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

Abstract

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 Pinus pinea was carried out, resulting in two Pseudomonas 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.

Keywords: Plant Growth Promoting Bacteria (PGPB); Pseudomonas; chlorosis reversion; iron nutrition; photosynthesis; siderophores.

Figure 1

Phylogenetic relationship among siderophore-producing PGPB based on 16S rDNA (60 strains). Neighbour joining was used to infer the evolutionary distances (numbers on the branches) with a bootstrap of 1000 replicates.

Figure 2

(A) Photosynthetic parameters (Fv/Fm; FPSR; NPQ) measured as relative units of fluorescence (RFU). (B) Content of photosynthetic pigments (chlorophyll “a”, chlorophyll “b”, and carotenoids), (μg·g⁻¹ of FW), in control plants in comparison to positive control plants. Data are expressed as the average (n = 3) ± the standard error. Asterisks show significant differences according to T-Student test (p < 0.05).

Figure 3

Dead, greenish, and yellow plants (%) in Pseudomonas Z8.8- and Z10.4-treated plants and in controls.

Figure 4

Photosynthetic parameters in inoculated and control plants. (A) Fv/Fm; (B) FPSII; (C) NPQ. RFU (n = 3) are expressed as the average ± the standard error. Asterisks show significant differences with controls according to T-Student test (p < 0.05).

Figure 5

Photosynthetic pigments concentration (μg·g⁻¹ fresh weight). Chlorophyll “a”, chlorophyll “b”, and carotenoids measured in control, and Z8.8- and Z10.4-inoculated plants (n = 3). Asterisks show significant differences with controls, according to T-Student test (p < 0.05).

Figure 6

Differential gene expression of iron uptake-related genes (HA1; FRO; IRT) in Pseudomonas (Z8.8 and Z10.4)-inoculated roots, expressed as M values with respect to transcript accumulation in control roots (n = 3). Asterisks show significant differences with control according to T-Student test (p < 0.05).