Cellulose, an abundant, crystalline polysaccharide, is central to plant morphogenesis and to many industries. Chemical and ultrastructural analyses together with map-based cloning indicate that the RSW1 locus of Arabidopsis encodes the catalytic subunit of cellulose synthase. The cloned gene complements the rsw1 mutant whose temperature-sensitive allele is changed in one amino acid. The mutant allele causes a specific reduction in cellulose synthesis, accumulation of noncrystalline beta-1,4-glucan, disassembly of cellulose synthase, and widespread morphological abnormalities. Microfibril crystallization may require proper assembly of the RSW1 gene product into synthase complexes whereas glucan biosynthesis per se does not.

To understand better the role of genes in controlling ovule development, a female-sterile mutant, aintegumenta (ant), was isolated from Arabidopsis. In ovules of this mutant, integuments do not develop and megasporogenesis is blocked at the tetrad stage. As a pleiotropic effect, narrower floral organs arise in reduced numbers. More complete loss of floral organs occurs when the ant mutant is combined with the floral homeotic mutant apetala2, suggesting that the two genes share functions in initiating floral organ development. The ANT gene was cloned by transposon tagging, and sequence analysis showed that it is a member of the APETALA2-like family of transcription factor genes. The expression pattern of ANT in floral and vegetative tissues indicates that it is involved not only in the initiation of integuments but also in the initiation and early growth of all primorida except roots.

An 8.5-kb cosmid containing the KORRIGAN gene complements the cellulose-deficient rsw2-1 mutant of Arabidopsis. Three temperature-sensitive alleles of rsw2 show single amino acid mutations in the putative endo-1,4-␤-glucanase encoded by KOR. The F 1 from crosses between kor-1 and rsw2 alleles shows a weak, temperature-sensitive root phenotype. The shoots of rsw2-1 seedlings produce less cellulose and accumulate a short chain, readily extractable glucan resembling that reported for rsw1 (which is defective in a putative glycosyltransferase required for cellulose synthesis). The double mutant (rsw2-1 rsw1) shows further reductions in cellulose production relative to both single mutants, constitutively slow root growth, and enhanced temperature-sensitive responses that are typically more severe than in either single mutant. Abnormal cytokinesis and severely reduced birefringent retardation in elongating root cell walls of rsw2 link the enzyme to cellulose production for primary cell walls and probably cell plates. The Rsw2 Ϫ phenotype generally resembles the Kor Ϫ and cellulose-deficient Rsw1 Ϫ phenotypes, but anther dehiscence is impaired in Rsw2-1 Ϫ. The findings link a second putative enzyme activity to cellulose synthesis in primary cell walls of Arabidopsis and further increases the parallels to cellulose synthesis in Agrobacterium tumefaciens where the celA and celC genes are required and encode a putative glycosyltransferase and an endo-1,4-␤-glucanase related to RSW1 and KOR, respectively.