Short_day (SD) induced the plant senescence in pumpkin ( Cucurbita pepo Linn.) strain 185. Structural assay, gene expression and a series of biochemical analyses were performed to analyze the senescence mechanism in pumpkin strain 185 exposed to SD. Two aspects of important changes initiated in SD exposure contributed to the senescence process. SD functionally led to the initiation of the apical transformation from vegetative to reproductive growth, and then programmed cell death (PCD) in the apical meristem, causing the loss of vigorous growth activity. Moreover, SD treatment resulted in the formation of a great number of dying cells in mesophyll tissue later in the development compared with the phenotype of plants under long_day (LD) conditions. During the senescence process, high expression of nuclease is an important molecular event. These results indicate that the initiation of senescence process in pumpkin stain 185 plants is closely related to the death of cells in apical meristem and mesophyll.
PPF1 is a vegetative growth related gene that encodes a putative membrane protein having high homology with Arabidopsis chloroplast thylakoid protein ALB3. Immunoelectron microscopic assay showed that PPF1 was mainly localized in the thylakold membrane and was highly expressed in well-developed chloroplasts of short day (SD) grown G2 pea while having a very low abundance in chloroplasts of long day (LD) grown plants two weeks after flowering. Comparison of the leaf senescence processes in transgenic Arabidopsis and wild type plants revealed that overexpression of PPF1 delayed leaf senescence, while the depression of its Arabidopsts homologue (ALB3) with PPF1 antisense mRNA accelerated leaf senescence obviously. Ultrastructural analysis of transgenic Arabidopsis plants showed that when PPF1 was overexpressed in Arabidopsis, the chloroplasts were bigger and had much more grana and stroma thylakoid membranes than those of wild type plants. On the contrary, when PPF1 was expressed in antisense orientation to reduce the level of PPF1 homologue in Arabidopsis, the transgenic plants had smaller chloroplasts With less grana. and poorly developed thylakoid membrane systems. These results suggested that the developmental status of chloroplasts was positively correlated with the level of PPF1 or its Arabidopsts homologue, ALB3. Our results suggested that PPF1 gene might regulate plant development by controlling chloroplast development.
