Enhancing citrus resilience to high temperature and intense light stress with Pseudomonas putida and Novosphingobium sp.

Adverse environmental conditions, aquifer contamination, and excessive fertilizer use have significantly impacted agricultural production, with these challenges worsening over the last few decades. Given the urgent need for more environmentally sustainable agricultural practices, recent studies have highlighted the potential of plant growth-promoting rhizobacteria (PGPR) as a biological alternative. PGPRs have been reported to function as biofertilizers and enhancers of plant tolerance to stress. Citrus, a globally important fruit crop, has been particularly affected by these environmental stressors. In this work, we aim to explore the effects of inoculating Cleopatra mandarin plants with PGPR strains Pseudomonas putida KT2440 and Novosphingobium sp. HR1a under combined conditions of heat stress and high light intensity (HSHL). Our findings demonstrate that inoculation with these strains provides significant protection against the detrimental effects of HSHL conditions. This protection is evident across multiple levels—phenotypically, physiologically, and molecularly—following the inoculation with Pseudomonas putida and Novosphingobium sp. Specifically, we observed under HSHL: (i) no leaf abscission in inoculated plants, (ii) improved photosynthesis efficiency under both non-stressed and stressed conditions, (iii) reduced malondialdehyde content, (iv) increased catalase activity, (v) upregulation of genes involved in stress tolerance (WRKY40, WRKY76, and HSF30), and (vi) increased sugar accumulation. Our study reveals that the use of Pseudomonas putida and Novosphingobium sp. offers a promising and environmentally sustainable approach for citriculture, particularly in mitigating the negative effects associated with climate change scenarios.