The synergistic relationship between bHLHm1 and the ammonium transporter AmF1 across the rhizobia and arbuscular mychorizzal fungi symbioses in legumes

Kaiser BN1, Ovchinnikova E1, Wen Z1, Chiasson D2 and Wu Y3

  1. School of Life and Environmental Sciences, The University of Sydney, 380 Werombi Road, Brownlow Hill, NSW.
  2. Ludwig-Maximilians-Universitat, Grobhaderner Str. 2-4 82152 Martinsried Germany.
  3. School of Agriculture Food and Wine, The University of Adelaide, Urrbrae, SA.

The regulation of ammonium transport in plants is traditionally aligned with the activities of AMT1 and AMT2 transport pathways. Previously we characterised the functional relationship between a soybean nodule transcription factor (bHLHm1) and a class of transport protein (AMF1) which permits the transport of ammonium when expressed in yeast and Xenopus laevis oocytes. Both proteins are required for an effective nodule symbiosis, where loss of activity disrupts nodule development and activity. We have further explored the post transcriptional relationship between bHLHm1 and AMF1 homologs in soybean and have confirmed using EMSA analysis and in planta transactivation assays that GmbHLHm1 recognises the promoter sequences of multiple AMF1homologs in soybean. In parallel studies, we have examined the relationship between bHLHm1 and AMF1 orthologs in Medicago truncatula. Both genes are induced upon arbuscular mychorrizal colonisation and the encoded proteins co-located within arbuscular containing root cortical cells. Loss of function of either bhlhm1 or amf1;3 in Medicago roots disrupts AM colonisation and alters the selected expression of other known AMT2 genes linked to AM mediated ammonium transport in arbuscular containing cortical cells.