Biochemical and genetic studies of variegated leaf mutants of soybean (Glycine max (L.) Merr.);

An unstable allele at a gene affecting chloroplast development in soybean (Glycine max (L.) Merr.) was originally identified in the cultivar Lincoln . Homozygous mutant plants produce green, yellow, or green/yellow variegated leaves. Chloroplasts from mutant plants exhibit qualitatively similar profiles for pigment and individual photosynthetic complexes; however, there is a reduction in total chlorophyll and the absence of several thylakoid membrane proteins. The oxygen-evolving complex of photosystem II (PSII) includes one polypeptide (OEC16) which was determined by Western blots to be absent in mutant chloroplasts, whereas, OEC33 and OEC23, two other components of the oxygen-evolving complex, are present. The mRNAs for all three products can be detected in equivalent amounts by Northern blots of RNA from both green and yellow tissues. The OEC16 mRNA derived from yellow leaf tissue is translatable in vitro; however, at reduced efficiency compared to mRNA isolated from green leaf tissue. Northern analysis of specific chloroplast-encoded (rbcL and 16S rRNA) and nuclear-encoded genes (rbcS and cab) revealed reduced levels of expression in mutant tissues. An unstable mutation in the soybean variety \u27Williams\u27 was generated by exposure of seeds to gamma ($\gamma$) radiation (25 kR; $\sp{60}$Co). The mutant (E25-10) plants carry abnormal chloroplasts in the yellow leaf sectors as determined by light and electron microscopy. Genetic studies indicate that a single nuclear-encoded gene is responsible for the mutation. In the defective chloroplasts, both the stroma and grana lamella systems are disrupted. LDS-PAGE analysis revealed 5-6 polypeptides with estimated molecular weights of 51, 44, 25, 15, 13, and 12 kD that are consistently absent in thylakoid membrane preparations from yellow tissue. Total chlorophyll content is significantly reduced in the mutant plants; however, there is no change in the chlorophyll a/b ratio. Major photosynthetic complexes are not detectable in mutant chloroplasts, whereas nuclear-encoded 33-, 23- and 16-kD proteins which function as the water splitting site of PSII are detectable. Both nuclear- (rbcS and cab) and chloroplast-encoded (rbcL, and 16S rRNA) photosynthetic genes were expressed at low levels in mutant tissue. An equivalent level of psbA mRNA is detectable from both green and yellow tissues; however, psaA transcripts are barely measurable in the mutant