The expression of phototropins in Arabidopsis leaves: developmental and light regulation.

Phototropins are blue light receptors, which play different roles during plant development. Two phototropins of Arabidopsis thaliana, phot1 and phot2, have strongly overlapping functions. In seedlings, both photoreceptors are responsible for phototropism. In mature leaves they redundantly regulate leaf shape, stomatal opening, and the accumulation of chloroplasts, whereas phototropin2 alone controls chloroplast avoidance response. Light not only activates phototropins, but also affects the level of their expression. In Arabidopsis seedlings, PHOT1 is downregulated and PHOT2 is upregulated by light. Since data on transcription levels of phototropins in mature Arabidopsis leaves is scarce, a comprehensive real-time PCR study of PHOT1 and PHOT2 expression during development was performed, from seedlings to senescing leaves. So far, neither the phototropin expression nor its modulation by light have been investigated during senescence. The results show that the general regulation pattern remains conserved during Arabidopsis lifecycle, whereas the level of transcripts fluctuates over time, pointing to the significance of the light control for functioning of phototropins. The second part of the study determined the influence of photosynthesis-derived signals and photoreceptor-activated transduction pathways on phototropin mRNA levels. The effects of blue and red light were examined using Arabidopsis mutant lines deficient in photoreceptors. The results reveal a complex network of interactions between these receptors in the regulation of phototropin transcription profiles. Cryptochrome1 and phytochromeB appear to be main photoreceptors involved in the regulation of PHOT1 transcript accumulation. The expression of PHOT2 is dependent on both cryptochromes and phytochromeA.