A Genome-Wide Screen for Genomic Regions Influencing Gravitropism and Tension Wood Formation in Populus

When exposed to gravitational stimuli, angiosperm trees have the remarkable ability to alter their normal growth patterns by forming a specialized tissue on the upper part of their stems and branches. This tissue, called tension wood, is composed of fibers containing a thick inner gelatinous cell wall layer, rich in cellulose, which creates a tensile force pulling the stem upright against gravity. In this study, we used a poplar hybrid population with irradiation-induced chromosomal lesions to investigate the genetic regulation of tension wood formation. We screened gravitropism response and tension wood formation in 240 unique genotypes carrying defined chromosomal lesions. Three replicates per genotype were grown in pots and then placed on their side for 2 weeks. Gravibending response was measured for each tree, and wood samples were collected from the midpoint of the gravibending stem for wood chemistry analysis and histological examination. The resulting gravibending data were used in a quantitative trait loci analysis, which identified five significant dosage QTL’s. Examples will be shown of putative mutants with altered wood anatomy and tension wood formation. Histology and wood chemistry data will be used in new quantitative trait loci scans, and also correlated with the gravibending data to understand how wood properties affect the ability to biomechanically reorient the stem toward the upright position. The results from this project will provide both genome-wide information about loci relevant to gravitropism and tension wood development, as well as individual mutants and candidate genes for future study.