Protein dynamics, activity and cellular localization of soybean lipoxygenases indicate distinct functional roles for individual isoforms

Vegetative lipoxygenases (VLXs) in soybean are hypothesized to function in nitrogen storage and partitioning. Isoform-specific antibodies for four of the five known VLX isoenzymes were used to investigate the influence of source-sink status on protein levels, as well as to analyze the tissue and subcellular localization of the different isoforms. VLXD responded most strongly to sink limitation, although the levels of VLXA, B and C increased as well. After sink limitation, VLXD and the vegetative storage protein, VSPalpha, accumulated in the vacuoles of bundle sheath and paraveinal mesophyll cells, while VLXA, B and C localized to the cytosol of these cells. All five known VLX isoenzymes were active with both linoleic and linolenic acid substrates after expression in Escherichia coli. The strong upregulation of VLXD levels after sink limitation as well as the localization of this isoform to the vacuoles of paraveinal mesophyll and bundle sheath cells (where VSPs are found) strongly suggest that VLXD should be considered as a major storage protein in soybean leaves. Furthermore, since VLXA, B and C also accumulate in sink-limited soybean leaves, are located in the cytosol of paraveinal mesophyll cells and are active at pH values typically found in this compartment, their activities may well contribute to lipid metabolism in this tissue. This multi-gene family is thus ideally poised to play a pivotal role in the balance of N deposition relative to lipid-based storage, defense or signaling, by modulating contributions to these processes in the transient storage cells of the paraveinal mesophyll.