Functional Genomics to Dissect Medicago Root Nodule Symbiosis

Date: Monday, July 13, 2015
Time: 4:10 PM
Room: Waterfront 3
Senjuti Sinharoy , The Samuel Roberts Noble Foundation, Ardmore, OK
Ivone Torres-Jerez , The Samuel Roberts Noble Foundation, Ardmore, OK
Yuhong Tang , The Samuel Roberts Noble Foundation, Ardmore, OK
Michael K. Udvardi , The Samuel Roberts Noble Foundation, Ardmore, OK
Symbiotic nitrogen fixation (SNF) injects 40-60 million tons of nitrogen into agricultural systems each year in an ecofriendly manner critical for sustainable agriculture1. Legumes can harbor a small group of nitrogen fixing bacteria called rhizobia in a specialized organ called the nodule. In response to signals produced by compatible soil rhizobia, plant cell division is triggered in root cortical cells, which generates a nodule primordium (pre-nodule) and, eventually, mature nitrogen-fixing nodules. Rhizobia are released, via endocytosis, into the cytoplasm of post meristematic cells of the nodule primordium. This results in bacteria surrounded by a plant membrane, which creates a novel organelle termed the symbiosome.

Root nodule development and cellular differentiation involve changes in the expression of thousands of plant genes2. Hundreds of transcription factor (TF) genes are expressed during nodule development2,3. We have taken a systematic, reverse-genetic approach to elucidate the role of uncharacterized M. truncatula TF genes in SNF. We have targeted several nodule-specific TFs using a tobacco (Nicotiana tabacum) retrotransposon (Tnt1) insertion mutant population of Medicago4. Knockout of a nodule specific Cysteine-2/Histidine-2 (C2H2) TF in M. truncatula resulted in a strong non-nitrogen fixing phenotype (Fix-). Functional characterization of this mutant indicated that the gene controls symbiosome development and hence the gene was named as REGULATOR OF SYMBIOSOME DIFFERENTIATION (RSD)5. Transcriptome analysis, of rsd mutant and wildtype nodules, using Affymetrix microarrays, revealed some of the gene regulatory networks behind the control of symbiosome development and helped us to identify target(s) of RSD activity. The phenotype of rsd nodules and the role of RSD in symbiosome development will be discussed in detail.

References

  1. Peoples, M.B., et al. (2009). The contributions of nitrogen-fixing crop legumes to the productivity of agricultural systems. Symbiosis 48: 1–17.
  2. Benedito, V.A., et al. (2008). A gene expression atlas of the model legume Medicago truncatula. Plant J. 55: 504–513.
  3. Young, N.D., et al. (2011). The Medicago genome provides insight into the evolution of rhizobial symbioses. Nature 480: 520–524.
  4. Tadege, M., et al. (2008). "Large-scale insertional mutagenesis using the Tnt1 retrotransposon in the model legume Medicago truncatula." The Plant Journal 54: 335-347.
  5. Sinharoy, S., et al., (2013) The C2H2 Transcription Factor REGULATOR OF SYMBIOSOME DIFFERENTIATION Represses Transcription of the Secretory Pathway Gene VAMP721a and Promotes Symbiosome Development in Medicago truncatula. 
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