MYB90 and MYB75 are two MYB transcription factors that regulate anthocyanin biosynthesis in Arabidopsis. Prior to this study a microarray experiment indicated that MYB90 was required for senescence associated anthocyanin biosynthesis. The role of MYB90 during senescence was investigated using a MYB90 knockout insertion mutant, IM28. Gene expression studies at different time points during the development of the seventh rosette leaf showed that MYB90 regulates anthocyanin biosynthesis genes and MYB75 during senescence. The absence of MYB90 expression reduced photosynthetic performance at two time points during development. Analysis of anthocyanin and flavonoid content showed that there was reduced anthocyanin biosynthesis in the absence of MYB90 expression.\ud The role of MYB90 and MYB75 during high light stress was investigated by analysing the photosynthetic performance and anthocyanin content of high light treated IM28 and MYB75 RNAi plants. MYB90 is required for increased resistance during high light stress, which reflected anthocyanin levels. However, there was eventual compensation for the absence of MYB90 expression in prolonged high light stress. MYB75 was also required for increased resistance to high light stress, but this was not reflected in the anthocyanin levels.\ud The spatial and temporal regulation of MYB90 and MYB75 was investigated using transgenic plants containing promoter: GUS fusions. MYB90 promoter activity was mainly localised to vascular tissue during senescence and low nitrogen/high glucose treatment. MYB75 showed differential tissue specificity in different treatments. The transcriptional regulation of the MYB90 promoter was analysed using promoter deletion GUS: fusions. A senescence specific activation region and a repressor region of the promoter were identified
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