteine proteases or the genes encoding cysteine proteases (assigned a central role in protein degradation) have been identified from various flower systems, but no cysteine protease has been identified from senescing Mirabilis jalapa flowers. In addition to proteases, the genes encoding ubiquitin (exhibiting proteasomal degradation by 26S proteasomes) have also been identified suggesting the two alternate pathways for protein degradation. Genes encoding specific nucleases have also been identified, but they displayed an early increase in transcript abundance before the senescence symptoms become evident and characterize the involvement of PCD during flower senescence. A range of transcription factors are described and their possible role in flower senescence has been discussed. A detailed description of genes involved in ethylene synthesis and the components involved in ethylene signaling have been presented.

Physiological and biochemical changes associated with flower development and senescence in so far unexplored Helleborus orientalis Lam. cv. Olympicus.

The so far unexplored H. Orientalis cv. Olympicus exhibits a unique pattern of flower senescence, involving re-greening of creamy white petaloid sepals at the later stages. The greenish sepals become photosynthetically competent immediately after pollination and persist until the seeds are set. After the seed set, the entire (green) flower abscises from the plant. Flower development of Helleborus orientalis cv. Olympicus growing in the open was divided into six stages (I-VI) from tight bud stage to the senescent stage. The average life span of an individual flower after it is fully open is about 6 days. Membrane permeability of sepal tissues estimated as electrical conductivity of leachates increased during senescence. The content of sugars and soluble proteins in the sepal tissues increased during flower opening and declined thereafter during senescence. The protease activity increased as the flower progressed towards senescence. From the present study, it becomes evident that decline in the sugar status and elevation in specific protease activity leading to degradation of proteins are the important factors regulating development and senescence in H. orientalis flowers. Although the tissue content of soluble proteins registered an overall quantitative decrease but SDS-PAGE of protein extract from sepal tissues suggested a decrease in the expression of high molecular weight proteins and an increase in low molecular weight proteins during flower development and senescence. At this stage it is not known whether the polypeptides that increased during senescence play an important role in the senescence of Helleborus orientalis flowers. The increase in these polypeptides during flower senescence is of particular interest because they may be linked to flower longevity. Understanding the nature of these proteins can provide new insights into the pathways that execute senescence and the post-transcriptional regulation of senescence in this flower system.