Transcriptional and Hormonal Control of Gravitropism and Tension Wood Formation in Populus

Angiosperm trees form tension wood, which represents a major ultrastructural and chemical remodeling of xylem cells, to counteract gravitational and mechanical stresses experienced by woody stems. Because tension wood can be experimentally induced and manipulated to produce characteristic changes in wood development, it serves as an excellent model system for understanding the genetic and hormonal regulation of wood development. In this talk, we present integrated results from experiments that quantify physiological, biochemical, ultrastructural and gene expression changes occurring during tension wood formation. Key hormone-related processes identified include initial response to gravity by auxin transport changes in the stem, and the stimulation of tension wood fiber production by GA. We show that the KNOX Homeobox transcription factor, ARBORKNOX2 (ARK2), influences both of these hormone responses. We developed extensive RNAseq datasets from wildtype and ARK2 mutants to model the transcriptional networks underlying tension wood formation, and to identify key processes and individual genes responsible for specific tension wood properties. Together, these results present an integrated view of the key changes in physiology and cell differentiation that are ultimately responsible for tension wood development and the generation of the forces underlying the  gravitropism of woody stems.