Improvement of Phosphate Solubilization and Medicago Plant Yield by an Indole-3-Acetic Acid-Overproducing Strain of Sinorhizobium meliloti

Author:

Bianco Carmen1,Defez Roberto1

Affiliation:

1. Institute of Genetics and Biophysics “Adriano Buzzati Traverso,” Via P. Castellino 111, 80131 Naples, Italy

Abstract

ABSTRACT Nitrogen (N) and phosphorus (P) are the most limiting factors for plant growth. Some microorganisms improve the uptake and availability of N and P, minimizing chemical fertilizer dependence. It has been published that the RD64 strain, a Sinorhizobium meliloti 1021 strain engineered to overproduce indole-3-acetic acid (IAA), showed improved nitrogen fixation ability compared to the wild-type 1021 strain. Here, we present data showing that RD64 is also highly effective in mobilizing P from insoluble sources, such as phosphate rock (PR). Under P-limiting conditions, the higher level of P-mobilizing activity of RD64 than of the 1021 wild-type strain is connected with the upregulation of genes coding for the high-affinity P transport system, the induction of acid phosphatase activity, and the increased secretion into the growth medium of malic, succinic, and fumaric acids. Medicago truncatula plants nodulated by RD64 ( Mt -RD64), when grown under P-deficient conditions, released larger amounts of another P-solubilizing organic acid, 2-hydroxyglutaric acid, than plants nodulated by the wild-type strain ( Mt -1021). It has already been shown that Mt -RD64 plants exhibited higher levels of dry-weight production than Mt -1021 plants. Here, we also report that P-starved Mt -RD64 plants show significant increases in both shoot and root fresh weights when compared to P-starved Mt -1021 plants. We discuss how, in a Rhizobium -legume model system, a balanced interplay of different factors linked to bacterial IAA overproduction rather than IAA production per se stimulates plant growth under stressful environmental conditions and, in particular, under P starvation.

Publisher

American Society for Microbiology

Subject

Ecology,Applied Microbiology and Biotechnology,Food Science,Biotechnology

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