New Arabidopsis cue mutants suggest a close connection between plastid- and phytochrome regulation of nuclear gene expression.

We searched for new components that are involved in the positive regulation of nuclear gene expression by light by extending a screen for Arabidopsis cue (chlorophyll a/b-binding [CAB] protein-underexpressed) mutants (H.-M. Li, K. Culligan, R.A. Dixon, J. Chory [1995] Plant Cell 7: 1599-1610). cue mutants display reduced expression of the CAB3 gene, which encodes light-harvesting chlorophyll protein, the main chloroplast antenna. The new mutants can be divided into (a) phytochrome-deficient mutants (hy1 and phyB), (b) virescent or delayed-greening mutants (cue3, cue6, and cue8), and (c) uniformly pale mutants (cue4 and cue9). For each of the mutants, the reduction in CAB expression correlates with the visible phenotype, defective chloroplast development, and reduced abundance of the light-harvesting chlorophyll protein. Levels of protochlorophyllide oxidoreductase (POR) were reduced to varying degrees in etiolated mutant seedlings. In the dark, whereas the virescent mutants displayed reduced CAB expression and the lowest levels of POR protein, the other mutants expressed CAB and accumulated POR at near wild-type levels. All of the mutants, with the exception of cue6, were compromised in their ability to derepress CAB expression in response to phytochrome activation. Based on these results, we propose that the previously postulated plastid-derived signal is closely involved in the pathway through which phytochrome regulates the expression of nuclear genes encoding plastid proteins.

Nucleotide sequence and temporal regulation of a seed-specific Brassica napus cDNA encoding a stearoyl-acyl carrier protein (ACP) desaturase.

The nucleotide sequence is reported for a cDNA containing the entire coding region of a stearoyl-ACP desaturase (EC 1.14.99.6) from Brassica napus L. cv. Jet neuf. The cDNA was obtained from a library constructed from poly(A)+ RNA purified from embryo tissue. The derived amino acid sequence demonstrates substantial similarity with those from other plant delta 9-desaturases. Comparative RNA-dot blot analyses using the delta 9-desaturase cDNA and a rapeseed oleosin cDNA as probes showed that although both these transcripts were seed-specific, they exhibited distinct patterns of temporal regulation. The desaturase message was induced by 25 days after anthesis (DAA), peaking at 45 DAA but decreasing considerably thereafter. In contrast, the oleosin transcript did not increase until 45-50 DAA, reaching a peak much later at about 70 DAA.

Cloning and characterisation of an oleosin gene from Brassica napus.

The sequence of an oleosin gene from Brassica napus has been determined. This gene contains a single intron of 437 bp and encodes a polypeptide of 195 amino acids. The oleosin gene product has an estimated molecular mass of 21.5 kDa and consists of a highly hydrophobic central domain flanked by relatively polar N- and C-terminal domains. The central domain is highly conserved between all oleosins sequenced to date and contains a run of periodically spaced leucine residues similar to that of a leucine-zipper motif. The gene has been shown to be expressed specifically in the embryo, maximally between 9 and 11 weeks after flowering, i.e. during the seed desiccation stage. Two transcriptional start sites have been mapped to -70 and -21 of the ATG and a putative ABA-responsive element and three repeated motifs have been identified in the promoter. These short promoter sequences could correspond to regulatory elements responsible for embryo-specific gene expression. Up to six genes exist in the oleosin gene family.