Several recessive, virescent mutants of maize were examined for high chlorophyll fluorescence during greening. Maize mutants which display high red fluorescence on irradiation with blue light have been shown to be defective in photosynthesis for all cases studied (C. D. Miles, Meth. Enz. 69:1, 1980). We have not found that this fluorescence can be induced in the case of greening, etiolated seedlings. Therefore virescent mutants which exhibit high chlorophyll fluorescence only during greening should have delayed synthesis of a chloroplast component(s) intimately involved in photosynthesis. Furthermore, we anticipate that the component(s) in question will also be critical to chloroplast biogenesis. It should be noted that there are several photosynthesis mutants in maize with high chlorophyll fluorescence which are green and are not virescent. Thus the absence of photosynthetic capacity per se does not block or slow formation of the photosynthetic apparatus.
Virescents that were screened for high chlorophyll fluorescence include:
Of the mutants examined to date, most do not display abnormally high chlorophyll fluorescence. One which did, mutant v*-424, was selected for further study. The v*-424 mutation is uncovered by TB-2L-1S and not by TB-1Sb, tentatively placing it on the long arm of chromosome 2. In seedlings which green rapidly, the phenotype is not lethal. Those which green slowly die unless rescued by sugar feeding (achieved by placing the cut edge of the second leaf to emerge in a small vial of sterile, 10% sucrose, with changes of the sucrose solution every other day). The presumed homozygotes for v*-424, both the rescued, slow-to-green individuals and rapidly greening individuals, grow to maturity in the field, producing small, fertile plants.
The extent of fluorescence in v*-424 was inversely proportional to the maturity of the tissue measured:
The time-variability of the induced fluorescence (see Figure 1) indicates that photosystems II and I are at least partially functional (see Miles ref. above).
Dodecyl sulfate-urea-polyacrylamide (10-18% gradient) electrophoresis revealed no significant differences between the protein compositions of thylakoids isolated from mesophyll chloroplasts of fluorescent and nonfluorescent siblings when equivalent amounts of chlorophyll are applied to gels. In all instances, the material employed was obtained from selfed heterozygotes for the v*-424 locus and segregated approximately one-fourth virescent phenotypes. Work is in progress to characterize the lesion in photosynthesis that has been inferred from the fluorescence data.
Mary Polacco and M. G. Neuffer
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