Chloroplast developmental timing: nuclear perturbation

Normally assembly of the major chlorophyll (a/b) light harvesting complex (LHCII) is a late event of thylakoid differentiation, both for angiosperms and algae. Mutation at an unmapped locus on the 2L chromosome arm delays greening and at the same time permits early assembly of LHCII (Polacco, Chang and Neuffer, 1985, Plant Physiol., in press). A visible phenotype is observed: virescence, with a developmentally conditional high yield of leaf chlorophyll fluorescence (dc-hcf). High chlorophyll fluorescence only occurs when virescent individuals are partially greened and not for fully greened leaves.

Four independently isolated mutant alleles have been found for this locus: v*-350, v*-424, v*-576A, v*-588A (see also MGNL 56:44, 1982). All four result in virescent individuals with the dc-hcf phenotype. Curiously, a second virescent phenotype, without the dc-hcf trait, also segregated in M3 progenies segregating v*-576A and v*-588A individuals that were dc-hcf. Over 25 virescent individuals have been examined for each of several v*-350 and v*-424 progenies and these were always dc-hcf. It was not clear whether both virescent phenotypes arise from mutation at the same locus. The original allelism tests were performed with material segregating both virescent phenotypes. Crosses performed in 1984 between +/v individuals of low-fluorescence progenies segregating only one virescent phenotype show that the virescent variants of v*-576A and v*-588A without the dc-hcf trait are allelic to v*-424, and in the case of 0-588A, to the dc-hcf variant of v*-588A. Ears segregating only dc-hcf virescent individuals are not yet available for v*-576A. The various combinations of virescent (not dc-hcf) with virescent (dc-hcf) yield varying degrees of fluorescence for the 4-10 individuals checked for each of several crosses. It should be noted that all of these mutations are in stocks that retain 12.5% of the original EMS-treated genome.

One explanation for the two virescent phenotypes is that v*-576A and v*-588A unmask loci that regulate the timing of LHCII assembly during thylakoid morphogenesis. Crosses have been made to test this hypothesis. Characterization of leaf chlorophyll fluorescence is being done for the early greening stages of siblings to dc-hcf v*-576A variants and non-dc-hcf, v*-576A variants.

Allelism tests of v*-424 with v4 and w3 were made and are being analyzed. Mapping of v*-424 relative to Ch w3 is in progress.

M. Polacco


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