Plant growth promoting actinobacteria from rhizosphere soils of Black pepper in Wayanad

Abstract

Black pepper (Piper nigrum L.) known as the ‘King of spices’ is one of the important export-oriented spice crops that provides major source of income and employment for rural households in Kerala. Wayanad is one of the main pepper growing tracts in Kerala. However, the production of black pepper in Wayanad has been declining over the past many years, mainly due to the poor soil health status, improper land management and changes in climatic factors. An increasing demand for low-input agriculture has resulted in a greater interest in soil microorganisms which are able to enhance plant health and soil quality. However, actinobacteria have not yet been exploited for this purpose. Hence the present study focused on isolation, screening and characterization of actinobacteria from rhizosphere soils of black pepper in Wayanad and evaluation of their plant growth promoting activity. Rhizosphere soil samples were collected from five different locations of flood affected and non-flood affected black pepper growing areas of Wayanad district. Enumeration of actinobacteria on three different media revealed that the population of actinobacteria ranged from 1.00 x104 cfu to 12.00 x104 cfu g -1 soil in non-flooded soil. Actinobacteria could not be detected in flooded soils, even at a dilution of 10-1 , except in Meppadi. Among the different media, starch casein agar recorded higher population of actinobacteria. A total of 35 isolates were purified and maintained for characterization and screening for plant growth promoting activities. Cultural, morphological and biochemical characters of all 35 isolates of actinobacteria were studied. All the isolates were Gram positive, and they varied in the colony morphology, colour of aerial mycelium, spore chain morphology and pigmentation. Two isolates Vlt-K and Plp-B produced yellow water-soluble pigment and Vlt-I produce purple water insoluble pigment. Screening of all the 35 isolates under in vitro conditions for plant growth promoting (PGP) activities revealed that the isolates were highly variable. Two isolates (Ptd-A and Amb-C) were superior to other isolates in IAA production. Significantly higher N-fixation was noticed in four isolates (Ptd-A, Ptd-E, Ptd-B and Ptr-A). Six isolates solubilized insoluble phosphate to available P, and also reduced the pH of the medium. Reduction in pH by phosphate solubilizers has been reported earlier and this is attributed to the production of organic and inorganic acids. None of the isolates solubilized K or Zn. Three isolates with multiple PGP activities were selected, based on ranking for PGP activities in vitro and identified by 16S rRNA gene sequencing (Ptd-A and Ptr-A as Streptomyces sp. and Ptd-E as Actinobacteria bacterium). These actinobacterial isolates evaluated in-planta for PGP activities with black pepper cuttings (variety Panniyur-1). Treatments also included PGPR mix-1 (KAU commercial formulation) and Organic Package of Practices Recommendations (2017). All the three native isolates exhibited significant increase in shoot length, number of leaves and internode length throughout the growth period from planting to five months. Significantly higher root growth was observed in treatment T2: Ptd-A (Streptomyces sp.), with significantly higher root volume, fresh and dry root weight. Significantly higher plant fresh weight was observed in T2: Ptd-A (Streptomyces sp.) and significantly higher plant dry weight was observed in T2: Ptd-A (Streptomyces sp.) and T3: Ptr-A (Streptomyces sp). The present investigation revealed that native actinobacteria have the potential to improve the growth of black pepper. These isolates may further be evaluated under field conditions before commercialization. Screening for other beneficial traits like disease suppression and drought tolerance may also be carried out. The compatibility of the actinobacteria with other PGP microorganisms may also be evaluated. Actinobacterial spores survive in soil for longer periods and this may be an added advantage. Actinobacterial diversity and the biochemical molecules produced by actinobacteria could be exploited to build up sustainable and eco-friendly agriculture.