The ToxA–Tsn1 system is an example of an inverse gene-for-gene relationship. The gene ToxA encodes a host-selective toxin (HST) which functions as a necrotrophic effector and is often responsible for the virulence of the pathogen. The genomes of several fungal pathogens (e.g., Pyrenophora tritici-repentis, Parastagonospora nodorum, and Bipolaris sorokiniana) have been shown to carry the ToxA gene. Tsn1 is a sensitivity gene in the host, whose presence generally helps a ToxA-positive pathogen to cause spot blotch in wheat. Cultivars lacking Tsn1 are generally resistant to spot blotch; this resistance is attributed to a number of other known genes which impart resistance in the absence of Tsn1. In the present study, 110 isolates of B. sorokiniana strains, collected from the ME5A and ME4C megaenvironments of India, were screened for the presence of the ToxA gene; 77 (70%) were found to be ToxA positive. Similarly, 220 Indian wheat cultivars were screened for the presence of the Tsn1 gene; 81 (36.8%) were found to be Tsn1 positive. When 20 wheat cultivars (11 with Tsn1 and 9 with tsn1) were inoculated with ToxA-positive isolates, seedlings of only those carrying the Tsn1 allele (not tsn1) developed necrotic spots surrounded by a chlorotic halo. No such distinction between Tsn1 and tsn1 carriers was observed when adult plants were inoculated. This study suggests that the absence of Tsn1 facilitated resistance against spot blotch of wheat. Therefore, the selection of wheat genotypes for the absence of the Tsn1 allele can improve resistance to spot blotch.
A halotolerant rhizobacteria, Klebsiella species (referred to MBE02), was identified that had a growth stimulation effect on peanut. To gain mechanistic insights into how molecular components were reprogrammed during the interaction of MBE02 and peanut roots, we performed deep RNAsequencing. In total, 1260 genes were differentially expressed: 979 genes were up-regulated, whereas 281 were down-regulated by MBE02 treatment as compared to uninoculated controls. A large component of the differentially regulated genes were related to phytohormone signalling. This included activation of a significant proportion of genes involved in jasmonic acid, ethylene and pathogendefense signalling, which indicated a role of MBE02 in modulating plant immunity. In vivo and in vitro pathogenesis assays demonstrated that MBE02 treatment indeed provide fitness benefits to peanut against Aspergillus infection under controlled as well as field environment. Further, MBE02 directly reduced the growth of a wide range of fungal pathogens including Aspergillus. We also identified possible molecular components involved in rhizobacteria-mediated plant protection. Our results show the potential of MBE02 as a biocontrol agent in preventing infection against several fungal phytopathogens. Roots not only facilitate plants to uptake minerals and water from the soil but also serve as a major source of nutrients for microorganisms residing in the rhizosphere 1-3. Root exudates are rich in organic compounds, which attract microbial population to colonize the plant roots 4. Most of these microbes are neutral; however, many are pathogenic by nature and negatively affect the plant growth 1,5. On the other hand, there are few microorganisms such as, mycorrhizal fungi, rhizobium and plant growth promoting rhizobacteria (PGPR), which produce beneficiary changes and stimulate plant growth via direct or indirect mechanisms 3,4,6-8. One of these beneficiary changes include improved root architecture by increasing the number of root hairs and lateral root numbers. This may be correlated with the signalling of phytohormones, mainly auxin 2. This enlargement in root surface facilitates plants to uptake more water and nutrient, and in turn leads to improved growth. In addition, colonization of plant roots by few PGPR strains trigger an induced systemic resistance (ISR) response that functions systemically throughout the plant and is often effective against a broad spectrum of plant pathogens 5,9,10. In contrast to the costly defenses that often, upon activation, restricts the plant growth, ISR is associated with a defence priming phenomenon, which simultaneously stimulates plant growth and immunity 5,6,11,12. These beneficial microbes could be an ideal choice for developing them into biocontrol agents that enhances disease resistance in crop plants. Over the past few years, our understandings about molecular mechanisms of PGPR mediated growth promotion and disease resistance in plants has improved significantly as several genetic determinants that regulate these pr...
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