Conservation and divergence of light-regulated genome expression patterns during seedling development in rice and Arabidopsis

phyA dominates in transduction of red-light signals to rapidly responding genes at the initiation of Arabidopsis seedling de-etiolation

Contrary to expectations based on the visible phenotypic behavior of seedlings undergoing de-etiolation in response to continuous red light (Rc), previous gene expression profiling showed that one or more of the five-membered phytochrome (phy) family of Arabidopsis, other than phyB, is predominantly responsible for transducing the Rc signals to light-responsive genes. To begin to identify which phys are involved, and to define potential primary targets of phy signaling, we have examined the genome-wide expression profiles of genes responding to Rc within 1 h (early response genes) of initial exposure of dark-grown wild-type, phyA, phyB and phyAphyB double mutant seedlings to the light signal. The data show that phyA has a quantitatively dominant role in Rc-induced expression of these early response genes, that phyB has minimal detectable regulatory activity in the presence of phyA, but assumes a quantitatively larger role in its absence, and that phyA and phyB combined are responsible for the full extent of Rc responsiveness of 96% of these genes. No evidence was obtained of a significant role for the remaining family members, phyC, phyD or phyE, in this process. In striking contrast, Rc-imposed repression of early response gene expression remains quantitatively strong in the phyAphyB double mutant, as well as the monogenic mutants, suggesting a significant role for one or more of the other three phys in this response. Examination of the established or predicted functional roles of the early response genes indicates that genes encoding transcription factors represent the largest single category, at a frequency three times their prevalence genome-wide. This dominance is particularly striking among those genes responding most robustly to the Rc signal, where >50% are classified as involved in transcriptional regulation, suggesting that these may have potentially primary regulatory roles at the interface between phy signaling and the light-responsive transcriptional network. Integration of the present data with those of a previous genome-scale transcriptional analysis of a pif3 mutant, suggests a complex network involving perception and transduction of inductive Rc signals by both phyA and phyB through both PIF3 and other undefined signaling partners to early response genes.

Let there be light in the nucleus!

Ambient light conditions trigger both developmental transitions, such as the induction of flowering, and a suite of adaptive responses, exemplified by the shade-avoidance syndrome. These responses are initiated by three families of photoreceptors that are conserved in all higher plants: the phototropins, cryptochromes and phytochromes (phyA--phyE, cry1--cry3, phot1 and phot2 in Arabidopsis). Molecular genetic studies performed mainly in Arabidopsis indicate that photon capture by these light sensors usually initiates rapid changes in the gene expression profile, leading to plant adaptation to their environment. Interestingly, numerous transcription factors are early targets of light regulation, both at the transcriptional and post-transcriptional levels.

Gene profiling of the red light signalling pathways in roots

Red light, acting through the phytochromes, controls numerous aspects of plant development. Many of the signal transduction elements downstream of the phytochromes have been identified in the aerial portions of the plant; however, very few elements in red-light signalling have been identified specifically for roots. Gene profiling studies using microarrays and quantitative Real-Time PCR were performed to characterize gene expression changes in roots of Arabidopsis seedlings exposed to 1 h of red light. Several factors acting downstream of phytochromes in red-light signalling in roots were identified. Some of the genes found to be differentially expressed in this study have already been characterized in the red-light-signalling pathway for whole plants. For example, PHYTOCHROME KINASE 1 (PKS1), LONG HYPOCOTYL 5 (HY5), EARLY FLOWERING 4 (ELF4), and GIGANTEA (GI) were all significantly up-regulated in roots of seedlings exposed to 1 h of red light. The up-regulation of SUPPRESSOR OF PHYTOCHROME A RESPONSES 1 (SPA1) and CONSTITUTIVE PHOTOMORPHOGENIC 1-like (COP1-like) genes suggests that the PHYA-mediated pathway was attenuated by red light. In addition, genes involved in lateral root and root hair formation, root plastid development, phenylpropanoid metabolism, and hormone signalling were also regulated by exposure to red light. Interestingly, members of the RPT2/NPH3 (ROOT PHOTOTROPIC 2/NON PHOTOTROPIC HYPOCOTYL 3) family, which have been shown to mediate blue-light-induced phototropism, were also differentially regulated in roots in red light. Therefore, these results suggest that red and blue light pathways interact in roots of seedlings and that many elements involved in red-light-signalling found in the aerial portions of the plant are differentially expressed in roots within 1 h of red light exposure.