| dc.description | The stratospheric ozone layer depletion causes an increase in solar ultraviolet-B radiation (UV-B, 290--320 nm) reaching the earth's surface. This may adversely affect agricultural and forestry production because enhanced UV-B may inhibit photosynthesis. The nuclear genes encoding light-harvesting complex II proteins (Ihcb), which are components essential for the first step of photosynthesis, are extremely sensitive to UV-B. It is possible that UV-B down-regulates lhcb genes by damaging chloroplasts, for the transcription of these genes requires intact chloroplasts, the intracellular organelles most fragile upon UV-B exposure. In order to test this hypothesis, pea (Pisum sativum L.) seedlings were treated with norflurazon, an inhibitor of carotenoid biosynthesis, to increase the sensitivity of chloroplasts to photo-oxidative stress. The norflurazon-treated plants were grown under dim red light to exclude complexity caused by photoreceptors other than phytochromes. Opposite to the hypothesis that damage to chloroplasts may increase the sensitivity of lhcb gene expression to UV-B, UV-B exposure increased rather than reduced lhcb transcript accumulation under dim red light in seedlings after chloroplasts were partially photobleached by norflurazon and white light exposure. Thus, mature, intact chloroplasts are required for UV-B to exert an inhibitory effect on pea lhcb mRNA accumulation Using reverse transcription polymerase chain reaction, all seven members of the lhcb family were found to respond similarly to UV-B; that is, mRNA accumulation for each gene decreased in green leaves but increased in etiolated buds. The UV-B response of these pea 1hcb genes divided them into the same two subfamilies as defined by their response to red light, suggesting a possible link between the signal transduction pathways mediating UV-B and red light effects on the genes. As determined by examination of lhcb1*2 and lhcb1*4 mRNA accumulation, a carotenoid-related component may mediate UV-B inhibition of both subfamilies, while a chlorophyll-related component may only mediate UV-B inhibition of the subfamily represented by lhcb1*2. However, a carotenoid-related component only mediated red light effects on the subfamily represented by lhcb1*2 while a chlorophyll-related component mediated the red light effects on both subfamilies. Moreover, UV-B protected the red light-linked chloroplast components against white light-induced photodamage, suggesting that the UV-B and red light pathways are not identical. In addition, cDNA clones of six UV-B-responsive genes were identified in pea using rapid amplification of 3' cDNA ends by the polymerase chain reaction. Among them, psUVzinc, psUVRub and psUVDeh were UV-B-repressible, and psUVaux, psUVglu , and psUVrib were UV-B-inducible. The nucleotide sequence of psUVzinc was not closely related to any gene registered in GenBank, representing a novel UV-B-responsive gene in pea. The other clones shared sequence similarity with genes encoding Rubisco activase, beta-1,3-glucanase, the pea dehydrin cognate B61, some 40S ribosomal proteins and a putative auxin-repressed protein of apricot, respectively. The cloning of these UV-B-responsive genes has generated new ideas concerning how UV-B affects plant growth and development. | en_US |
