Aberrant expression of the Arabidopsis circadian-regulated APRR5 gene belonging to the APRR1/TOC1 quintet results in early flowering and hypersensitiveness to light in early photomorphogenesis.

In Arabidopsis thaliana, the transcripts of the APRR1/TOC1 family genes each start accumulating after dawn rhythmically and one after another at intervals in the order of APRR9-->APRR7-->APRR5-->APRR3-->APRR1/TOC1 under continuous light. Except for the well-characterized APRR1/TOC1, however, no evidence has been provided that other APRR1/TOC1 family genes are indeed implicated in the mechanisms underlying circadian rhythms. We here attempted to provide such evidence by characterizing transgenic plants that constitutively express the APRR5 gene. The resulting APRR5-overexpressing (APRR5-ox) plants showed intriguing properties with regard to not only circadian rhythms, but also control of flowering time and light response. First, the aberrant expression of APRR5 in such transgenic plants resulted in a characteristic phenotype with regard to transcriptional events, in which free-running rhythms were considerably altered for certain circadian-regulated genes, including CCA1, LHY, APRR1/TOC1, other APRR1/TOC1 members, GI and CAB2, although each rhythm was clearly sustained even after plants were transferred to continuous light. With regard to biological events, APRR5-ox plants flowered much earlier than wild-type plants, more or less, in a manner independent of photoperiodicity (or under short-day conditions). Furthermore, APRR5-ox plants showed an SRL (short-hypocotyls under red light) phenotype that is indicative of hypersensitiveness to red light in early photomorphogenesis. Both APRR1-ox and APRR9-ox plants also showed the same phenotype. Therefore, APRR5 (together with APRR1/TOC1 and APRR9) must be taken into consideration for a better understanding of the molecular links between circadian rhythms, control of flowering time through the photoperiodic long-day pathway, and also light signaling-controlled plant development.