An Arabidopsis callose synthase

Abstract

β-1,3-glucan polymers are major structural components of fungal cell walls, while cellulosic β-1,4-glucan is the predominant polysaccharide in plant cell walls. Plant β-1,3-glucan, called callose, is produced in pollen and in response to pathogen attack and wounding, but it has been unclear whether callose synthases can also produce cellulose and whether plant cellulose synthases may also produce β-1,3-glucans. We describe here an Arabidopsis gene, AtGsl5, encoding a plasma membrane-localized protein homologous to yeast β-1,3-glucan synthase whose expression partially complements a yeast β-1,3-glucan synthase mutant. AtGsl5 is developmentally expressed at highest levels in flowers, consistent with flowers having high β-1,3-glucan synthase activities for deposition of callose in pollen. A role for AtGsl5 in callose synthesis is also indicated by AtGsl5expression in the Arabidopsis mpk4 mutant which exhibits systemic acquired resistance (SAR), elevated β-1,3-glucan synthase activity, and increased callose levels. In addition, AtGsl5 is a likely target of salicylic acid (SA)-dependent SAR, since AtGsl5mRNA accumulation is induced by SA in wild-type plants, while expression of the nahG salicylate hydroxylase reduces AtGsl5 mRNA levels in the mpk4 mutant. These results indicate that AtGsl5is likely involved in callose synthesis in flowering tissues and in the mpk4 mutant.