A WUSCHEL-Independent Stem Cell Specification Pathway is Repressed by PHB, PHV and CNA in Arabidopsis.

Abstract

A tight balance between stem cell specification and differentiation is important to maintain a functional meristem and continuously initiate new organs throughout the lifespan of plants. The Arabidopsis WUSCHEL (WUS) transcription factor has been thought to be essential for specification of stem cell identity at shoot and flower meristems. CLAVATA (CLV) signaling acts through POL/PLL1 to limit WUS transcription and thus promote stem cell differentiation. To further dissect the CLV signaling pathway, I performed a modifier mutagenesis on pol-6 seeds and identified a novel allele of AGO10 (ago10-15). AGO10 prevents the degradation the HD-zip III family of transcription factors. I used a triple mutant of three of the HD-zip III members (PHB, PHV and CNA) which have superficially been described as having post-embryonic meristem enhancement. I found that phb phv cna mutants accumulate stem cells at both the shoot and flower meristems, along with changes to CLV3 and WUS cis element activation. When I generated the clv3 phb phv cna quadruple mutant, I observed dramatic enhancement of the clv stem cell accumulation phenotypes at the shoot and flower meristem. Remarkably, wus phb phv cna quadruple mutants developed functional meristems despite the absence of the central stem cell factor WUS. This conclusion is based on the ability of the triple mutant to engage in continuous organogenesis, the formation of meristem-like structures, as well as the retention of cell layering and WUS cis element activation within these meristem-like structures. These observations allow for a number of conclusions. First, WUS is dispensable for shoot meristem formation and organogenesis. Second, PHB/PHV/CNA represent a second, previously unknown, pathway for stem cell specification at shoot and flower meristems. Third, PHB/PHV/CNA act to repress this novel stem cell specification. When de-repressed, this second stem cell pathway leads to an accumulation of stem cells and an enlargement of the stem cell niche. When de-repressed in a wus mutant background, this second stem cell pathway leads to functional meristems. These results indicate a set of key stem cell specification factors remains unidentified.