Culm Brittleness of Barley (Hordeum vulgare L.) Mutants Is Caused by Smaller Number of Cellulose Molecules in Cell Wall

Abstract

The physicochemical nature of the cell wall was determined in the fourth internode of three isogenic brittle mutants of barley (Hordeum vulgare L.) and corresponding nonbrittle strains. Cellulose contents of the brittle culms were 17.5 to 20.3% of those of corresponding nonbrittle strains. No major difference was found in lignin and noncellulose components (except glucose) between brittle and nonbnttle strains. Maximum bending stresses of brittle culms were 38.0 to 54.2% of those of corresponding nonbrittle strains. The degree of polymerization of cellulose, measured by viscometry, was similar between the brittle and the nonbrittle strains. Mole number of cellulose molecules in a unit length of brittle culms, calculated by dividing cellulose mass by molecular weight, was 7.7 to 17.3% of those of the nonbrittle strains. These results indicate that brittleness of mutant culms is due to fewer numbers of cellulose molecules in the cell walls.Our previous study (15) of brittle culms of three barley mutants generated by a mutagen and Kamairazu mutant whose genetic background was not known revealed that the mutants contain less cellulose in the cell wall than the normal strains. Comparison between normal and isogenic brittle strains is necessary for genetic and biochemical analyses.The low cellulose content in the mutants could be due to a lower level of UDP-Glc, breakdown of cellulose-forming enzymes, disordered cellulose-forming complex (terminal complex), and low production of cellulose-forming enzymes. Determination of mol wt of cellulose produced in the mutants affords a clue to the mechanism of brittleness and decreased cellulose content. If the mol wt of cellulose in the mutants is less than that in normal strains, less cellulose content results from the lower mol wt. Alternatively, if the mol wt ofcellulose of the mutant is similar to that in the normal strains, then fewer cellulose-forming complexes is the cause. Furthermore, determination of mol wt of cellulose can afford a clue to the complex mechanism of cellulose synthesis including assembly ofcellulose-forming enzymes, interaction ofthe enzymes with plasma membrane, and movement of the enzyme assembly, '

Collapsed xylem phenotype of Arabidopsis identifies mutants deficient in cellulose deposition in the secondary cell wall.

“…In the brittle culm mutants of barley, there is a correlation between cellulose content and the maximum load at yield (Kokubo et al, 1989(Kokubo et al, , 1991, and it is similar to that observed in this study. The irx mutations appear to affect the bending modulus more dramatically than the maximum load at yield.…”

“…The physical properties of the stem, which reflect the properties of the cell wall (Kokubo et al, 1989(Kokubo et al, , 1991; are radically altered. The stem material of irx plants is both weaker and much less stiff than that of the wild type (Figure 7).…”

“…Among the few mutants known to affect the polysaccharide component of cell walls are the brittle culm mutants of maize, rice, and barley. The barley lines have a reduction in the amourx of cellulose in the cell wall to -20% of that in the wild type, and the walls exhibit a twofold decrease in breaking strength (Kokubo et al, 1989(Kokubo et al, , 1991. The biochemical basis for the defect in the brittle culm mutants has not been determined.…”

Multiple Cellulose Synthase Catalytic Subunits Are Required for Cellulose Synthesis in Arabidopsis

“…Freeze-fracture studies of the plasma membrane have shown unique "rosette" structures associated with the ends of microfibrils, suggesting that these rosettes are the site of cellulose synthesis (reviewed in Brown, 1996). Confirmation of these results has come from Arabidopsis ( rsw1 ) and barley (brittle culm) mutants that have less cellulose than their respective wild types (Kokubo et al, 1991;Arioli et al, 1998a). Plasma membranes from the cells affected in these mutants exhibit greatly decreased numbers of rosettes.…”

Prospects for improving the nutritive value of temperate, perennial pasture grasses

“…Cellulose content has been correlated with brittleness in barley internodes. Brittle mutants contained onefifth normal levels of cellulose (Kokubo et al 1989(Kokubo et al , 1991. Bailey (1964) showed that higher weight gains are associated in ryegrass with lower cellulose content which is related to tensile strength.…”