Year of Publication

2015

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Agriculture, Food and Environment

Department

Plant and Soil Sciences

First Advisor

Dr. Hongyan Zhu

Abstract

Legumes form symbiotic and pathogenic interactions with microbes. Understanding the molecular mechanisms underlying the legume-microbe interactions would help us to improve crop production in a sustainable manner. This thesis covers two independent research projects. The first project was to study the role of alternative splicing in RCT1-mediated disease resistance. RCT1 is a TIR-NBS-LRR-type plant resistance (R) gene in Medicago truncatula that confers broad-spectrum resistance to Colletotrichum trifolii, a fungal pathogen that causes anthracnose disease in Medicago. RCT1 undergoes alternative splicing at both coding and 3'-untranslated regions, thereby producing multiple transcript variants in its expression profile. Alternative splicing of RCT1 in the coding region results from the retention of intron 4. The transcript with retention of intron 4 is predicted to encode a truncated protein lacking the C-terminal domain of the full-length protein. We showed that the RCT1 function requires the combined presence of the regular and alternative transcripts. This study, in addition to the reports on the tobacco N and Arabidopsis RPS4 genes, adds another significant example showing the involvement of alternative splicing in R gene-mediated plant immunity. The second project was to study the symbiotic specificity in the soybean-rhizobial interaction. It is well known that legume plants can make their own nitrogen fertilizer by forming a root nodule symbiosis with nitrogen-fixing soil bacteria, called rhizobia. One remarkable property of this symbiosis is its high level of specificity, which occurs at both inter- and intra-species levels and takes place at multiple phases of the interaction, ranging from initial bacterial infection and nodulation to late nodule development associated with nitrogen fixation. In this study, we performed fine mapping of the Rj4 gene that controls nodulation specificity in soybean. The Rj4 allele prevents the host plant from nodulation with many strains of Bradyrhizobiumelkanii, which are frequently present in soils of the southeastern USA. Since B. elkanii strains are poor symbiotic partners of soybean, cultivars containing an Rj4 allele are considered favorable. We have delimited the Rj4 locus within a 47-kb genomic region on soybean chromosome 1 and identified the candidate genes. We are in the process to validate the candidate genes.

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