Nuclear and cytoplasmic “stay‐green” mutations of soybean alter the loss of leaf soluble proteins during senescence

In soybean ( Glycine max [L.] Merr.) the homozygous combination of the recessive alleles dI and d2 (i.e., dldld2d2 ) at two different nuclear loci or the cytoplasmic gene cytG inhibit chlorophyll degradation during senescence; i.e. their leaves are green when they are shed. The main objectives of the present work were: (J) to determine whether these “stay‐green” genes also interfere with the loss of the bulk of leaf soluble proteins and ribulose bisphospnate carboxylase/oxygensase (Rubisco; EC 4.1.1.39) during senescence and (2) to relate this to alterations in leaf proteolytic activity. Leaves of the normal. Yellowing cvs Clark and Harosoy lost about 90% of their soluble proteins before abscission. The abscising leaves of these cultivars contained no detectable Rubisco. By contrast, protein degradation was significantly less in leaves of near‐isogenic lines of Clark and Harosoy carrying dIdId2d2 , with or without G (a dominant nuclear gene in a third locus causing green seed coats). These leaves still retained 50% of the soluble protein and large amounts of both subunits of Rubisco at the time of abscission. Alone, neither dl nor d2 had any effect. The cytoplasmic gene cytG slowed the loss of Rubisco. although eventually when leaves were shed they contained as little Rubisco as Clark. Despite inhibition (i.e. dIdId2d2 and GGdIdId2d2 ) or retardation (i.e. cytG ) of protein loss, these mutant genotypes did not differ from Clark in the breakdown of endogenous Rubisco by leaf extracts (“autodigestion”). The wild‐type alleles in the dI and d2 loci may control a central regulatory process of the senescence syndrome.