Wood development regulators involved in apical growth in Pinus canariensis

The shoot apical meristem is responsible of seasonal length increase in plants. In woody plants transition from primary to secondary growth is also produced during seasonal apical growth. These processes are controlled by different families of transcription factors. Levels of transcriptomic activity during apical growth were measured by means of a cDNA microarray designed from sequences related to meristematic activity in Pinus canariensis. The identification of differentially expressed genes was performed using a time-course analysis. A total of 7170 genes were differentially expressed and grouped in six clusters according to their expression profiles. We identified master regulators, such as WUSCHEL-like HOMEOBOX (WOX), to be involved in the first stages of apical development, i.e. growth of primary tissues, while other transcription factors, such as Class III HOMEODOMAIN-LEUCINE ZIPPER (HD-ZIP III) and KNOTTED-like (KNOX) and BEL1-like (BELL) HOMEODOMAIN proteins, were found to be induced during last stages of apical seasonal development, already with secondary growth. Our results reveal the main expression patterns of these genes during apical development and the transition from primary to secondary stem growth. In particular, the regulatory factors identified play key roles in controlling stem architecture and constitute candidate genes for the study of other development processes in conifers.