Isolation and Assessment of Potential Rhizobacteria for Biological Control of Barnyard Grass (Echinochloa Crus-Galli (L.) P. Beauv), a Problematic Weed in Rice Fields

Abstract

<p>Barnyard grass (<i>Echinochloa crus-galli</i> (L.) P. Beauv) is a significant weed commonly found in rice fields. Currently, the primary approach to controlling this weed involves the application of herbicides. However, this method can have adverse effects on human health, the environment, and food safety. Also, excessive herbicide use can contribute to the development of herbicide-resistant weeds. To address these concerns, this study explored the potential of beneficial rhizobacteria as an eco-friendly alternative for managing barnyard grass. The research focused on evaluating the bioherbicidal effects of rhizobacterial strains obtained from the rhizosphere of barnyard grass.</p> <p>Barnyard grass and rice displayed diverse reactions to the rhizobacterial strains. Several strains, namely <i>Bacillus subtilis</i> (XNT4), <i>B. cereus</i> (CNT15), <i>Streptomyces luteogriseus</i> (CNT26h), <i>Pseudomonas aeruginosa</i> strain SMVIT (11CG), <i>P. aeruginosa</i> (6BF), and <i>Enterobacter cloacae</i> (6BG), exhibited the capability to inhibit the growth of barnyard grass shoots and roots, while concurrently promoting the growth of rice root or shoot. This distinct response indicates that these strains possess selective inhibitory effects specifically targeting barnyard grass, without compromising the growth of rice plants.</p> <p>A collection of rhizobacteria (387 isolates) was isolated from the rhizosphere of barnyard grass. In laboratory settings, 106 of these isolated strains significantly reduced both the shoot and root length of barnyard grass seedlings by more than 50% compared to the control. Furthermore, 26 strains exhibited minimal inhibitory effect on rice, causing less than a 5% reduction in shoot and root length of rice seedlings compared to the barnyard grass. During a laboratory bioassay conducted with barnyard grass and rice plants grown together, nine rhizobacterial strains (T220, T230, T232, T233, T246, T254, T268, T291, and T294) were identified as significantly reducing the growth of barnyard grass while leaving the growth of rice unaffected. Among these isolates, four strains derived from the soil rhizosphere exhibited detrimental effects on barnyard grass in greenhouse conditions. Molecular identification revealed that the four rhizosphere bacteria potentially possessing herbicidal activity were<i> Pseudomonas fluorescens</i> (T230) and three isolates of <i>Enterobacter ludwigii</i> (T220, T232, and T233). This study represents the first documentation of <i>P. fluorescens</i> and <i>E. ludwigii</i>, isolated from the rhizosphere of barnyard grass, exhibiting varying degrees of growth suppression on barnyard grass while not affecting the growth of rice.</p> <p>Pseudomonas fluorescens (T230) and <i>E. ludwigii</i> (T220, T232, and T233) isolates demonstrated a consistent decrease in barnyard grass shoot length, number of leaves, and dry weight with an increase in the concentration of the tested rhizobacterial strains. Furthermore, the effectiveness of these soil microbes was found to be higher when applied at an earlier stage. Consequently, an early application of the rhizobacterial strains at a concentration of 107 cfu/g of soil resulted in a significant reduction in barnyard grass dry weight. The study revealed that <i>P. fluorescens</i> and <i>E. ludwigii</i> strains exhibited superior efficacy in reducing barnyard grass dry weight in black Vertosol clay compared to sandy loam Chromosol and red Ferrosol clay soils. Additionally, it was observed that the non-flooded treatment provided favourable conditions for the rhizobacterial strains to reduce barnyard grass growth, in contrast to the flooded and intermittent flooding treatments. </p> <p>The microbial communities present in the rhizosphere and endosphere of barnyard grass, growing in different agricultural environments, were observed to be strongly influenced by soil type and the growth stage of the plants. Among the sampled rhizosphere of barnyard grass, four bacterial groups, namely Actinobacteria, Proteobacteria, Acidobacteria, and Chloroflexi, were identified as the most prevalent taxa. Notably, the structure of the rhizosphere community was significantly impacted by soil type, with the Chromosol soil displaying a threefold higher abundance of bacterial communities compared to the Vertosol soil, regardless of the rhizosphere or endosphere.<i> Enterobacter</i> exhibited an increase in abundance as the barnyard grass plants advanced in growth and was found in both soil types. Conversely,<i>Pseudomonas</i> displayed a greater dominance in the Chromosol soil compared to the Vertosol soil. </p> <p>In conclusion, the rhizobacterial strains subjected to testing displayed diverse reactions to barnyard grass and rice plants. Notably, four of these strains -<i> Pseudomonas fluorescens</i> (T230) and three isolates of <i>Enterobacter ludwigii</i> (T220, T232, and T233) - demonstrated adverse effects on barnyard grass while maintaining the growth of rice plants. The results highlighting the selective inhibition of weed growth by particular rhizobacterial strains, including <i>P. fluorescens</i> and <i>E. ludwigii</i>, offer compelling evidence for their potential inclusion in future weed management strategies. </p>