Protein compositions of mesophyll and paraveinal mesophyll of soybean leaves at various developmental stages

Mesophyll and paraveinal mesophyll protoplasts (PVMP) were isolated from leaves of soybean (Glycine max) at various stages of physiological development, and protein compositions of the two protoplast types were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblotting. Polypeptides of 27, 29 (previously shown to be storage proteins), and 94 kilodaltons were found to be PVMP-specific proteins and were present in both nodulated and nonnodulated plants. The 27 and 94 kilodalton polypeptides were major PVMP constituents. All three polypeptides accumulate as early as one-quarter leaf expansion. Immunoblotting and immunocytochemical studies using antibodies against the 27/29 kilodalton proteins confirmed that they are specific to the paraveinal mesophyll (PVM) and that they are localized in the PVM vacuole. The 27 kilodalton polypeptide increased significantly by two weeks depodding, and this accumulation was restricted to the PVM vacuole. Radiolabeling experiments showed that the difference in relative amounts of the 27 and 29 kilodalton polypeptides was due to a greater rate of synthesis of the 27 kilodalton polypeptide. The 94 kilodalton polypeptide accumulated to a maximum at anthesis, but was absent at 2 weeks postanthesis in both depodded and podded nodulated plants, probably because they were nitrogen limited. In nonnodulated plants, it was present through 2 weeks postanthesis. The results confirm that the 27 and 29 kilodalton proteins of soybean leaf are stored in the PVM vacuole and show that they are accumulated early during leaf development while they are still strong sinks for nitrogen. The 94 kilodalton protein, previously found to accumulate in leaves after depodding, is also a PVM protein and is likely a third vegetative storage protein, although its accumulation appears to be more dependent on excess nitrogen availability. The results further support the hypothesis that the PVM is a specialized leaf tissue that functions in synthesis and compartmentation of storage proteins.