The Emerging Role of Proline in the Establishment and Functioning of Legume-Rhizobium Symbiosis

Abstract

High levels of some enzymes involved in proline synthesis and utilization were early found in soybean nodules, and rhizobial knockout mutants were shown to be defective in inducing nodulation and/or fixing nitrogen, leading to postulate that this amino acid may represent a main substrate for energy transfer from the plant to the symbiont. However, inconsistent results were reported in other species, and several studies suggested that proline metabolism may play an essential role in the legume-Rhizobium symbiosis only under stress. Different mechanisms have been hypothesized to explain the beneficial effects of proline on nodule formation and bacteroid differentiation, yet none of them has been conclusively proven. Here, we summarize these findings, with special emphasis on the occurrence of a legume-specific isoform of δ1-pyrroline-5-carboxylate synthetase, the enzyme that catalyses the rate-limiting step in proline synthesis. Data are discussed in view of recent results connecting the regulation of both, the onset of nodulation and proline metabolism, to the redox status of the cell. Full comprehension of these aspects could open new perspectives to improve the adaptation of legumes to environmental stress.

Drought stress effects on Lathyrus seedlings: A morpho-physiological and molecular study

“…The catabolism of proline in mitochondria is facilitated by two essential enzymes, namely proline dehydrogenase (PDH) and P5C dehydrogenase (P5CDH), ultimately yielding glutamine. Studies have indicated that the modulation of proline synthesis, evidenced by the upregulation of P5CS transcription, and the downregulation of PDH under conditions of drought stress in leguminous plants like Lathyrus, could contribute to an elevation in proline content within PEG-treated Lathyrus (both at the seed and seedling stages) (Ruszkowski et al 2015, Sabbioni andForlani 2022).…”