Phytohormone signaling and its regulation in bread wheat (Triticum aestivum L.) during pathogenesis

Kundu, Pritha (2019) Phytohormone signaling and its regulation in bread wheat (Triticum aestivum L.) during pathogenesis. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Pritha Kundu (14RS065)) 14RS065.pdf - Submitted Version Restricted to Repository staff only Download (13MB)

Abstract

Bread wheat (Triticum aestivum L.) is the second most important food crop in the developing countries like India after rice. The crop, however persistently faces a plethora of biotic and abiotic stresses that severely reduces its gross annual production. Wheat being favorably grown in the warmer part of the world is often attacked by several deadly fungal pathogens that include spot blotch, stem rust and blast disease. While the role of phytohormones in defending fungal pathogens is prominent, very little is known about the modulation of their signaling network in wheat. In this study, the functioning of the major phytohormones, like Salicylic Acid (SA), Jasmonic Acid (JA) and Ethylene (ET) have been demonstrated along with their regulators that fine-tune defense mechanism during the fungal attack in wheat. In the hemibiotrophic interactions that include spot blotch and blast, both SA and JA act synergistically with ET to provide resistance against spot blotch, whereas ET signaling is attenuated in response to blast disease. However, during the biotrophic wheat-rust-rice interaction, SA acts synergistically with ET thereby promoting resistance, whereas JA signaling is attenuated in rice. In the spot blotch disease, biochemical characterization of the JA pathway through complementation and pharmacological blocking assays confirms its positive immune response. This was further validated by the loss of function study through VIGS of Lipoxygensase2 (LOX2) gene. Metabolite profiling of wheat post infection also supports the role of phenylpropanoid pathway in the disease resistance. To validate the role of the key transcription factors, WRKY33 and NPR1-3 in immunity, overexpression constructs were prepared. The protein involved in flowering time regulation and light signaling, GIGANTEA (GI), was also studied to unravel its role of susceptibility to spot blotch. Further, in order to elucidate the molecular mechanism that differentially regulates the phytohormone signaling genes in host and non-host, the black stem rust disease of wheat was studied. Homology mining of the major 15 defense related genes (12 genes of phytohormone signaling and 3 of phenylpropanoid pathway) was carried out in rice. It was followed by the comparative study of their transcript abundance to decipher the role of SA, JA, ET and EDS1/PAD4 pathways in host (wheat) and non-host (rice) resistance mechanism during stem rust infection. All the important molecular markers of SA, ET and EDS1/PAD4 pathways were elicited in the non-host but not in the host. Interestingly, JA pathway was upregulated in the susceptible wheat. Similarly, to characterize the blast disease, 105 wheat genotypes were screened against the Magnaporthe oryzae strain Ka. Comparative profiling of the key phytohormone signaling genes and the quantitative analysis of the secondary metabolites and SA were also carried out in the contrasting lines. In 2017 a major outbreak of wheat blast occurred in India that was further confirmed by several molecular and morphological analyses. Further, to understand the geographic origin of the recovered field pathogens and its relationship to other isolates whole genome sequencing was carried out. Therefore, deeper understanding of the molecular mechanisms underpinning the differential elicitation of defense-signaling may provide us with novel targets for better management of these intractable diseases in future.

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