No abstract

SummaryAlthough soluble sugar levels affect many aspects of plant development and physiology, little is known about the mechanisms by which plants respond to sugar. Here we report the isolation of 13 sugarinsensitive (sis) mutants of Arabidopsis that, unlike wild-type plants, are able to form expanded cotyledons and true leaves when germinated on media containing high concentrations of glucose or sucrose. The sis4 and sis5 mutants are allelic to the ABA-biosynthesis mutant aba2 and the ABAinsensitive mutant abi4, respectively. In addition to being insensitive to glucose and sucrose, the sis4/ aba2 and sis5/abi4 mutants also display decreased sensitivity to the inhibitory effects of mannose on early seedling development. Mutations in the ABI5 gene, but not mutations in the ABI1, ABI2 or ABI3 genes, also lead to weak glucose-and mannose-insensitive phenotypes. Wild-type and mutant plants show similar responses to the effects of exogenous sugar on chlorophyll and anthocyanin accumulation, indicating that the mutants are not defective in all sugar responses. These results indicate that defects in ABA metabolism and some, but not all, defects in ABA response can also alter response to exogenous sugar.

A gene from the flowering plant Arabidopsis thaliana that encodes an omega-3 desaturase was cloned on the basis of the genetic map position of a mutation affecting membrane and storage lipid fatty acid composition. Yeast artificial chromosomes covering the genetic locus were identified and used to probe a seed complementary DNA library. A complementary DNA clone for the desaturase was identified and introduced into roots of both wild-type and mutant plants by Ti plasmid-mediated transformation. Transgenic tissues of both mutant and wild-type plants had significantly increased amounts of the fatty acid produced by this desaturase.

Previous genetic evidence suggested that the fad8 and fad7 genes of Arabidopsis thaliana encode chloroplast membrane-associated 0-3 desaturases. A putative fad8 cDNA was isolated by heterologous hybridization using a gene encoding an endoplasmic reticulum-localized W-3 desaturase (fad3) as a probe. The cDNA encodes a protein of 435 amino acid residues with a molecular mass of 50,134 D. Constitutive expression of the cDNA in transgenic plants of a fad7 mutant resulted in genetic complementation of the mutation, indicating that the fad7 and fad8 gene products are functionally equivalent. Expression of the fad8 cDNA in transgenic plants often resulted in the co-suppression of both the endogenous fad7 and fad8 genes in spite of the fact that these two genes share only about 75% nucleotide identity. In contrast to all other known plant desaturases, including fad7, the steady-state level of fad8 mRNA is strongly increased in plants grown at low temperature. This suggests that the role of fad8 is to provide increased w-3 desaturase activity in plants that are exposed to low growth temperature. The fads-1 mutation created a premature stop codon 149 amino acids from the amino-terminal end of the fad8 open reading frame, suggesting that this mutation results in a complete loss of fad8 activity.The chloroplast membranes of higher plants have unusually high concentrations of tienoic fatty acids, with linolenic (Clk3) or a combination of linolenic and hexadecatienoic (C16:3), making up more than 80% of the fatty acids found in this organelle (Harwood, 1982). These fatty acids are synthesized by sequential insertion of double bonds into derivatives of stearic (Clk0) and hexadecanoic (C,,,,) acids. The formation of the first double bond in 18-carbon fatty acids is generally catalyzed within chloroplasts by one of the few soluble plant desaturases, the stearoyl-acyl carrier protein desaturase (McKeon and Stumpf, 1982). The formation of the second and third double bonds in 18-carbon fatty acids can then ~ ~