Plant Growth Promoting Actinobacteria from Rhizosphere Soils of Black Pepper in Wayanad

Abstract

Actinobacteria isolated from the rhizosphere soils of black pepper comprising both flood affected and non-flood affected areas of Wayanad district. Among different soil samples analysed, Puttad (Ptd) recorded significantly superior actinobacterial population on starch casein aga, Kenknight & Munaier’s agar and actinomycetes isolation agar. Actinobacterial colonies could not be detected in any of the flooded soil samples on any medium, even at a dilution of 10_^{^-1}, except in Meppadi soil, which recorded a low population of 0.3 x 10_^¹ cfu g_^{^-1} soil. Starch casein agar is best media to isolate actinobacteria from soil samples compared to other two media. The cultural, morphological and biochemical characterization of thirty-five isolates was carried out. Further the isolates were evaluated for their plant growth promoting traits such as IAA production, nitrogen fixation, P, K and Zn solubilization. The isolates Ptd-A and Amb-C were found to be significantly superior to all other isolates, with IAA production of 15.9 g ml_^{^-1} and 15.38 g ml_^{^-1} respectively. The four isolates <i>viz</i>. Ptd-A, Ptd-E, Ptd-B and Ptr-A recorded significantly superior nitrogen fixation and the phosphate solubilized was significantly higher in Ptd-E, Ptd-D, Ptr-E, Ptd-A and Ptr-A, as compared to other isolates. All isolates were negative to K and Zn solubilization. Based on <i>in vitro </i>evaluations, three isolates were shortlisted (Ptd-A, Ptd-E and Ptr-A) and subjected to <i>in vivo </i>evaluation for growth promotion in black pepper (variety Panniyur 1). Rooted plants of black pepper were raised in sterile potting mixture. Bioinoculants applied at the time of planting and 45 days after planting. The PGPR Mix-1 and Organic Package of Practices Recommendations (2017) were used for comparison with the microbial inoculants along with control. In the <i>in-planta</i> experiment, biometric characters were recorded at monthly intervals, up to five months. The actinobacterial treatment, T1: Ptd-E, T2: Ptd-A and T3: Ptr-A showed significant increase in shoot length, number of leaves and internode length throughout the growth period from planting to five MAP. Significantly higher root growth was observed in treatment T2: Ptd-A, with significantly higher root volume, fresh and root weight. The potential actinobacteria were identified Ptd-A and Ptr-A as <i>Streptomyces </i>sp. and Ptd-A as <i>Actinobacteria bacterium</i> using 16S r RNA gene sequencing.