Repression of callus initiation by the mi RNA ‐directed interaction of auxin–cytokinin in Arabidopsis thaliana

Summary In tissue culture systems plant cells can be induced to regenerate to whole plants. A particularly striking example of cellular reprogramming is seen in this regeneration process, which typically begins with the induction of an intermediate cell mass referred to as callus. The identity of the key genetic cues associated with callus formation is still largely unknown. Here a micro RNA ‐directed phytohormonal interaction is described which represses callus initiation and formation in Arabidopsis thaliana . miR160 and ARF 10 ( At2g28350 ), a gene encoding an auxin response factor, were shown to exhibit a contrasting pattern of transcription during callus initiation from pericycle‐like cells. The callus initiation is faster and more prolific in a miR160 ‐resistant form of ARF 10 ( mARF 10 ), but slower and less prolific in the transgenic line over‐expressing miR160c ( At5g46845 ), arf10 and arf10 arf16 mutants than that in the wild type. ARF 10 repressed the expression of Arabidopsis Response Regulator15 ( ARR 15 , At1g74890 ) via its direct binding to the gene's promoter. The loss of function of ARR 15 enhanced callus initiation and partly rescued the phenotype induced by the transgene Pro35S:miR160c . Overexpression of ARR 15 partly rescues the callus initiation defect of mARF 10 plants. Our findings define miR160 as a key repressor of callus formation and reveal that the initiation of callus is repressed by miR160 ‐directed interaction between auxin and cytokinin.