The root hair ‘infectome’ of Medicago truncatula: secrets of hair removal

Nutrient exchange between the model legume Medicago truncatula and its nitrogen fixing symbiont Sinorhizobium meliloti occurs in indeterminate nodules initiated in the root cortex.  Plant derived infection threads provide the microbe with safe passage between the site of entry at the curled root hair and the nodule primordium. We have used microarray analysis of isolated root hairs to establish the transcriptome at key developmental milestones of early infection.  In addition, we profiled the hyperinfected sickle mutant and three transcription factor mutants with defects in infection thread formation (nin, ern1 and nf-ya1). Our data revealed suites of hormone related genes associated with infection thread initiation and progression. Promoter GUS fusions of three upregulated auxin responsive genes demonstrated expression at sites of infection and in nodule primordia. One of these, the orthologue of Arabidopsis Auxin Response Factor 16 was shown to be required for maximal rhizobial infection confirming a role for auxin in this process. Profiling of the transcription factor mutants highlighted NIN as a key player, required for the upregulation of approximately one quarter of all infection associated genes. The assembly of a gene regulatory network indicates overlap between the NIN and NF-YA1 regulons while ERN1 appears to function independently.