PGPB Improve Photosynthetic Activity and Tolerance to Oxidative Stress in Brassica napus Grown on Salinized Soils

Massimiliano Rossi,Cinzia Forni,Maddalena Del Gallo,Concetta Capo,Fabrizio Pietrini,Ilaria Borromeo,Bernard R Glick

doi:10.3390/app112311442

Abstract

Soil salinization, one of the most common causes of soil degradation, negatively affects plant growth, reproduction, and yield in plants. Saline conditions elicit some physiological changes to cope with the imposed osmotic and oxidative stresses. Inoculation of plants with some bacterial species that stimulate their growth, i.e., plant growth-promoting bacteria (PGPB), may help plants to counteract saline stress, thus improving the plant’s fitness. This manuscript reports the effects of the inoculation of a salt-sensitive cultivar of Brassica napus (canola) with five different PGPB species (separately), i.e., Azospirillum brasilense, Arthrobacter globiformis, Burkholderia ambifaria, Herbaspirillum seropedicae, and Pseudomonas sp. on plant salt stress physiological responses. The seeds were sown in saline soil (8 dS/m) and inoculated with bacterial suspensions. Seedlings were grown to the phenological stage of rosetta, when morphological and physiological features were determined. In the presence of the above-mentioned PGPB, salt exposed canola plants grew better than non-inoculated controls. The water loss was reduced in inoculated plants under saline conditions, due to a low level of membrane damage and the enhanced synthesis of the osmolyte proline, the latter depending on the bacterial strain inoculated. The reduction in membrane damage was also due to the increased antioxidant activity (i.e., higher amount of phenolic compounds, enhanced superoxide dismutase, and ascorbate peroxidase activities) in salt-stressed and inoculated Brassica napus. Furthermore, the salt-stressed and inoculated plants did not show detrimental effects to their photosynthetic apparatus, i.e., higher efficiency of PSII and low energy dissipation by heat for photosynthesis were detected. The improvement of the response to salt stress provided by PGPB paves the way to further use of PGPB as inoculants of plants grown in saline soils.

Highlights

The continued increase in land affected by salinization [1] makes it necessary to select crops that are salt tolerant, to avoid the yield loss that would otherwise be associated with high soil salinity
All strains grew at a high salt concentration (160 mM NaCl), A. brasilense was able to grow at 320 mM NaCl; the Pseudomonas sp. strain could grow at the highest salinity tested (640 mM NaCl in tryptic soy broth (TSB) medium) (Figure 1)
Plant Growth in Saline Conditions The seed germination phase of canola is sensitive to salinity; plant growth-promoting bacteria (PGPB) inoculation could help the seeds overcome this salt stress

Summary

Introduction

The continued increase in land affected by salinization [1] makes it necessary to select crops that are salt tolerant, to avoid the yield loss that would otherwise be associated with high soil salinity. Photosynthesis is one of the physiological processes that are extremely susceptible to environmental stress, and for this reason, photosynthetic efficiency can be a good biomarker of environmental pressure in stressed plants [6]. In this context, chlorophyll fluorescence imaging can be analyzed to evaluate plant stresses, allowing in vivo analysis that it is non-destructive [7]. It allows the evaluation of the heterogeneity in photosynthetic functions throughout a leaf, owing to image analysis of the quantum efficiency of photosystem II (PSII) in plants affected by several stresses, including heavy metals [8,9], salinity [10], pharmaceuticals [11], and emerging contaminants [12]

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