A Medicago truncatula Cystathionine-β-Synthase-like Domain-Containing Protein Is Required for Rhizobial Infection and Symbiotic Nitrogen Fixation

Abstract

The symbiosis between leguminous plants and soil rhizobia culminates in the formation of nitrogen-fixing organs called nodules that support plant growth. Two Medicago truncatula Tnt1-insertion mutants were identified that produced small nodules, which were unable to fix nitrogen effectively due to ineffective rhizobial colonization. The gene underlying this phenotype was found to encode a protein containing a putative membrane-localized domain of unknown function (DUF21) and a cystathionine-b-synthase domain. The cbs1 mutants had defective infection threads that were sometimes devoid of rhizobia and formed small nodules with greatly reduced numbers of symbiosomes. We studied the expression of the gene, designated M. truncatula Cystathionine-b-Synthase-like1 (MtCBS1), using a promoter-b-glucuronidase gene fusion, which revealed expression in infected root hair cells, developing nodules, and in the invasion zone of mature nodules. An MtCBS1-GFP fusion protein localized itself to the infection thread and symbiosomes. Nodulation factor-induced Ca 2+ responses were observed in the cbs1 mutant, indicating that MtCBS1 acts downstream of nodulation factor signaling. MtCBS1 expression occurred exclusively during Medicago-rhizobium symbiosis. Induction of MtCBS1 expression during symbiosis was found to be dependent on Nodule Inception (NIN), a key transcription factor that controls both rhizobial infection and nodule organogenesis. Interestingly, the closest homolog of MtCBS1, MtCBS2, was specifically induced in mycorrhizal roots, suggesting common infection mechanisms in nodulation and mycorrhization. Related proteins in Arabidopsis have been implicated in cell wall maturation, suggesting a potential role for CBS1 in the formation of the infection thread wall.

The Nodule-Specific PLAT Domain Protein NPD1 Is Required for Nitrogen-Fixing Symbiosis

“…Nodule development and symbiotic metabolism are complex processes that involve the coordinated expression of thousands of plant and bacterial genes. Development of genetic and genomic resources for model legumes such as M. truncatula and Lotus japonicus has facilitated the discovery of some of the genes that are essential to these processes (Benedito et al, 2008;Sato et al, 2008;Tadege et al, 2008;Young et al, 2011;Couzigou et al, 2012;Fukai et al, 2012;Kryvoruchko et al, 2016;Sinharoy et al, 2016). Having elucidated much of the NF signaling pathway and many of the early steps preceding symbiosome release into host cells (Oldroyd et al, 2011), interest has shifted toward later aspects of symbiosis, including bacteroid differentiation, maintenance, senescence, and nitrogen fixation efficiency (Bourcy et al, 2013;Sinharoy et al, 2013;Berrabah et al, 2014a).…”

A snapshot of functional genetic studies inMedicago truncatula

“…cbs1 plants have more micro-colonies but less propagating infection threads. CBS1 encodes for a putative membrane-localized domain of unknown function (DUF21) and a cystathionine-β-synthase domain, and is has been speculated that CBS1 participate in infection thread cell wall development (Sinharoy et al, 2016).…”

Plant Genetic Control over Infection Thread Development during Legume-Rhizobium Symbiosis

“…M. truncatula Cystathionine-β-synthase-like1 (MtCBS1) is also involved in the development of an IT [93]. Mutants in this gene formed inefficient small white nodules.…”

“…The protein MtCBS1 was localized in ITs and symbiosomes. Based on data on homologous proteins in A. thaliana, the function of MtCBS1 in the maturation of the IT wall was suggested [93].…”