| Summary: | <p>Abstract</p> <p>Background</p> <p>Chlorophyll <it>b </it>is a major photosynthetic pigment in green plants that is synthesized by chlorophyllide <it>a </it>oxygenase (CAO). The regulation of chlorophyll <it>b </it>biosynthesis is an important determinant for the antenna size of photosystems. Chlorophyll <it>b </it>synthesis is partly regulated on a transcriptional level by the expression of the <it>CAO </it>gene. In addition, the synthesis of chlorophyll <it>b </it>is strictly regulated on a protein level by the stability of the CAO enzyme. CAO consists of three domains, which are sequentially named from the N terminus as the A, B and C domains. The A domain of CAO participates in the regulation of the CAO protein stability.</p> <p>Results</p> <p>In order to clarify the physiological function of the A domain, we constructed transgenic Arabidopsis (<it>Arabidopsis thaliana</it>) plants which either overexpressed the complete CAO or a truncated version of CAO lacking the A domain. The transgenic plants overexpressing the A-domain-deleted CAO accumulated an excess amount of chlorophyll <it>b </it>during greening. The transgenic plants which lacked the A domain either died or were obviously retarded when they were exposed to continuous light immediately after etiolation. In addition, the loss of the A domain in CAO impaired another step of chlorophyll biosynthesis, namely the conversion of divinyl-protochlorophyllide <it>a </it>to monovinyl protochlorophyllide <it>a </it>under dark conditions.</p> <p>Conclusion</p> <p>The A domain of CAO regulates the level of CAO, and thus prevents the excess accumulation of chlorophyll <it>b</it>. This function of the A domain is especially important during the greening stage of etiolated seedlings. At this stage, the plants are vulnerable to photodamages which could be caused by excessive chlorophyll <it>b </it>accumulation. In addition, de-regulation of the CAO level affects monovinyl-protochlorophyllide biosynthesis in darkness by unknown mechanisms. In conclusion, the A domain of CAO is essential in the control of chlorophyll biosynthesis and in the survival of seedlings during de-etiolation especially under strong illumination.</p>
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