Hierarchical coupling of phytochromes and cryptochromes reconciles stability and light modulation of Arabidopsis development

In plants, development is a continuing process that takes place under stron g fluctuations of the light environment. Here we show that in Arabidopsis t haliana plants grown under intense white light, coupling of the photorecept or cryptochrome 2 to developmental processes is broader than previously app reciated. Compared to the wild type, the cry2 mutant showed reduced activit y of a Lhcb1*2 promoter fused to a reporter, and delayed flowering. The cry 2 mutation also reduced the inhibition of hypocotyl growth, the unfolding o f the cotyledons, the rate of leaf production during the vegetative phase, and the pace of development after transition to the reproductive stage; but these effects were obvious only in the absence of cryptochrome 1 and in so me cases phytochrome A and/or phytochrome B, Complementary, the cry2 mutati on uncovered novel roles for cryptochrome 1 and phytochrome A. The activity of the Lhcb1*2 promoter was higher in the cry1 cry2 mutant than in the cry 2 mutant, suggesting that cry1 could be involved in blue-light repression o f photosynthetic genes. Surprisingly, the phyA cry1 cry2 triple mutant flow ered earlier and showed better response to photoperiod than the cry1 cry2 d ouble mutant, indicating that phyA is involved in light repression of flowe ring. Growth and development were severely impaired in the quadruple phyA p hyB cry1 cry2 mutant. We propose that stability and light modulation of dev elopment are achieved by simultaneous coupling of phytochrome A, phytochrom e B, cryptochrome 1 and cryptochrome 2 to developmental processes, in combi nation with context-dependent hierarchy of their relative activities.