Abstract

Components of the circadian clock as deduced from studies in Arabidopsis thaliana have counterparts in the sequenced genome of the deciduous tree species Populus trichocarpa. Functional studies have however been lacking. Recently we carried out a study on the role of the putative homologues of Arabidopsis LATE ELONGATED HYPOCOTYLS (LHY) and TIMING OF CAB EXPRESSION1 (TOC1) in Populus using trees with reduced expression of these genes (Ibáñez et al., (2010) Plant Physiol 153:1823-1833). We found that the Populus homologues indeed function in the circadian clock mechanism, and investigated further their role in the seasonal regulation of induction and exit of winter dormancy. Both LHY and TOC1 were shown to help define the critical day length, that decide if trees are able to grow or not. Also, unexpectedly, LHY expression was needed in order to resume growth in response to favorable temperature under Spring-like conditions. Moreover, during dormancy when perennials must be able to endure freezing temperatures, we found that trees need LHY expression to tolerate freezing. In accordance with this finding the cold response was also compromised in these trees since they were unable to up-regulate C-REPEAT BINDING FACTOR1 (CBF1) in response to low temperature. Thus, we have shown that the central circadian clock and its components needs to be functional to accurately time growth and for building of freezing tolerance, both traits necessary for survival and a perennial life style at higher latitudes.