An AM‐induced, MYB‐family gene of Lotus japonicus (LjMAMI) affects root growth in an AM‐independent manner

Summary The interaction between legumes and arbuscular mycorrhizal ( AM ) fungi is vital to the development of sustainable plant production systems. Here, we focus on a putative MYB ‐like ( L j MAMI ) transcription factor ( TF ) previously reported to be highly upregulated in Lotus japonicus mycorrhizal roots. Phylogenetic analyses revealed that the protein is related to a group of TF s involved in phosphate ( P i) starvation responses, the expression of which is independent of the P i level, such as PHR 1. GUS transformed plants and quantitative reverse transcription PCR revealed strong gene induction in arbusculated cells, as well as the presence of L j MAMI transcripts in lateral root primordia and root meristems, even in the absence of the fungus, and independently of Pi concentration. In agreement with its putative identification as a TF, an eGFP ‐ L j MAMI chimera was localized to the nuclei of plant protoplasts, whereas in transgenic Lotus roots expressing the eGFP ‐ L j MAMI fusion protein under the control of the native promoter, the protein was located in the nuclei of the arbusculated cells. Further expression analyses revealed a correlation between L j MAMI and L j PT 4 , a marker gene for mycorrhizal function. To elucidate the role of the L j MAMI gene in the mycorrhizal process, RNA i and overexpressing root lines were generated. All the lines retained their symbiotic capacity; however, RNA i root lines and composite plants showed an important reduction in root elongation and branching in the absence of the symbiont. The results support the involvement of the AM ‐responsive L j MAMI in non‐symbiotic functions: i.e. root growth.