PHABULOSA Controls the Quiescent Center-Independent Root Meristem Activities in Arabidopsis thaliana

Abstract

Plant growth depends on stem cell niches in meristems. In the root apical meristem, the quiescent center (QC) cells form a niche together with the surrounding stem cells. Stem cells produce daughter cells that are displaced into a transit-amplifying (TA) domain of the root meristem. TA cells divide several times to provide cells for growth. SHORTROOT (SHR) and SCARECROW (SCR) are key regulators of the stem cell niche. Cytokinin controls TA cell activities in a dose-dependent manner. Although the regulatory programs in each compartment of the root meristem have been identified, it is still unclear how they coordinate one another. Here, we investigate how PHABULOSA (PHB), under the posttranscriptional control of SHR and SCR, regulates TA cell activities. The root meristem and growth defects in shr or scr mutants were significantly recovered in the shr phb or scr phb double mutant, respectively. This rescue in root growth occurs in the absence of a QC. Conversely, when the modified PHB, which is highly resistant to microRNA, was expressed throughout the stele of the wild-type root meristem, root growth became very similar to that observed in the shr; however, the identity of the QC was unaffected. Interestingly, a moderate increase in PHB resulted in a root meristem phenotype similar to that observed following the application of high levels of cytokinin. Our protoplast assay and transgenic approach using ARR10 suggest that the depletion of TA cells by high PHB in the stele occurs via the repression of B-ARR activities. This regulatory mechanism seems to help to maintain the cytokinin homeostasis in the meristem. Taken together, our study suggests that PHB can dynamically regulate TA cell activities in a QC-independent manner, and that the SHR-PHB pathway enables a robust root growth system by coordinating the stem cell niche and TA domain.

SHORTROOT-Mediated Intercellular Signals Coordinate Phloem Development in Arabidopsis Roots

“…Although scr-4 showed a reduction in the number of procambial cells, similar to that of shr, it constantly showed two SEs in at least one of the two phloem poles ( Figures 2C, G). A similar case was also observed when we examined the SEs in transgenic plants harboring ProCRE1:PHBem-GFP, which expresses a microRNA-resistant version of PHB, under the promoter of the stele-specific gene CRE1 (Sebastian et al, 2015). This line overexpresses functional PHB throughout the stele, as in the shr mutant.…”

PHABULOSA Mediates an Auxin Signaling Loop to Regulate Vascular Patterning in Arabidopsis

“…However, the root vascular phenotypic effect of 35S::IAA30 is more similar to phenotypes resulting from PHB or CNA gain of function than from HD-ZIP III loss of function. Furthermore, overexpression of IAA20 and IAA30 resulted in terminated root meristems (Sato and Yamamoto, 2008a), a phenotype also observed in 1-week-old phb-7d plants (Sebastian et al, 2015; A.E. Valdés, C.J.…”

Boron deficiency inhibits root growth by controlling meristem activity under cytokinin regulation

“…The Quiescent Center (QC) of the RAM ensures post-embryonic root growth providing stem cells that then will divide and elongate/differentiate, leading to root growth [46,47]. The failure in root meristem maintenance observed under B deficiency could be related to a rapid cell differentiation of stem cell daughters or to the lack of QC activity.…”