K + regulates bacteroid-associated functions of Bradyrhizobium

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Abstract
Cowpea Bradyrhizobium 32H1 cells, when grown under 0.2% O2, synthesize nitrogenase, as well as a methylammonium (ammonium) transport system and an electrogenic K+/H+ antiporter. This effect was seen in growth medium containing 8-12 mM K+ but not with 50 .mu.M K+. Addition of K+ to cells growing under low O2 tensions in low-K+ medium led to various phenotypic properties associated with bacterioids, including the ability to reduce acetylene, induction of an ammonium transport carrier and the K+/H+ antiporter, and increased synthesis of two heme-biosynthetic enzymes, .delta. aminolevulinate synthase and .delta.-aminolevulinate dehydratase. K+ addition caused the repression of glutamine synthetase and of capsular polysaccharide synthesis, functions related to the free-living state. A similar pattern of regulation was observed in Bradyrhizobium japonicum. In addition, K+-mediated derepression in Bradyrhizobium 32H1 was inhibited by exudate of Vigna unguiculata, its host plant. We conclude that K+ ions, in addition to low O2 tension, are needed for the expression of several bacteroid-related functions in bradyrhizobia and thus are a major controlling influence in bacteroid development.