From Cellulose to Mechanical Strength: Relationship of the Cellulose Synthase Genes to Dry Matter Accumulation in Maize

Abstract

Stalk strength in maize is primarily determined by the amount of cellulose in a unit length of the internode. An increase in cellulose concentration of the cell wall, aside from allowing simultaneous improvements in stalk strength and harvest index, will in- crease the value of stover as a feedstock for ethanol production. Sucrose synthase makes UDP-glucose, substrate for cellulose formation, from uridine diphosphate (UDP) and sucrose whereby it conserves the energy of the glycosidic bond. The alternative route of UDP-glucose formation through UDP-glucose pyrophosphorylase, in contrast, con- sumes two equivalents of uridine triphosphate (UTP), making it an energy-intensive process. In vivo, the reaction catalyzed by sucrose synthase operates in the direction of UDP-glucose formation because of deviation of the relationship between mass action ratio (in vivo ratio of products to substrates) and Keq from unity. A reduction in the amount of enzyme could be compensated by this mechanism without affecting the magnitude of net flux. Since cellulose is crystallized into microfibrils immediately after synthesis, the reaction of cellulose synthase is considered to be far from equilibrium. Cellulose synthase may thus exert considerable control on carbon flux into cellulose. We isolated 12 members of the CesA gene family from maize. Upon phylogenetic analy- sis, three of the maize CesA genes, ZmCesA10-12, clustered with the Arabidopsis CesA sequences that had previously been shown to be involved in secondary wall formation. These three genes were coordinately expressed across multiple tissues, suggesting that they might interact with each other to form a functional cellulose synthase complex. Isolation of the expressed CesA genes from maize and their association with primary or secondary wall formation has made it possible to test their respective roles in cellulose synthesis in different cell types through association genetics, mutational genetics, or a transgenic approach. This information would be useful in improving stalk strength in cereals.