Activity of chlorophyll synthetase and chlorophyll b reductase in the chlorophyll-deficient plastome mutant of sunflower

The activity of chlorophyll (Chl) synthetase was determined in the leaves o f a light-grown Chl-deficient mutant of sunflower (Helianthus annuus L,, li ne 2-24, albina mutant) and the cotyledons of its wild type (line 3629). Ex ogenous chlorophyllide a (Chlide a) and chlorophyllide b (Chlide b) were us ed as substrates. Chi synthetase activity was measured by conversion of inf iltrated Chlide into Chl, In etiolated cotyledons and white leaves of the m utant incubated for 6 h in darkness, 10 and 0.6% of exogenous Chlide a were converted into Chl a, and the efficiency of esterification was 30 and 3%, respectively In the etiolated cotyledons, both Chl b and Chl a accumulated, indicating the presence of Chl b reductase transforming Chl b into Chl a. As assessed by low-temperature fluorescence, some parts of white mutant lea ves contained some amount of Chl. Dark resynthesis of protochlorophyllide ( Pchlide) was observed in these leaf parts as well. Residual activity of Chl synthetase is suggested to be present there. In the Chl-less leaf parts, P chlide could not be synthesized, apparently because of the loss of enzyme a ctivities catalyzing the synthesis of 5-aminolevulinic acid (ALA), Coupled inhibition of the initial and final stages of ehl biosynthesis in the mutan t leaves is likely to indicate a common mechanism controlling Chl formation in plants.