Genomewide characterization of the light-responsive and clock-controlled output pathways in Lotus japonicus with special emphasis of its uniqueness.

During the last decade, tremendous progress has been made in understanding the molecular mechanisms underlying the plant circadian clock in Arabidopsis thaliana, mainly taking advantage of the availability of its entire genomic sequence. It is also well understood how the clock controls the photomorphogenesis of seedlings, including the shade avoidance response, and how the clock controls the photoperiodic flowering time in the spring annual long-days herb A. thaliana. Based on this, here we attempt to shed light on these clock-controlled fundamental and physiological events in Lotus japonicus, which is a perennial temperate legume with a morphological nature quite different from Arabidopsis. In the Lotus database, we first compiled as many clock-, light-, and flowering-associated coding sequences as possible, which appear to be orthologous or homologous to the Arabidopsis counterparts. Then we focused on the PHYTOCHROME INTERACTING FACTOR4 (PIF4)-mediated photomorphogenic pathway and the FLOWERING LOCUS T (FT)-mediated photoperiodic flowering pathway. It was shown in L. japonicus that the putative LjPIF4 homologue is expressed in a manner dependent on the circadian clock, and the putative LjFT orthologue is expressed coincidentally and especially in the long-days conditions, as in the case of A. thaliana. LjFT is capable of promoting flowering in A. thaliana, whereas the function of LjPIF4 seems to be divergent to a certain extent from that of AtPIF4. These results are discussed with emphasis on the intriguing differences between these model plant species.