Transcriptional Response to Nod Signaling

Abstract

Nitrogen fixation occurs in the root nodules of legumes that form as part of a symbiotic process with rhizobial bacteria. Although the transcriptional regulators of the GRAS family were identified as important for mediating the plant response to the bacterial Nod factor that induces nodule formation, these GRAS proteins may not directly bind DNA. Middleton et al . identify ERN, a member of the ERF subfamily of transcription factors with the AP2 domain that bind DNA, as a participant in Nod signaling in Medicago truncatula . The authors screened plants deficient in nodule formation and identified two alleles called bit1-1 and bit1-2 . Plants carrying either of these alleles not only failed to form nodules in response to rhizobial infection but also failed to form nodules in response to a constitutively active form of calcium- and calmodulin-dependent protein kinase (CCaMK) that causes spontaneous nodule formation. The bit1-1 plants were also defective in activation of a Nod factor reporter gene. Genetic mapping of these alleles revealed that they encoded point mutations in the DNA binding domain ( bit1-2 ) or deletions ( bit1-1 ) in a gene that encodes a protein with sequence similarity to ERF transcription factors. They named this protein ERN for ERF Required for Nodulation. Rhizobial inoculation stimulated the expression of ERN , and this required a functional Nod signaling response. Consistent with a role in transcriptional response to Nod signaling, microarray analysis of plants with the bit1-1 allele showed decreased transcriptional responsiveness to rhizobial infection. Whether ERN interacts with GRAS transcriptional regulators and whether ERN binds to the promoters of Nod-regulated genes remains to be determined. P. H. Middleton, J. Jakab, R. V. Penmetsa, C. G. Starker, J. Doll, P. Kaló, R. Prabhu, J. F. Marsh, R. M. Mitra, A. Kereszt, B. Dudas, K. VandenBosch, S. R. Long, D. R. Cook, G. B. Kiss, G. E. D. Oldroyd, An ERF transcription factor in Medicago truncatula that is essential for Nod factor signal transduction. Plant Cell 19 , 1221-1234 (2007). [Abstract] [Full Text]