Wood is formed from secondary xylem tissue consisting of cells with a heavily thickened secondary cell wall that is enriched in lignin and cellulose. In contrast, primary cell walls are composed mainly of polysaccharides (including cellulose), but contain very little lignin. Wood formation occurs through a complex series of steps involving cell division and expansion and the biosynthesis of lignin and cellulose. In addition, heartwood (darker wood at the center of the trunk) forms in many tree species through a highly regulated process of programmed cell death. Although herbaceous plants by definition do not form wood as in trees and shrubs, they nonetheless form secondary xylem tissue that is in many respects similar to that of their woody relatives. In two separate studies published in The Plant Cell, researchers identify key transcription factors that control the expression of genes related to secondary wall formation in the model herbaceous plant Arabidopsis. The presence of similar genes and pathways in tree species suggests that they may play a role in the regulation of wood formation in trees.
In the first paper, Masaru Ohme-Takagi and colleagues at the National Institute of Advanced Industrial Science and Technology in Japan, the Japan Science and Technology Agency, Nagoya University, and the RIKEN Institute (Mitsuda et al., 2007) show that two plant-specific transcription factors, designated NAC SECONDARY WALL THICKENINGS PROMOTING FACTOR1 (NST1) and NST3, regulate the formation of secondary walls in woody tissues (e.g. stem and hypocotyl) of Arabidopsis. Experiments with both loss- and gain-of-function mutants of these genes suggest that NST1 and NST3 function redundantly to promote secondary wall thickening in xylem tissue. Mutations disrupting the function of NST1 or NST3 alone did not produce any obvious abnormalities in mutant plants, but double knock-out lines of NST1 and NST3 showed a significant loss of thickening and lignification of
Contact: Nancy Eckardt
American Society of Plant Biologists