| It is well known that light significantly affects development of plant diseases. However, influence of light in nonhost resistance and gene-for-gene resistance and its mechanisms are largely unknown. In this thesis research effect of light on nonhost resistance of tobacco to Colletotrichum adlay isolate and N/p50-dependent gene-for-gene resistance and the mechanisms were studied. The main results are as follows:1. Effect of photosynthesis pathway on N/p50-dependent gene-for-gene resistance and nonhost resistance of tobacco to Colletotrichum adlay isolate was investigated using diuron as photosystem II (PSII) electron transport inhibitor. Itwas found that treatment with 23.3 mg· L-1 diuron accelerated the occurrence of N/p50-dependent hypersensitive response (HR). Colletotrichum adlay isolate could successfully infect the otherwise nonhost tobacco, resulting in small lesions. However, enlargement of these lesions was limited, and no plentiful conidia were observed in the lesions. These data indicate that PSII-mediated pathway attenuates N/p50-dependent HR, but enhances nonhost resistance of tobacco to Colletotrichum adlay isolate, which suggests that PSII-mediated pathway affects various types of disease resistance in different ways.2. Influence of light on N/p50-dependent HR and its molecular mechanisms were analysed. Light/dark regime and light duration significantly affected HR development in N/p50 tobacco seedlings. The time required for HR development was tightly negatively related with the duration time of light/dark regime. HR was observed in N/p50 tobacco seedlings since 21 hours post shift (hps) from 28℃ to 22℃ and maintained under continuous dark condition, the whole seedlings wilt 24 hps. The earliest time of HR development in N/p50 seedlings under light/dark regimes 4h/20h, 8h/16h, and 12h/12h, was 23, 26 and 32 hps respectively;while the time for appearance of whole seedling wilt under these light/dark regimes was 32, 40 and 52 hps respectively. Under continuous light condition, HR and seedling wilt did not appear until 48 hps and 72 hps. These results indicate that short light/dark regime anddark accelerate and enhance A^J0-dependent HR.Relationship between effect of high temperature and dark on N/p5 O-depemdent HR was examined. It was found that under high temperature, effect of dark on iV/￡?50-dependent HR was strikingly alleviated. When N/p50 seedlings were grown at 32°C under different light/dark regimes, no HR occurred under light/dark regimes 4h/20h, 8h/16h, and 12h/12h or continuous light condition. HR developed under continuous dark condition, but it was initiated 7 days post growth, which was significantly later in comparison! with low temperature condition (22°C). These data indicate that effect of high temperature on iV/￡>50-depemdent HR is much stronger that that of dark.Influence of light on expression of p50, HR marker gene HSR203J and defense-related gene PR6 was studied employing Northern blot assay. Dark siginificantly increased expression of all these genes, while light strongly repressed expression of p50.Effect of light on N gene expression was analysed by Real Time-PCR technique. In plants homozygous for the N gene, expression of this gene under continuous light and dark conditions was similar, except at 12 hours post growth (hpg). At this timepoint, N gene expression under continuous dark was 1.67 times as high as under continuous light. However, in N/p50 seedlings, transcripts of the N gene accumulated at significantly different levels under the two conditions. Before 12 hpg, abundance of N gene transcripts was significantly higher under dark than under light, especially at 4 hpg, the former was 2.89 times as high as the latter. However, at 22 hpg, abundance of N gene transcripts under dark dramatically dropped to only 1/4 as high as that under light.The role of increased expression of the p50 and N genes in enhanced A^tf-dependent HR remains further study.
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