Online

Chakraborty, Sanhita

Plant Biology

2018

Ph. D.

Legumes contribute significantly to sustainable agriculture because of their high protein content. This intrinsic nitrogen is the product of a mutualistic association between their roots, and a group of soil bacteria, collectively known as rhizobia. The rhizobium-legume symbiosis is a complex developmental process that involves rhizobial entry and differentiation, coupled in space and time, with the development of a root organ (the nodule) and is subject to modulation by environmental factors. Salt stress is a strong negative regulator of rhizobium-legume symbiosis, but despite its agricultural impact, the mechanism of salt regulation of rhizobium-legume symbiosis is poorly understood. This dissertation addresses this issue with focus on early rhizobium-legume signaling and maturation of nodules, using the model legume Medicago truncatula. The rhizobium-legume symbiosis is initiated with a molecular dialogue between the symbiotic partners, followed by the entry of rhizobia inside the host. These processes are under tight genetic regulation and involve the symbiotic induction of several host genes. I used a candidate gene-approach to locate the temporal intersection of salt and rhizobium-legume signaling and found that the ionic component of salinity hyperinduces Early Nodulin 11 (ENOD11) and ERF Required for Nodulation 1 (ERN1). I also found that the hyperinduction of ENOD11 requires two symbiotic rhizobial signals- Nod factors (NF) and succinoglycan and the host transcription factor Nodulation Signaling Pathway 2 (NSP2). In order to explore the possibility of an extensive transcriptional re-programming during rhizobium-legume signaling in the presence of salinity, I profiled the host root transcriptome and discovered strong transcriptional upregulation by salt, of several host genes associated with NF perception, rhizobial infection and ubiquitination. Curiously, I found that the hyperinduction of these genes correlate with an inhibition of rhizobial entry into the host. These findings highlight that early rhizobium-legume signaling and rhizobial colonization of the host are under tight transcriptional regulation of the host. Despite the early inhibition of infection, few nodules develop under salt stress. However, these nodules are morphologically abnormal, indicating that salinity must interfere with nodule maturation, a prerequisite for nitrogen fixation. Consistent with the theme of early symbiotic signaling, I found host genes involved during the early stages of nodule differentiation to remain highly expressed in nodules that developed under salt stress, suggesting a delay in maturation. This inference was further corroborated when I found that genes associated with the later stages of nodule development and nitrogen fixation showed decreased expression in the presence of salt. Additionally, I observed that this developmental shift of nodules under salinity was associated with differential accumulation of osmolytes, such as, proline betaine and homostachydrine in the salt-stressed nodules. Together, my work provides a mechanistic understanding of the intersection between nodulation and salt signaling, a question of major agricultural impact.