Regulation of cellulose synthesis--another player in the game?

Cellulose is a central component in plant cell walls. In the primary cell wall (deposited in cells that are still expanding), it is a vital component of the load-bearing network and because of its physical properties is important in determining the orientation of cell expansion. After a period of cell expansion, some cells lay down a thick secondary cell wall inside the primary wall. The secondary cell wall provides plants with the mechanical properties that allow them to stand upright, and is a major component in properly functioning xylem vessels. Cellulose is one of the major components of secondary cell walls. The importance of cellulose in plant cell walls is reflected in it being the world’s most abundant biopolymer, with an estimated 180 billion tonnes synthesized annually (Englehardt, 1995). Despite this importance, our understanding of how cellulose is synthesized, and how this synthesis is regulated, is still incomplete. In this issue of New Phytologist (pp. 386–396), Correa-Aragunde et al. describe how the signalling molecule nitric oxide (NO) modulates cellulose synthesis in tomato (Solanum lycopersicum) roots. Pharmaceutical application of the NO donor sodium nitroprusside (SNP) was used to investigate incorporation of radiolabelled glucose into cellulose. Low (pmolar) concentrations of NO increased incorporation of radiolabelled glucose into the cellulose fraction in roots, whereas higher (nmolar) concentrations reduced incorporation into cellulose. These effects were transient and reversible, as determined by use of an NO scavenger. Microscopic analysis of root structure suggested that these differences were caused by effects on primary cell wall synthesis. Root length was reduced in plants treated with higher concentrations of NO and was accompanied by reduced cortical cell length and an apparent swelling of the root, phenotypes that are frequently observed in Arabidopsis mutants affected in primary cell wall cellulose synthesis. Three different cellulose synthase (CesA) catalytic subunits are generally considered to be required for cellulose synthesis, the three subunits being different in primary and secondary cell walls. Correa-Aragunde et al. identified three CesA transcripts from tomato cDNA libraries that are likely to be involved in primary cell wall cellulose synthesis, based on the similarity to three genes involved in this process in potato (Solanum tuberosum). The level of transcript of these three genes was slightly reduced by treatment with high concentrations of NO, suggesting that it may be at a transcriptional level that NO affects cellulose synthesis. Nitric oxide signalling in plants is an area in which there is still much to learn, and has been covered in a recent review (Wilson et al., 2008). It is clear, however, that NO is important as a signalling molecule in a number of processes in plants, and the description in this issue of its effect on modulating cellulose synthesis adds to a growing list of pathways that are affected by NO.