Plant growth–promoting traits of culturable seed microbiome of citrus species from Purvanchal Himalaya

Despite Northeastern India being “Treasure House of Citrus Genetic Wealth,” genetic erosion of citrus diversity poses severe concern with a corresponding loss in seed microbial diversity. The seed microbiome of citrus species unique to the Purvanchal Himalaya is seldom explored for their use in sustainable orchard management. Isolation and characterization of culturable seed microbiomes of eight citrus species, namely, Citrus reticulata Blanco, C. grandis (L.) Osbeck, C. latipes Tanaka, C. megaloxycarpa Lushaigton, C. jambhiri Lush, C. sinensis (L.) Osbeck, C. macroptera Montr, and C. indica Tanaka collected from NE India were carried out. The isolates were then screened for an array of plant growth–promoting (PGP) traits [indole acetic acid (IAA) production, N2 fixation, phosphate and zinc complex dissolution, siderophores, and Hydrogen Cyanide (HCN) production]. The pure culture isolates of seed microbiomes were capable of dissolving insoluble Ca3(PO4)2 (1.31–4.84 µg Pi ml-1 h-1), Zn3(PO4)2 (2.44–3.16 µg Pi ml-1 h-1), AlPO4 (1.74–3.61 µg Pi ml-1 h-1), and FePO4 (1.54–4.61µg Pi ml-1 h-1), mineralized phytate (12.17–18.00 µg Pi ml-1 h-1) and produced IAA-like substances (4.8–187.29 µg ml-1 h-1). A few isolates of the seed microbiome were also able to fix nitrogen, secrete siderophore-like compounds and HCN, and dissolve ZnSO4 and ZnO. The 16S ribosomal Ribonucleic Acid (rRNA)–based taxonomic findings revealed that Bacillus was the most dominant genus among the isolates across citrus species. Isolates CG2-1, CME6-1, CME6-4, CME6-5, CME6-9, CJ7-1, CMA10-1, CI11-3, and CI11-4 were identified as promising bioinoculants for development of microbial consortium having multifaceted PGP traits for nutritional benefits of nitrogen, phosphorus and zinc, and IAA hormonal benefits to citrus crops for better fitness in acid soils.