Figure 4.—

A model for BAM function through signal transduction and ligand sequestration. (A) In wild-type cell layer three (L3) meristem cells, CLV1 is expressed at higher levels than BAM proteins and is the primary transducer of CLE signal. CLV1/CLV1 and CLV1/BAM complexes are consistent with the dominant-negative function of many clv1 alleles and their interaction with bam mutations. In clv1-1 L3 cells, BAM signaling may be blocked by interference of the mutant clv1-1 kinase domain. Residual signaling may come from a partially active BAM/clv1-1 heterodimer and/or the presumably small amount of BAM/BAM homodimers (not shown). In the absence of CLV1 (clv1-null), BAM proteins are available to transduce CLE signal, albeit at a reduced level compared to WT cells. To simplify the diagram, CLV2 and other components have not been included. (B) In a wild-type shoot meristem, CLV3 signals primarily through CLV1, but also through BAM1 and BAM2 to regulate the WUS expression domain. BAM1 and BAM2 also function on the flanks of the meristem where they sequester CLV3-related CLE ligands, creating a buffer around the meristem center. (C) In a clv3 apex, WUS repression is lost, leading to an expansion of the WUS expression domain and an enlargement of the meristem, while CLE ligands outside of the meristem are sequestered. (D) In a bam1 bam2 apex, CLE ligands are no longer sequestered, leading to overactivation of CLV1 and reduction in the meristem size, even meristem termination in bam1 bam2 bam3 triple mutants. (E) In a clv3 bam1 bam2 apex, loss of CLV3 is partially balanced through activation of CLV1 by unsequestered CLE ligands resulting in partial WUS expression control. (F) A model depicting genetic pathways through which CLV1, BAM1, and BAM2 function.