P Metabolism in the Bean-Rhizobium tropici Symbiosis

Abstract

Nodulated legumes require more P than legumes growing on mineral nitrogen, but little is known about the basis for the higher P requirement. Experiments were conducted to determine how Rhizobium tropici responds to P limitation and to understand how P is partitioned between the symbionts under conditions of adequate or limiting P. Free-living R. tropici responds to P stress by increasing P transport capacity and inducing both an acid and an alkaline phosphatase. This P-stress response occurs when the medium P concentration decreases below 1 p~. Both P-stressinducible phosphatases are found in bacteroids taken from plants growing with adequate P, suggesting that P levels in the symbiosome space is low enough to induce the expression of these enzymes. Bacteroid alkaline phosphatase-specific activity was highest during vegetative growth of the bean plant, but decreased approximately 75% during the host reproductive stages. In hydroponic experiments 32P-tracer studies showed that in vivo rates of P accumulation were significantly higher in bacteroids from P-limited plants compared with those from plants that had been supplied with adequate P. In contrast, label accumulation in leaves was greatest in plants grown with adequate P.On a worldwide basis, most cultivated soils have insufficient P for maximum crop yields. Legumes are particularly affected because they are typically cultured symbiotically, and it has been shown that legumes dependent on symbiotic nitrogen fixation have higher P requirements than legumes grown with a nitrogen fertilizer. Soybean (Cassman et al., 1981~;Israel, 1987), clover (Powell, 1977), common bean Bliss, 1987, 1989), pigeon pea (Itoh, 1987), and cowpea (Cassman, et al., 1981~) a11 respond positively to P fertilization. Growth and symbiotic parameters increased by P fertilization include whole-plant N concentration, plant dry matter, nodule number, nodule mass, nitrogenase activity, and specific nodule nitrogenase activity.The reasons for this P response are poorly understood. In soybean symbiotic N, fixation places a P tax on the host plant (Israel, 1987). Field studies with peanuts have found that P concentrations in the stems and seeds of nodulating ' This work was initiated while T.R.M. was a National Science Foundation postdoctoral fellow in plant biology (BIR-9203796) lines are lower than those in nonnodulating isolines (Sahrawat, et al., 1988), suggesting that P distribution within the plant is different when the legume plant grows symbiotically. The higher P requirement of symbiotically grown legumes is apparently not due to different abilities of roots in the two N environments to absorb P (Israel, 1987), which implies that an optimum symbiotic interaction between the host plant and rhizobia depends on an efficient allocation and use of available P.Most P metabolism studies with rhizobia have used freeliving cells. P requirements for growth (Keyser and Munns, 1979;Cassman et al., 1981aCassman et al., , 1981b, P transport rates, and P storage (Cassman et al., 1981b;Beck and Mun...

Characterization of the Rhizobium (Sinorhizobium) meliloti high- and low-affinity phosphate uptake systems

“…strain NGR234 were also low, at 1.3 and 0.4 nmol/min/mg (dry weight) of cells, respectively. Similar numbers were reported by Al-Niemi et al (1) for P i uptake in P i -sufficient cells and bacteroids of Rhizobium tropici. Moreover, in many of the experiments, we observed that despite the apparent low rates of P i transport, the cells showed the expected rates of succinate uptake (30 to 60 nmol/min/mg of protein), confirming that the cells were physiologically active.…”

Routes of pyruvate synthesis in phosphorus‐deficient lupin roots and nodules

“…physiological studies (Sa & Israel, 1991;Al-Niemi et al, 1997;Al-Niemi et al, 1998) it is apparent that bacteroids in the nodule almost always operate at low P concentrations and that bacteroids can fulfil their P requirements by scavenging from the host cells (Al-Niemi et al, 1997Colebatch et al, 2004).…”

“…Additionally, it was demonstrated that nodules can take up P from the host, but do not readily release its P reserves to the host root (Al-Niemi et al ., 1998) suggesting that nodules form strong sinks for P. Furthermore, this important role of P in nodules is supported by the threefold higher P concentrations compared with other tissues (Vadez et al ., 1997). Therefore, under conditions of P limitation, the optimum symbiotic interaction between the host plant and rhizobia would depend on efficient allocation and use of available P (Al-Niemi et al ., 1997.…”

The effect of inoculation of pea plants with mycorrhizal fungi and Rhizobium on nitrogen and phosphorus assimilation

“…It was established that nodulating legumes require more P than legumes growing on mineral nitrogen (Al-Niemi et al 1997). The AM fungi associated with legumes are an essential link for effective phosphorus nutrition, leading to enhanced nitrogen fixation that in turn promotes root and mycorrhizal growth.…”