The structural, biochemical, and genetic characterization of a new radiation-induced, variegated leaf mutant of soybean [Glycine max (L.) Merr].

Abstract

A variegated leaf mutant in soybean [Glycine max (L.) Merr.] has been identified and characterized. E25-10 was derived by exposure of seeds of the "Williams' 82" cultivar to gamma-radiation. In this mutant, yellow leaf sectors contain defective chloroplasts, in which the thylakoid membranes are presented as long, parallel structures with little or no overlap. No starch grains have been detected in the mutant chloroplasts. Small vesicles and plastoglobuli can be found within the defective chloroplasts. Genetic studies revealed that a single nuclear-encoded gene is responsible for the mutation in E25-10. The total chlorophyll content is reduced in yellow leaf tissue by 70-80%. However, the chlorophyll a/b ratio is not altered. The absorbance spectrum of pigments in the mutant leaf tissue differed from that of the green extracts in the range of 400-500 nm. This reduction in total chlorophyll and the change in the absorbance spectrum pattern in the yellow tissue is related to a loss of certain photosynthetic complexes. Green gel analysis revealed that four major pigment-protein complexes (CP1, LHCP1, LHCP2, and CPa) of the thylakoid membranes were absent in the E25-10 mutant. Lithium dodecyl sulphate polyacrylamide gel analysis showed that at least 5-6 polypeptides (51, 44, 25, 15, 13, and 12 kDa) were missing in the thylakoid membranes of chloroplasts from the yellow tissue. Changes in chloroplast- and nuclear-encoded gene message levels were detected. The psaA transcripts which code for the P700 apoprotein in PSI were reduced in chloroplasts from the E25-10 mutant yellow tissue. The levels of the large subunit of ribulose bisphosphate carboxylase (rbcL) and light harvest complex protein (LHCP) of PSII mRNA appeared to be reduced slightly in the mutant plants. However, a much more significant reduction in the 16S rRNA and the small subunit of ribulose bisphosphate carboxylase (rbcS) expression was detected in the yellow leaf sectors. Our results suggest that the possible lesion in E25-10 is located in the photosystem I even though fewer grana were observed in the defective chloroplasts.