Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Biology

Supervisor

Szczyglowski, Krzysztof

2nd Supervisor

Huner, Norman P.A.

Co-Supervisor

Abstract

The ability to host nitrogen-fixing bacteria, which convert atmospheric di-nitrogen to ammonia, inside root cells is a special adaptation of most leguminous and only some non-leguminous plants. It engenders the development of new, root-derived organs, called nodules. Significant research effort has been dedicated to understanding how these ultimate nitrogen-fixing plant organs are built. Nonetheless, important gaps with respect to pertinent knowledge still remain.

My thesis work has focused on deciphering the role of the SHORT INTERNODES/STYLISH (SHI/STY) transcription factor gene family during nodule formation in Lotus japonicus, a model legume plant. I show here that the SHI/STY gene family comprises at least nine members, called STY1 to STY9. The RNAseq, qRT-PCR and promoter-GUS localization experiments revealed that expression of all nine STY genes associates with nodule development which in most cases required NF-YA1, encoding a nodule-specific subunit of a heterotrimeric transcription factor. The activity of the STY promoters was associated with nodule primordia and with the vasculature and vascular parenchyma in fully mature nodules. Mutant analysis and a dominant negative approach were used to assess the functional relevance of the STY genes during nodule formation. Based on the outcome of experiments with the STY3-SRDX presumed dominant repressor, STY genes are shown to be required for both, symbiotic infection and nodule formation in L. japonicus. Using in silico analysis and a candidate gene approach, YUCCA1 and YUCCA11 genes, likely involved in regulation of auxin biosynthesis rates, were identified as potential targets of the STY-dependent regulation. My data suggest that L. japonicus STYs participate, in a highly redundant manner, in a cascade of transcriptional reprograming that is initiated with the perception of bacterially-encoded nodulation factors by the host roots and leads to local auxin signalling which is required for the differentiation of a fully functional, nitrogen-fixing nodule. The summative findings of my thesis work contribute toward better understanding of the requisite mechanisms for nitrogen-fixing symbiosis, with refinement on the signal transduction pathway that is generated during root nodule organogenesis.

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