Identification of Pseudomonas mechanisms contributing to maize (Zea mays L.) protection against Fusarium graminearum

Abstract

Contamination of maize (Zea mays L.) with fumonisins in the field occurs when conditions favourable to the growth of Fusarium spp. such as high water activity by the time the plant reach near physiological maturity. The main source of the contamination is airborne conidia, mainly dispersed by wind, insects or rain. Fusarium spp. enters maize ear mainly via the silks or via ear wounds caused by birds or insects. This fungus is the most important ear and kernel rotting pathogen of maize. This fungus causes several infections in maize plants that cause significant quality and yield losses. The fungus also produces toxins that result in significant damage to the maize plant. In this study, three rhizospheric bacterial strains were isolated from two (Animal Health Center (AHC), North-West University, Mafikeng Campus, South Africa and (ii) North-West University Agricultural Farm, Molelwane (NWUAFM), South Africa) maize fields. Biochemical and morphological characteristics, nucleotide sequence analysis of the 16rDNA revealed that all the isolates belong to members of the genus Pseudomonas. The bacterial isolates were tested for their antagonistic activity against the growth of F. graminearum in-vitro. Various Fusarium growth inhibition traits (genes) including those for the production of the antibiotic phenazines (phz), pyoluteorine (phltb), pyrrolnitrin (prnD) and 2.4- diacetylphloroglucinol (2.4-DAPG). Moreover, all the Pseudomonas isolates were tested positive for the production of siderophore, indole-3-3acetic acid (IAA) and the enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase. All isolates (P. mediterranea, P. putida and P. fluorescens) had the ability to produce siderophore with the highest production of 51.90% observed in P. putida. All isolates also produced indole-3-acetic acid, ACC deaminase activity and ammonia, while two isolates (P. mediterranea and P. fluorescens) solubilized phosphate and one (P. mediterranea) produced hydrogen cyanide. P. putida was the most abundant and best performing isolate with the highest IAA production (9.57±0.66 μg/ml) and the highest ACC deaminase activity (0.87±0.12 μmol α-ketobutyrate mg protein⁻¹ hour⁻¹). A gnotobiotic study was undertaken to test the antagonistic effect of P. mediterranea and P. putida against F. graminearum using three maize seed cultivars. The result indicated that the inoculation of maize seeds with the three Pseudomonas spp. significantly suppressed the growth of F. graminearum and also resulted in significant increase in important physiological parameters including root growth. The significance of this study is that it generated valuable baseline data and information on the biocontrol activities of Pseudomonas mediterranea strain B5 and Pseudomonas putida strain S6 against Fusarium graminearum infection. With further screening in the field trials, these Pseudomonas strains could be developed into agents for effective control of F. graminearum diseases in maize plants.