Bergamo, Italy

Istituto Sperimentale per la Cerealicoltura

KÖln, Germany

Max-Planck-Institut für Züchtungsforschung

Developmentally regulated and tissue-specific expression of the glossy2 gene of maize --Velasco, R, Korfhage, C, Salamini, A, Tacke, E, Schmitz, J, Motto, M, Salamini, F, Döring, HP In maize, a large system of genes mapping at 17 separate loci (the glossy or gl loci) has been identified that affect the quantity and/or composition of cuticular waxes on the surface of the seedling leaves (Bianchi et al., Maydica 30: 179-198, 1985). Mutations at the Gl2 locus reduce the amount of surface wax to one-fifth of wild-type levels in the first to sixth plant leaves (Bianchi et al., Maydica 20: 165-173, 1975). While waxes of gl2 plants have a chemical composition similar to those of adult leaves, seedlings homozygous for this mutation accumulate wax compounds two to four carbons shorter compared to wild type. Data available indicate that the Gl2 gene product could be specifically involved in the chain elongation step from C30 to C32 (Bianchi et al., 1975). The Gl2 locus was recently cloned and shown to encode a polypeptide of 426 amino acids of unknown function (Tacke et al., Plant J. 8: 907-917, 1995). Because waxes play a role in resistance of plants to environmental stresses their biosynthesis and secretory processes have relevance both to basic and applied studies. We have considered in this study several regulatory aspects concerning the expression specificity of the Gl2 gene.

The expression of the gene was analysed on the RNA and protein level. The Gl2 gene is transcribed in young leaves, in coleoptiles, in husks, in immature ears and in silks of wild type plants. No transcripts were found in roots of wild type plants or in tissues of plants homozygous for a recessive gl2-ref allele. Antibodies against the GL2 protein revealed predominant presence in the leaves with a juvenile wax phenotype. Protein detection was consistent with transcriptional activity of the gene in wild type coleoptiles, husks, immature ears and silks but not in roots, anthers and seeds. Sequence data bank analysis revealed homologies of the GL2 protein of maize with eighteen sequences from several plant species.

A domain of 27 amino acids is highly conserved in the proteins of different plant species. This domain, PLx3QxTxFxCGGx8Hx3D, is novel and with a function still unknown. Interestingly, the domain is present in the four proteins found in Clarika breweri, Cantharanthus roseus, Gentiana triflora and Dianthus caryophyllus which are credited to have a coenzymeA-dependent acyl transferase activity. Based on this finding, already St. Pierre et al. (Plant J. 14: 703-713, 1998) proposed that the GL2 protein belongs to plant proteins which function as acetyl transferases, an enzymatic activity which has still to be demonstrated for gl2.

For the Gl2 gene the highest sequence similarity was observed with the Cer2 Arabidopsis gene, having both gene mutant alleles conditioning a defect in wax biosynthesis (Tacke et al., 1995; Negruk et al., Plant J. 9: 137-145, 1996). Their mutant waxes are defective in the accumulation of C30 and C32 fatty acids, respectively. The fact that both mutants are defective in the last step of fatty acid elongation might indeed support a role of Cer-2 and Gl2 genes in acyl-CoA-dependent fatty acid biosynthesis. This putative role of Gl2 and Cer-2 as structural genes involved in wax biosynthesis, integrates the current knowledge on this process based on molecular studies of the Gl1 and Gl8 maize loci. These genes exhibit sequence similarities to other plant wax synthetic genes coding a putative transfer protein (Hansen et al., Plant Physiol. 113: 1091-1100, 1997) and a ß-ketoacyl reductase (Xu et al., Plant Physiol. 115: 507-510, 1997). It is interesting to note that all the putative proteins with homology to the GL2 protein contain from 420 to 470 amino acids. This detail may indicate their common origin from a precursor gene, which later on evolved acquiring specific functions in different biochemical pathways.

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