Genome-wide Comparison Of Two Poplar Genotypes With Different Growth Rates And Functional Identification Of Poplar CYCD Genes

Rapid growth is one of the primary advantageous features of Populus, and extensive physiological research has been carried out on the growth of Populus throughout the growing period among different clones. However, the molecular information related to the mechanisms of rapid growth is rather limited. In this study, an Affymetrix poplar genome array was employed to analyze the transcriptomic changes from the pre-growth to the fast-growth phase in two poplar clones(P.deltoides x P.nigra, DN2, and P.nigra×(P.deltoides×P. nigra), NE19) with different growth rates. A total of 1695 differently expressed genes were identified between two time points in NE19 and DN2 (two-way ANOVA, p<0.01 and fold change≥2). Except for genes changing in common for both clones, many transcripts were regulated specifically in one genotype. After functional analysis of the differentially expressed genes, distinct biological strategies seemed to be utilized by the two genotypes to accommodate their fast-growth phase. The faster-growing clone NE19, which has a higher photosynthetic rate and larger total leaf area, emphasized growth-related primary metabolism. However, the slower-growing DN2 tended to have more up-regulated genes involved in defense-related secondary metabolism and stress response. Emphasis of such divergent biological processes in two clones may explain their significant growth differences during the fast-growth phase. The results of microarray show that the expression level of D-type cyclin (CYCD3;1) gene in NE19 is higher than that in DN2 through out the whole growth season. CYCDs play critical roles in controlling the progression through the Gl into the S phase across the G1/S boundary. In this study, we identified six putative CYCD genes which were designate as PdCYCDl-7 from NE19 by complementing a yeast mutant lacking G1 cyclins. Sucrose and phytohormone treatments on in vitro-grown poplar seedlings leading to the alteration of morphological traits in roots and the effects on the transcript levels of PdCYCDs were monitored. Together, these results suggest that the six populus CYCD genes are functional in rescuing yeast cyclin mutant but may still have group-specific functions during the development of plants.