Mechanism of transport of vegetative storage proteins to the vacuole of the paraveinal mesophyll of soybean leaf

Abstract

Microscopy techniques were used to identify the pathway of transport of soybean leaf vegetative storage proteins (VSPα/β and VSP94) to the vacuoles of a specialized cell type, the paraveinal mesophyll (PVM), where they accumulate. PVM cells are enriched in endoplasmic reticulum and Golgi bodies relative to surrounding mesophyll cells. The margins of medial and trans Golgi cisternae had attached or closely associated noncoated vesicles with densely staining membranes and lumenal contents of the same appearance as material that accumulated in the vacuole. These vesicles appeared to be transported preferentially to the tonoplast, where fusion with the membrane released the granular contents into the vacuole. Cytochemical staining with phosphotungstic acid and silver methenamine supported this interpretation as both the Golgi vesicles and the tonoplast stained intensely with these reagents, unlike the tonoplast of mesophyll cells which do not accumulate VSP. Immunocytochemical localization for VSPα/β labeled the Golgi bodies and associated vesicles, and vacuolar material in PVM cells, but not in mesophyll. Similar labeling was seen in PVM of another legume species previously found to accumulate antigenically similar VSPs. Immunolocalization for VSP94, a lipoxygenase, labeled the PVM cytosol and material in the PVM vacuole, but not the Golgi or vesicles. The results of this study demonstrate that the Golgi pathway is utilized for transport of VSPα/β in the PVM, which follows the mechanism of deposition demonstrated for certain seed storage proteins. VSP94 appeared to follow a separate path for accumulation in PVM vacuoles.