# Observations Concerning Rhizobium tropici Bacteroid Phosphorus Stress Response During Symbiosis With Phaseolus vulgaris

## TL;DR

This study explores how Rhizobium tropici bacteroids respond to phosphorus stress during symbiosis with bean plants.

## Contribution

The study reveals the role of alkaline phosphatase and high-affinity Pi transporters in Pi metabolism and carbon exchange in determinate nodules.

## Key findings

- Alkaline phosphatase mutant bacteroids did not show defects in Pi acquisition but had altered carbon uptake.
- The high-affinity Pi transporter is crucial for Pi acquisition and symbiotic performance in determinate nodules.
- Pi taken up into the nodule is redistributed to the host plant rather than remaining in the nodule.

## Abstract

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.

14CO2 and 32PO4 labelling of bean was used with
Rhizobium tropici
mutants. An alkaline phosphatase mutant was not affected for bacteroid P acquisition, though bacteroid and nodule 14C‐carbon uptake was altered. The high‐affinity P transporter mutant was defective in nodule P acquisition and symbiotic performance. Plant image by Brgfx/Freepik.