Unravelling the synergistic interaction of Thrips tabaci and newly recorded, Thrips parvispinus with Alternaria porri (Ellis.) Cif., inciting onion purple blotch.

Onion purple blotch is the most indispensable foliar disease of crop and has become a major concern for farmers and research fraternity. An attempt to investigate the role of injury in parasitism by Alternaria porri indicated that disease incidence and severity enhance considerably with injury. Thrips injured plants inoculated with A. porri presented 100% incidence and 52-72% severity while mechanically injured plants inoculated with A. porri showed 60-70% incidence and 28-34% severity. The uninjured plants showed considerably less disease incidence (30-40%) and severity (10-16%). Injured inoculated plants presented reduced leaf length and leaf area while the leaf diameter remained unaffected. The lesion number, lesion length and size was substantially enhanced with concomitant infestation of pest and pathogen. Thrips tabaci injury led to more pronounced symptoms of purple blotch compared to Thrips parvispinus injury. There was substantial decrease in photosynthetic rate and chlorophyll content with stress imposed on plant whilst the relative stress injury was enhanced. The induction of injury and inoculation of A. porri had an impact on the concentration of total phenolics, total soluble sugars, total proteins and hydrogen peroxide in onion leaves. A. porri combined with injury caused a more pronounced decrease in total soluble sugars and total protein content while enhancement in total phenolics and hydrogen peroxide content compared to uninjured plants. The dynamic nature of morpho-physiological and biochemical changes owing to stress conditions imposed on onion plant adds an extra layer of complexity in understanding the onion plant physiology and their ability to work out in response to challenging environment conditions.

Comparative Transcriptome Analysis of Onion in Response to Infection by Alternaria porri (Ellis) Cifferi.

Purple blotch (PB) is one of the most destructive foliar diseases of onion and other alliums, caused by a necrotrophic fungal pathogen Alternaria porri. There are no reports on the molecular response of onion to PB infection. To elucidate the response of onion to A. porri infection, we consequently carried out an RNAseq analysis of the resistant (Arka Kalyan; AK) and susceptible (Agrifound rose; AFR) genotype after an artificial infection. Through differential expression analyses between control and pathogen-treated plants, we identified 8,064 upregulated and 248 downregulated genes in AFR, while 832 upregulated and 564 downregulated genes were identified in AK. A further significant reprogramming in the gene expression profile was also demonstrated by a functional annotation analysis. Gene ontology (GO) terms, which are particularly involved in defense responses and signaling, are overrepresented in current analyses such as "oxidoreductase activity," "chitin catabolic processes," and "defense response." Several key plant defense genes were differentially expressed on A. porri infection, which includes pathogenesis-related (PR) proteins, receptor-like kinases, phytohormone signaling, cell-wall integrity, cytochrome P450 monooxygenases, and transcription factors. Some of the genes were exclusively overexpressed in resistant genotype, namely, GABA transporter1, ankyrin repeat domain-containing protein, xyloglucan endotransglucosylase/hydrolase, and PR-5 (thaumatin-like). Antioxidant enzyme activities were observed to be increased after infection in both genotypes but higher activity was found in the resistant genotype, AK. This is the first report of transcriptome profiling in onion in response to PB infection and will serve as a resource for future studies to elucidate the molecular mechanism of onion-A. porri interaction and to improve PB resistance in onions.

Electron Beam Irradiated Chitosan elicits enhanced antioxidant properties combating resistance to Purple Blotch Disease (Alternaria porri) in Onion (Allium cepa).

PURPOSE: This study was carried out to assess the effect of irradiated chitosan as an elicitor on the biochemical traits associated with resistance to purple blotch disease in onion. MATERIALS AND METHODS: Chitosan was electron beam irradiated at 100 kGy dose to obtain low molecular weight chitosan. Irradiated chitosan at 20 and 0.04% concentration and different time intervals was used as a biological elicitor cum antimicrobial agent against purple blotch disease in onion. Field grown onion (Variety Basanvant 780) plants were foliar sprayed with irradiated chitosan and the biochemical responses were monitored using parameters namely chlorophylls, carotenoids, antioxidant enzymes, phenols, and antifungal enzyme ß-1,3 Glucanase using standard methods. RESULTS: Compared to control treatment, a positive correlation with irradiated chitosan treatment was observed for an increase in ß-1,3-glucanase, peroxidase activity, and contents of total phenolics, chlorophylls, and carotenoids, which cumulatively contributed to resistance response against the purple blotch disease. Irradiated chitosan (0.04%) treated onion plants at 30, 45, and 60 DAT showed a higher total phenolics, ß-1,3-glucanase activity, and peroxidase activity besides enhanced antioxidant properties. CONCLUSION: The results suggest that irradiated chitosan has elicited resistance responses against purple blotch disease in onion. The increased production of antioxidant metabolites may provide value addition to onion as a food commodity.

Identification of Novel Source of Resistance and Differential Response of Allium Genotypes to Purple Blotch Pathogen, Alternaria porri (Ellis) Ciferri.

Purple blotch, caused by Alternaria porri (Ellis) Cifferi, is a serious disease incurring heavy yield losses in the bulb and seed crop of onion and garlic worldwide. There is an immediate need for identification of effective resistance sources for use in host resistance breeding. A total of 43 Allium genotypes were screened for purple blotch resistance under field conditions. Allium cepa accession 'CBT-Ac77' and cultivar 'Arka Kalyan' were observed to be highly resistant. In vitro inoculation of a selected set of genotypes with A. porri, revealed that 7 days after inoculation was suitable to observe the disease severity. In vitro screening of 43 genotypes for resistance to A. porri revealed two resistant lines. An additional 14 genotypes showed consistent moderate resistance in the field as well as in vitro evaluations. Among the related Allium species, A. schoenoprasum and A. roylei showed the least disease index and can be used for interspecific hybridization with cultivated onion. Differential reaction analysis of three A. porri isolates (Apo-Chiplima, Apn-Nasik, Apg-Guntur) in 43 genotypes revealed significant variation among the evaluated Allium species (P = 0.001). All together, the present study suggest that, the newly identified resistance sources can be used as potential donors for ongoing purple blotch resistance breeding program in India.