ANGUSTIFOLIA mediates one of the multiple SCRAMBLED signaling pathways regulating cell growth pattern in Arabidopsis thaliana

Two ANGUSTIFOLIA genes regulate gametophore and sporophyte development in Physcomitrella patens

In Arabidopsis thaliana the ANGUSTIFOLIA (AN) gene regulates the width of leaves by controlling the diffuse growth of leaf cells in the medio-lateral direction. In the genome of the moss Physcomitrella patens, we found two normal ANs (PpAN1-1 and 1-2). Both PpAN1 genes complemented the A. thaliana an-1 mutant phenotypes. An analysis of spatiotemporal promoter activity of each PpAN1 gene, using transgenic lines that contained each PpAN1-promoter- uidA (GUS) gene, showed that both promoters are mainly active in the stems of haploid gametophores and in the middle to basal region of the young sporophyte that develops into the seta and foot. Analyses of the knockout lines for PpAN1-1 and PpAN1-2 genes suggested that these genes have partially redundant functions and regulate gametophore height by controlling diffuse cell growth in gametophore stems. In addition, the seta and foot were shorter and thicker in diploid sporophytes, suggesting that cell elongation was reduced in the longitudinal direction, whereas no defects were detected in tip-growing protonemata. These results indicate that both PpAN1 genes in P. patens function in diffuse growth of the haploid and diploid generations but not in tip growth. To visualize microtubule distribution in gametophore cells of P. patens, transformed lines expressing P. patens α-tubulin fused to sGFP were generated. Contrary to expectations, the orientation of microtubules in the tips of gametophores in the PpAN1-1/1-2 double-knockout lines was unchanged. The relationships among diffuse cell growth, cortical microtubules and AN proteins are discussed.

QUIRKY regulates root epidermal cell patterning through stabilizing SCRAMBLED to control CAPRICE movement in Arabidopsis

SCM, a leucine-rich repeat receptor-like kinase, is required for root epidermal cells to appropriately interpret their location and generate the proper cell-type pattern during Arabidopsis root development. Here, via a screen for scm-like mutants we describe a new allele of the QKY gene. We find that QKY is required for the appropriate spatial expression of several epidermal cell fate regulators in a similar manner as SCM in roots, and that QKY and SCM are necessary for the efficient movement of CPC between epidermal cells. We also show that turnover of SCM is mediated by a vacuolar degradation pathway triggered by ubiquitination, and that QKY prevents this SCM ubiquitination through their physical interaction. These results suggest that QKY stabilizes SCM through interaction, and this complex facilitates CPC movement between the epidermal cells to help establish the cell-type pattern in the Arabidopsis root epidermis.

SCM is a receptor-like kinase that ensures proper root patterning. Here, Song et al. find that SCM promotes the movement of CPC from non-hair cells to neighboring root epidermal cells and identify QKY as an additional facilitator of CPC mobility that acts by preventing vacuolar degradation of SCM.

ANGUSTIFOLIA contributes to the regulation of three-dimensional morphogenesis in the liverwort Marchantia polymorpha

Arabidopsis thaliana mutants deficient in ANGUSTIFOLIA (AN) exhibit several phenotypes at the sporophyte stage, such as narrow and thicker leaves, trichomes with two branches, and twisted fruits. It is thought that these phenotypes are caused by abnormal arrangement of cortical microtubules (MTs). AN homologs are present in the genomes of diverse land plants, including the basal land plant Marchantia polymorpha, and their molecular functions have been shown to be evolutionarily conserved in terms of the ability to complement the A. thaliana an-1 mutation. However, the roles of ANs in bryophytes, the life cycle of which includes a dominant haploid gametophyte generation, remain unknown. Here, we have examined the roles of AN homologs in the model bryophyte M. polymorpha (MpAN). Mpan knockout mutants showed abnormal twisted thalli and suppressed thallus growth along the growth axis. Under weak blue light conditions, elongated thallus growth was observed in wild-type plants, whereas it was suppressed in the mutants. Moreover, disordered cortical MT orientations were observed. Our findings suggest that MpAN contributes to three-dimensional morphogenesis by regulating cortical MT arrangement in the gametophytes of bryophytes.

TORNADO1 regulates root epidermal patterning through the WEREWOLF pathway in Arabidopsis thaliana

Cell fate in the root epidermis of Arabidopsis thaliana is determined in a position-dependent manner. SCRAMBLED (SCM), an atypical leucine-rich repeat receptor-like kinase, mediates this positional regulation via its effect on WEREWOLF (WER) expression, and subsequently, its downstream transcription factor, GLABRA2 (GL2), which are required for nonhair cell development. Previously, TORNADO1 (TRN1), a plant-specific protein with a leucine-rich repeat ribonuclease inhibitor-like domain, was shown to be required for proper epidermal patterning in Arabidopsis roots. In this work, we analyzed the possible involvement of TRN1 in the known root epidermal gene network. We discovered that the trn1 mutant caused the ectopic expression of WER and the randomized expression of GL2 and EGL3. This suggests that TRN1 regulates the position-dependent cell fate determination by affecting WER expression in Arabidopsis root epidermis. Additionally, the distinct phenotypes of the aerial parts of the trn1-t and scm-2 mutant suggest that TRN1 and SCM might have different functions in the development of aerial parts.