C ₄ -dicarboxylate transport mutants of Rhizobium trifolii form ineffective nodules on Trifolium repens

Abstract

Mutants of Rhizobium trifolii strain 7012 defective in C4-dicarboxylate transport were isolated by using a selective procedure based on pH indicator media. The mutant strains CR7098 and CR7099 failed to grow on or transport succinate, fumarate, or malate, but grew at wild-type rates on several other carbon sources.The C4-dicarboxylate transport system was inducible in strain 7012, but was expressed constitutively in four out of five succinate-positive revertants of strain CR7098. In the fifth CR7098 revertant (strain CR8008) the system was inducible. However, in contrast to strain 7012, strain CR8008 failed to use the C4-dicarboxylates in the presence of a second carbon source. Revertants of strain CR7099 were similar to strain 7012. Both strains CR7098 and CR7099 nodulated white and red clover at a rate similar to that ofstrain 7012, but nodules formed by the mutant strains were white and ineffective. Microscopic examination showed that the pattern of development of white clover nodules formed by strain CR7098 was similar to that observed with nodules formed by strain 7012, except that large amounts of starch accumulated in bacteroid-filled cells and senescence occurred earlier. Revertant strains were effective, except for strain CR8008, which formed ineffective nodules. The results show that a supply of C4-dicarboxylates to bacteroids is essential for nitrogen fixation in clover nodules. However, rhizobia within plant cells must also utilize other carbon sources to support growth and division.

Alfalfa malate dehydrogenase (MDH): molecular cloning and characterization of five different forms reveals a unique nodule‐enhanced MDH

“…Malate is also tightly linked to nodule nitrogen assimilation (Rosendahl et al, 1990). The compound is highly abundant in effective nodules (4-7 µmoles gfw -1 ) and is readily transported into bacteroids via a dicarboxylic acid transporter (Ronson et al, 1981;Rosendahl et al, 1990;Salminen and Streeter, 1992;Udvardi et al, 1988). Furthermore, Rhizobium mutants defective in either dicarboxylic acid transport or NAD ϩ malic enzyme are ineffective (Driscoll and Finan, 1993;Finan et al, 1983;Ronson et al, 1981).…”

Factors contributing to the accumulation of glutamate in Bradyrhizobium japonicum bacteroids under microaerobic conditions

“…Generation of reductant and ATP for nitrogenase is thought to depend on the operation of the TCA cycle (Duncan & Fraenkel, 1979;Ronson et al, 1981;Salminen & Streeter, 1987). Thus, a mechanism which would permit conversion of 2-oxoglutarate to succinate, bypassing OGDH and permitting continued operation of the TCA cycle, would be highly important.…”

How rhizobial symbionts invade plants: the Sinorhizobium–Medicago model

“…In a mixed infection, phbC and bdhA mutants are less competitive for nodule occupancy than the wild type, suggesting that the ability to synthesize and use PHB might allow the bacteria to compete in situations of reduced carbon availability 109,111 . Fixed carbon from the plant is provided to the bacteria in the form of dicarboxylic acids, such as malate, through the bacterial Dct uptake system 101,112 . NAD + -malic enzyme activity, which produces pyruvate directly from malate, is required for nitrogen fixation by S. meliloti 113 .…”