The receptor-like kinases BAM1 and BAM2 are required for root xylem patterning

Significance Plant miRNAs can move from cell to cell through cytoplasmic channels called plasmodesmata, transmitting positional information that is decoded to make developmental decisions; one well-studied example of this is xylem patterning in the root of the model plant Arabidopsis thaliana, which depends on the formation of opposing gradients of two miRNAs and their target transcripts. Here, we show that two plasma membrane- and plasmodesmata-localized proteins, BAM1 and BAM2, are required for maintaining appropriate levels and distribution of the miRNA target transcripts, and hence for proper root xylem patterning, uncovering additional players in the regulation of this developmental process and adding one more function, direct or indirect, to the ever-growing portfolio of BAM1 and BAM2. Xylem patterning in the root is established through the creation of opposing gradients of miRNAs and their targets, transcripts of the HD-ZIP III family of transcriptions factors, enabled by the cell-to-cell spread of the former. The miRNAs regulating xylem patterning, miR165/6, move through plasmodesmata, but how their trafficking is regulated remains elusive. Here, we describe that simultaneous mutation of the plasma membrane- and plasmodesmata-localized receptor-like kinases (RLKs) BARELY ANY MERISTEM (BAM) 1 and 2 or expression of the geminivirus-encoded BAM1/2-interactor C4 results in higher accumulation and broader distribution of the HD-ZIP III transcripts despite normal total accumulation of miR165/6, and ultimately causes defects in xylem patterning, which depend on the function of the aforementioned miRNA targets. Taken together, our results show that BAM1 and BAM2 are redundantly required for proper xylem patterning in the Arabidopsis root, by ensuring the proper distribution and accumulation of miR165/6-targeted transcripts.