Co-inoculation of native multi-trait plant growth promoting rhizobacteria promotes plant growth and suppresses Alternaria blight disease in castor (Ricinus communis L.)

Abstract

Background: Castor (Ricinus communis L.) is cultivated for seed oil and to feed (leaves) Eri silkworm, Samia ricini (Donovan) Hutt. Alternaria blight affects castor cultivation resulting substantial yield loss (∼30%). Uses of synthetic fertilizers and agrochemicals for disease management have serious concerns as the castor leaves are fed to eri silkworms for rearing. Application of plant growth promoting rhizobacteria for disease suppression and to enhance plant growth will be a healthier choice in castor cultivation. The aim of this study was to assess the efficacy of Alternaria blight disease suppression by native rhizobacteria isolated from wasteland castor and their ability on plant growth promotion.

Methodology: We isolated 50 bacterial antagonists from castor rhizosphere using the dilution plate method and evaluated their antagonistic activity against the castor blight pathogen, Alternaria ricini. Based on antimicrobial bioassay and plant growth promotion (PGP) traits (phosphate solubilization, ACC deaminase activities, production of IAA, GA₃, HCN, NH₃ and siderophore), salt and acid tolerance; we have chosen ten potential isolates and identified them through 16SrRNA gene sequencing and analysis. Disease suppression and plant growth studies were evaluated in pot experiments.

Results and conclusion: Three isolates namely, Enterobacter hormaechei (LRP-2), Bacillus mycoides (HF-1) and B. aryabhattai (UR-6) showed potential antagonistic activities and PGP traits which were selected for disease suppression and PGP studies. Application of PGPR consortia (LRP-2+HF-1) could suppress the plants from A. ricini infection in challenged inoculation. Mix inoculation of LRP-2 and UR-6 showed synergistic effect and enhanced plant growth in pot experiments. Combinations of E. hormaechei (LRP-2), B. mycoides (HF-1) and B. aryabhattai (UR-6) can be applied as bio-control and bio-fertilizer formulation to protect castor from Alternaria blight and also to enhance plant growth.

Keywords: Alternaria blight; Antagonists; Castor; Plant growth promoting rhizobacteria (PGPR); Wasteland.

Figure 1

Bacterial 16SrRNA sequences based homology study of the selected bacterial sequences indicating the position of strain LRP-2, HF-1 and UR-6 (antagonist and plant growth promoting strains) using Neighbour Joining method.

Figure 2

Identification of fungal pathogen (LH01) of castor (Alternaria ricini). A-C, Infected castor leaf, fungal mycelium and septate spore of the fungus; D – Dendrogram was drawn using the homology study among the closest sequencess through BLAST search in NCBI database (using MEGA).

Figure 3

Antimicrobial bioassay of the selected bacterial antagonists against A. ricini (dual culture method). Values followed by a different letter are significantly different (P ≤ 0.01). Error bars represent standard deviations.

Figure 4

Indole-3-Acetic Acid (IAA) and Gibberellic acid (GA₃) production by the selected bacterial antagonists. Values followed by a different letter are significantly different (P ≤ 0.01). Error bars represent standard deviations.

Figure 5

ACC (1-aminocyclopropane-1-carboxylic acid) deaminase activity of the selected bacterial antagonists. Values followed by a different letter are significantly different (P ≤ 0.01). Error bars represent standard deviations.

Figure 6

Plant growth parameters of castor in different treatment combinations (PGPT₁: Normal soil, PGPT₂: Cow dung, PGPT₃: LRP-2, PGPT₄: LRP-2 + UR-6, PGPT₅: LRP-2 + UR-6 + 50% of recommended NPK). A - Shoot length (m), B - Leaves per plant (nos.), C - Leaf biomass (g). Values followed by a different letter are significantly different (P ≤ 0.01). Error bars represent standard deviations.