Iron Deficiency in Tomatoes Reversed by Pseudomonas Strains: A Synergistic Role of Siderophores and Plant Gene Activation

An alkaline pH in soils reduces Fe availability, limiting Fe uptake, compromising plant growth, and showing chlorosis due to a decrease in chlorophyll content. To achieve proper Fe homeostasis, dicotyledonous plants activate a battery of strategies involving not only Fe absorption mechanisms, but also releasing phyto-siderophores and recruiting siderophore-producing bacterial strains. A screening for siderophore-producing bacterial isolates from the rhizosphere of <i>Pinus pinea</i> was carried out, resulting in two <i>Pseudomonas</i> strains, Z8.8 and Z10.4, with an outstanding in vitro potential to solubilize Fe, Mn, and Co. The delivery of each strain to 4-week-old iron-starved tomatoes reverted chlorosis, consistent with enhanced Fe contents up to 40%. Photosynthesis performance was improved, revealing different strategies. While Z8.8 increased energy absorption together with enhanced chlorophyll "a" content, followed by enhanced energy dissipation, Z10.4 lowered pigment contents, indicating a better use of absorbed energy, leading to a better survival rate. The systemic reprogramming induced by both strains reveals a lower expression of Fe uptake-related genes, suggesting that both strains have activated plant metabolism to accelerate Fe absorption faster than controls, consistent with increased Fe content in leaves (47% by Z8.8 and 42% by Z10.4), with the difference probably due to the ability of Z8.8 to produce auxins affecting root structure. In view of these results, both strains are effective candidates to develop biofertilizers.