Elisa Gamalero

The Mediterranean area is one of the geographic zones most affected by land degradation and desertification and these conditions appear to be even more exacerbated by climate change. Based on this idea, this work aimed to isolate, identify, characterize, and select bacterial strains able to tolerate salinity and drought, which could possibly be used in agriculture as plant biofertilizers. The sampling of rhizosphere soil was performed in two Algerian regions, Ghardaïa and Djelfa (arid and semi-arid zones, respectively) in six provinces, targeting fourteen native plant species, known for their therapeutic use. A total of 288 bacterial strains were isolated, identified, and characterized for their growth at different temperatures and salt tolerance. Based on these capabilities, 95 isolates were selected. These strains underwent further evaluation for their plant-beneficial traits, including siderophore synthesis, auxin production, and phosphate solubilization. Additionally, we assessed their impact on tomato, cucumber, and sorghum seed germination. In a final screening step, nine bacterial strains were tested for their potential plant growth-promoting activity on tomato plants grown in semi-controlled conditions. Our results demonstrated that three strains (Bacillus simplex AH24, Microbacterium arborescens PU10, and Microbacterium paludicola AEA23) showed plant growth promotion activities on tomato.

Tomatoes are one of the most important crops worldwide and also play a central role in the human diet. Microbial consortia are microorganism associations, often employed as bioinoculants, that can interact with the native rhizosphere microbiota. The aim of this study was to evaluate the impact of a bacterial-based biostimulant (Pseudomonas fluorescens and Bacillus amyloliquefaciens) (PSBA) in combination, or not, with a commercial inoculum Micomix (Rhizoglomus irregulare, Funnelliformis mosseae, Funnelliformis caledonium, Bacillus licheniformis, Bacillus mucilaginosus) (MYC) on the native rhizosphere communities and on tomato production. The trial was carried out using Solanum lycopersicum in an open field as follows: control full NPK (CFD), control reduced NPK (CRD), MYC, PSBA, PSBA + MYC. Bacterial population in the different samples were characterized using a next generation sequencing approach. The bioinocula effect on the native rhizosphere microbiota resulted in significant variation both in alpha and beta diversity and in a specific signature associated with the presence of biostimulants, especially in the presence of co-inoculation (PSBA + MYC). In particular, the high initial biodiversity shifts in the community composition occurred and consisted in the increase in the abundance of genera correlated to the soil acidification and in an enhanced density of nitrogen-fixing microbes. The results also highlighted the well-known rhizosphere effect.

Here, a brief summary of the biosynthesis of 1-aminocyclopropane-1-carboxylate (ACC) and ethylene in plants, as well as overviews of how ACC and ethylene act as signaling molecules in plants, is presented. Next, how the bacterial enzyme ACC deaminase cleaves plant-produced ACC and thereby decreases or prevents the ethylene or ACC modulation of plant gene expression is considered. A detailed model of ACC deaminase functioning, including the role of indoleacetic acid (IAA), is presented. Given that ACC is a signaling molecule under some circumstances, this suggests that ACC, which appears to have evolved prior to ethylene, may have been a major signaling molecule in primitive plants prior to the evolution of ethylene and ethylene signaling. Due to their involvement in stimulating ethylene production, the role of D-amino acids in plants is then considered. The enzyme D-cysteine desulfhydrase, which is structurally very similar to ACC deaminase, is briefly discussed and the possibility that ACC deaminase arose as a variant of D-cysteine desulfhydrase is suggested.