ROLE OF PLANT MIRNAS IN DISEASE RESISTANCE

Abstract

Plants have evolved a strong innate immunity system to defend themselves against pathogen infection. The defense responses require a fine-tune reprogramming of gene expression and transcriptional regulation of genes coding for antimicrobial and signaling proteins, transcription factors and proteins associated with hormone-regulation and defense responses. The transcriptional regulation of protein coding genes is essential for a proper defense against pathogens, but evidences have also demonstrated the important role of small RNAs in post-transcriptional regulation. MicroRNAs (miRNAs) are small non-coding RNAs which are regulators of gene expression at post-transcriptional level. The regulatory mechanisms of miRNAs are based on degradation or translational repression of their target gene transcripts. The function of miRNAs in plant development processes has been widely studied, however, there is still a lack of information about the involvement of miRNAs in defense responses against pathogens._x000D_ This thesis comprises the study of miRNAs in plant innate immunity. The work has been developed in rice (Chapter I and Chapter II) and Arabidopsis (Chapter III). These plants are respectively monocotyledonous and dicotyledonous model systems for functional genomics studies. _x000D_ Chapter I describes the functional role of a polycistronic miRNA in the interaction with the pathogenic fungus Magnaporthe oryzae. This fungus is responsible of rice blast disease, one of the most devastating diseases worldwide. Previous identification of a new rice polycistronic miRNA (miR166k-166h) let us to investigate how it is regulated during the infection and which genes are under its regulation. The availability of a T-DNA insertion transgenic line has made possible to demonstrate that the transcriptional activation of this polycistronic miRNA confers resistance to M. oryzae, thus, acting as positive regulator of innate immunity. _x000D_ The Chapter II elucidates the importance of expression regulation of the miRNA biogenesis gene Dicer-like 1a (DCL1a) in resistance against M. oryzae. In this chapter it is demonstrated that overexpression of DCL1a triggers rice susceptibility to blast disease. The mechanisms by which DCL1a overexpression renders susceptibility are diverse, from a down-regulation of defense-related genes and diterpenoid phytoalexins genes to a misregulation of reactive oxygen species (ROS) and miRNA homeostasis._x000D_ In Chapter III Arabidopsis miR773 functional role in immunity has been described. This miRNA promotes the cleavage of methyltransferase2 (MET2) transcripts. When the action of miR773 by a target mimic mechanism is repressed, plants (MIM773) are more resistance to fungal infection (Plectosphaerella cucumerina, Fusarium oxysporum f. sp. conglutinans and Colletotrichum higginsianum). On the other hand, the miR773 overexpression and met2 knock-down mutation confers susceptibility to fungal infection._x000D_ Altogether, the results obtained during this PhD thesis demonstrate that miRNA and miRNA biogenesis genes are important components in the plant innate immunity responses to pathogens in rice and Arabidopsis. A better knowledge of miRNA function could be useful for the designing of new crop protection strategies.

Date of Award	2 Mar 2018
Original language	English
Supervisor	Blanca San Segundo de los Mozos (Director)