Observations Concerning <scp> <i>Rhizobium tropici</i> </scp> Bacteroid Phosphorus Stress Response During Symbiosis With <scp> <i>Phaseolus vulgaris</i> </scp>

Bacteroid inorganic phosphorus (Pi) metabolism in the Rhizobium-legume symbiosis differs between indeterminate and determinate legume nodules. In contrast to alfalfa bacteroids, bean (Phaseolus vulgaris) bacteroids exhibit high levels of alkaline phosphatase (AP), the native reporter enzyme for the bacterial Pi stress response. ¹⁴C and ³²Pi whole plant labelling techniques were used in conjunction with diagnostic mutants (lacking AP or lacking high affinity Pi transport) to assess the relative importance of the Pi stress response in Rhizobium tropici bacteroids during symbiosis. The AP- mutant was not defective for symbiosis and did not differ from wildtype bacteroids for Pi acquisition. ¹⁴C-CO₂ feeding to host plants revealed ¹⁴C-carbon uptake and accumulation in AP- mutant bacteroids, and their nodules were increased relative to wildtype bacteroids, implying that organo-P compounds may account for meaningful levels of carbon exchange between symbionts. ³²Pi tracer experiments implied that the high affinity transporter is important to bacteroid Pi acquisition and symbiotic performance in determinate nodules, but that the symbiosome Pi concentration does not meet the capacity of the high affinity transporter. ³²P tracer work also illustrated that Pi taken up into the nodule does not remain in the nodule, but rather is redistributed to the host.