Prospecting potential of endophytes for modulation of biosynthesis of therapeutic bioactive secondary metabolites and plant growth promotion of medicinal and aromatic plants.

Medicinal and aromatic plants possess pharmacological properties (antidiabetes, anticancer, antihypertension, anticardiovascular, antileprosy, etc.) because of their potential to synthesize a wide range of therapeutic bioactive secondary metabolites. The concentration of bioactive secondry metabolites depends on plant species, local environment, soil type and internal microbiome. The internal microbiome of medicinal plants plays the crucial role in the production of bioactive secondary metabolites, namely alkaloids, steroids, terpenoids, peptides, polyketones, flavonoids, quinols and phenols. In this review, the host specific secondry metabolites produced by endophytes, their therapeutic properties and host-endophytes interaction in relation to production of bioactive secondry metaboloites and the role of endophytes in enhancing the production of bioactive secondry metabolites is discussed. How biological nitrogen fixation, phosphorus solubilization, micronutrient uptake, phytohormone production, disease suppression, etc. can play a vital role in enhacing the plant growth and development.The role of endophytes in enhancing the plant growth and content of bioactive secondary metabolites in medicinal and aromatic plants in a sustainable mode is highlighted.

Black rot disease incited by Indian race 1 of Xanthomonas campestris pv. campestris in Brassica juncea L. cv. Pusa Bold in India.

Mustard (Brassica juncea L.) is an important oil seed crop in the Brassicaceae family. It is widely cultivated in India for its edible leaves, oil and medicinal properties. In January 2022, we noticed necrotic symptoms typical black rot disease on Brassica juncea (L.) cv. Pusa Bold grown in Indian Agricultural Research Institute, India. Initially, chlorotic lesions emerged on the leaf margin, which progressed to angular V-shaped necrotic lesions and blackened veins. Disease progression became a necrotic appearance in the leaf results browning and papery leaf texture appeared. The suspected causal agent was isolated from three different diseased plants of Pusa Bold on nutrient sucrose agar medium that formed pale yellow, mucoid, and fluidal colonies. Three representative isolates originated from three different plants were sub-cultured on YGCA medium. These isolates are Gram-negative, oxidase negative, KOH positive, nonfluorescent on King's Medium B agar, and positive for starch hydrolysis test (Schaad and White 1974). The 16S ribosomal RNA gene and avirulence genes - AvrBs1 and AvrGf1 were amplified and sequenced in these three isolates with other Xanthomonas campestris pv. campestris (Xcc) isolates. The DNA sequence analysis revealed that these isolates are within the species of X. campestris. The race 1 specific marker namely xcc-b100_4389 was used to characterized the race by detection of 1090bp fragment respectively from gDNA of Xcc isolates (Rubel et al., 2017). The pathogenicity of these isolates was tested twice on youngest leaves of 30-day-old plants of Pusa Bold to convey Koch postulates. Inoculum of three isolates were prepared in nutrient broth at 28°C for 48-h. The pathogenicity test was conducted by small scissors dipped in a bacterial suspension (~ 108 cfu/ml) to cut leaf near margins at 10 points per leaf and the three youngest leaves per plant with three replications. The number of infected points per leaf and the severity of symptoms were assessed 15 and 30 days after inoculation (Singh et al., 2011; 2016). The chlorotic lesions with V-shaped symptoms were appeared on all inoculated plants after 15 and 30 dpi (days post-inoculation). The bacteria were reisolated from inoculated plants and has the same identity as original isolates by using 16S rRNA, avr genes and race 1 specific marker PCR, thereby confirming Koch's postulates. The bacterial inoculation was repeated and the same symptoms appear. Most of the crucifers are infected with black rot disease e.g., cauliflower, cabbage, Brussels, sprout etc. (Vicente et al., 2001). The nucleotide BLAST analysis of 16S rRNA, AvrBs1, AvrGf1 showed a 100% identity with different Xcc strains reported from Germany (B100; AM920689), Brazil (ATCC 33913; AE008922), India (Xcc-C7; CP077958), France (CFBP 5817; CM002673) and China (8004; CP000050) (Singh et al. 2022). Whilst, the nBLAST analysis of xcc-b100_4389 showed 100% nucleotide identity with Xcc race 1 (B100; AM920689), Germany. The sequences were deposited in GenBank (16S rRNA: OM839780; AvrBs1: OM994397; AvrGf1: OM994398; xcc-b100_4389: OM994399). The XccAK1 strain (ITCCBH_0014) was deposited in Indian Type Culture Collection, ICAR-IARI, New Delhi, India. Presently, it is a first report of necrotic black rot on B. juncea cv. Pusa Bold incited by Xcc race 1, India. Previous research reported the black rot disease on other species of the Brassica genus e.g., B. oleracea, and B. napus in Serbia (Popovic et al., 2013) and Argentina (Gaetan et al., 2005).

Assessing the efficacy of phyllospheric growth-promoting and antagonistic bacteria for management of black rot disease of cauliflower incited by Xanthomonas campestris pv. campestris.

The study aimed to assess the potential of phyllospheric bacterial strains isolated from cauliflower plants as biocontrol agents against black rot disease caused by Xanthomonas campestris pv. campestris, through both in vitro and in vivo evaluations. A total of 46 bacterial strains were isolated from healthy and infected cauliflower leaves of both resistant and susceptible plants, and evaluated them for various traits, including plant growth-promoting activities and in vitro antagonistic activity against Xanthomonas campestris pv. campestris. Further, a pot experiment was conducted with the susceptible cauliflower genotype (Pusa Sharad) and 10 selected phyllospheric bacterial isolates to assess their biocontrol efficacy against the disease. The results showed that 82.60% of phyllospheric bacterial isolates were positive for phosphate solubilization, 63.04% for ammonia production, 58.69% for HCN production, 36.95% for siderophore production, and 78.26% had the capacity to produce IAA. Out of the 46 isolates, 23 exhibited in vitro antagonistic activity against X. campestris pv. campestris and 10 isolates were selected for a pot experiment under glasshouse conditions based on their good plant growth-promoting activities and antagonistic assay. The results revealed that bacterial isolate CFLB-27 exhibited the highest biocontrol efficiency (65.41%), followed by CFLB-24 (58.30%), CFLB-31 (47.11%), and CFLB-26 (46.03%). These four isolates were identified as Pseudomonas fluorescens CFLB-27, Bacillus velezensis CFLB-24, Bacillus amyloliquefaciens CFLB-31, and Stenotrophomonas rhizophila CFLB-26. This study provides valuable insights into the potential of phyllospheric bacteria as an effective tool for disease management in sustainable agriculture.