Abstract
BackgroundThe bacterial-spot disease caused by different Xanthomonas species is one of the major tomato diseases that reduce crop production and quality. Pesticides indiscriminate usage has resulted in an increase in resistant bacterial strains as well as contamination of farmers, consumers and the environment. Plant growth-promoting bacteria and humic acids can act as elicitors of plant defence mechanism causing extensive transcriptional and metabolic reprogramming which, in turn, produce a range of plant chemical defences. The purpose of this study was to study how humic acids and plant growth-promoting bacteria, when applied to the substrate, affected the severity of bacterial spot symptoms in tomato leaves.Materials and methodsOne-month-old Micro-Tom tomato (Solanum lycopersicum L.) were transferred to 3 L pots filled with a sterile mixture of sand and vermiculite (2:1, v:v) and treated or not (control) with 250 mL of 4.5 mmol C. L−1 of humic acids, Herbaspirillum seropedicae (108 CFU. mL−1) and the combination of humic acids plus H. seropedicae. One day after substrate treatment, the leaves were inoculated (or not) with X. euvesicatoria (Xe). The area below the disease progression curve based on severity scores and the number of symptomatic leaflets was used to assess phytopathogen virulence. The concentration of oxalic, citric and succinic acids in leaf extracts were determined using HPLC analysis.ResultsSole or combined H. seropedicae (BAC) and humic acids (HA) application promoted shoot and root growth related to control when plants were challenged with Xe pathogen. For plants inoculated with Xe, more significant plant-growth promotion results were obtained for HA + BAC treatment. The first visible symptoms were observed 16 days after inoculation with 2 × 104 CFU. g−1 of Xe cells in leaves of control plants. HA and BAC applied alone or combined reduced disease severity. Only plants treated with HA were able to reduce disease incidence (number of the leaflets with symptoms). Organic acids, such as oxalic, citric and succinic acids, rose in Xe-inoculated leaves. The reduced amount of organic acids in diseased leaves treated with HA + BAC may be linked to a decrease in disease progression.ConclusionHumic acids and H. seropedicae increased growth by modulating the content of organic acids in leaf tissue, attenuating the symptoms of the bacterial spot disease.Graphic abstract
Highlights
Biofertilization with soluble humates and plant growthpromoting bacteria has attracted attention in recent years due to its beneficial effects on nutrient uptake and protection of plants against abiotic stress [1, 2]
For plants inoculated with X. euvesicatoria (Xe), more significant plant-growth promotion results were obtained for humic acids (HA) + Suspension with HA plus BAC (BAC) treatment
We evaluated the population size of H. seropedicae cells established in the plant growth substrate using the most probable number (MPN) methodology at 5 and 15 days after suspension application
Summary
Biofertilization with soluble humates and plant growthpromoting bacteria has attracted attention in recent years due to its beneficial effects on nutrient uptake and protection of plants against abiotic stress [1, 2]. This protection is generally due to increase of enzymatic and nonenzymatic antioxidant defenses, increase in compatible solutes production and changes in ion balance as well as boost in metabolism and plant growth. The purpose of this study was to study how humic acids and plant growth-promoting bacteria, when applied to the substrate, affected the severity of bacterial spot symptoms in tomato leaves
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