This could be caused by: (1) the different models and approaches used, including the expression of hmp from different promoters (i.e. a nodule-specific promoter in our work vs the strong and constitutive p35S promoter in Shimoda's work); (2) a putative specific role played by the class I Hb to promote the initial infection process; and/or (3) the need for a set concentration range of NO for successful establishment of the symbiotic relationship. Here, IMMT is linked to infection.