Asymmetric Redundancy of ZERZAUST and ZERZAUST HOMOLOG in Different Accessions of Arabidopsis thaliana.

Divergence among duplicate genes is one of the important sources of evolutionary innovation. But, the contribution of duplicate divergence to variation in Arabidopsis accessions is sparsely known. Recently, we studied the role of a cell wall localized protein, ZERZAUST (ZET), in Landsberg erecta (Ler) accession, lack of which results in aberrant plant morphology. Here, we present the study of ZET in Columbia (Col) accession, which not only showed differential expression patterns in comparison to Ler, but also revealed its close homolog, ZERZAUST HOMOLOG (ZETH) Although, genetic analysis implied redundancy, expression analysis revealed divergence, with ZETH showing minimal expression in both Col and Ler In addition, ZETH shows relatively higher expression levels in Col compared to Ler Our data also reveal compensatory up-regulation of ZETH in Col, but not in Ler, implying it is perhaps dispensable in Ler However, a novel CRISPR/Cas9-induced zeth allele confirmed that ZETH has residual activity in Ler Finally, the synergistic interaction of the receptor-like kinase gene, ERECTA with ZET in ameliorating morphological defects suggests crucial role of modifiers on plant phenotype. The results provide genetic evidence for accession-specific differences in compensation mechanism and asymmetric gene contribution. Thus, our work reveals a novel example for how weakly expressed homologs contribute to diversity among accessions.

The cell wall-localized atypical ß-1,3 glucanase ZERZAUST controls tissue morphogenesis in Arabidopsis thaliana.

Orchestration of cellular behavior in plant organogenesis requires integration of intercellular communication and cell wall dynamics. The underlying signaling mechanisms are poorly understood. Tissue morphogenesis in Arabidopsis depends on the receptor-like kinase STRUBBELIG. Mutations in ZERZAUST were previously shown to result in a strubbelig-like mutant phenotype. Here, we report on the molecular identification and functional characterization of ZERZAUST We show that ZERZAUST encodes a putative GPI-anchored ß-1,3 glucanase suggested to degrade the cell wall polymer callose. However, a combination of in vitro, cell biological and genetic experiments indicate that ZERZAUST is not involved in the regulation of callose accumulation. Nonetheless, Fourier-transformed infrared-spectroscopy revealed that zerzaust mutants show defects in cell wall composition. Furthermore, the results indicate that ZERZAUST represents a mobile apoplastic protein, and that its carbohydrate-binding module family 43 domain is required for proper subcellular localization and function whereas its GPI anchor is dispensable. Our collective data reveal that the atypical ß-1,3 glucanase ZERZAUST acts in a non-cell-autonomous manner and is required for cell wall organization during tissue morphogenesis.

The C2-domain protein QUIRKY and the receptor-like kinase STRUBBELIG localize to plasmodesmata and mediate tissue morphogenesis in Arabidopsis thaliana.

Tissue morphogenesis in plants requires communication between cells, a process involving the trafficking of molecules through plasmodesmata (PD). PD conductivity is regulated by endogenous and exogenous signals. However, the underlying signaling mechanisms remain enigmatic. In Arabidopsis, signal transduction mediated by the receptor-like kinase STRUBBELIG (SUB) contributes to inter-cell layer signaling during tissue morphogenesis. Previous analysis has revealed that SUB acts non-cell-autonomously suggesting that SUB controls tissue morphogenesis by participating in the formation or propagation of a downstream mobile signal. A genetic screen identified QUIRKY (QKY), encoding a predicted membrane-anchored C2-domain protein, as a component of SUB signaling. Here, we provide further insight into the role of QKY in this process. We show that like SUB, QKY exhibits non-cell-autonomy when expressed in a tissue-specific manner and that non-autonomy of QKY extends across several cells. In addition, we report on localization studies indicating that QKY and SUB localize to PD but independently of each other. FRET-FLIM analysis suggests that SUB and QKY are in close contact at PD in vivo. We propose a model where SUB and QKY interact at PD to promote tissue morphogenesis, thereby linking RLK-dependent signal transduction and intercellular communication mediated by PD.

ANGUSTIFOLIA is a central component of tissue morphogenesis mediated by the atypical receptor-like kinase STRUBBELIG.

BACKGROUND: During plant tissue morphogenesis cells have to coordinate their behavior to allow the generation of the size, shape and cellular patterns that distinguish an organ. Despite impressive progress the underlying signaling pathways remain largely unexplored. In Arabidopsis thaliana, the atypical leucine-rich repeat receptor-like kinase STRUBBELIG (SUB) is involved in signal transduction in several developmental processes including the formation of carpels, petals, ovules and root hair patterning. The three STRUBBELIG-LIKE MUTANT (SLM) genes DETORQUEO (DOQ), QUIRKY (QKY) and ZERZAUST (ZET) are considered central elements of SUB-mediated signal transduction pathways as corresponding mutants share most phenotypic aspects with sub mutants. RESULTS: Here we show that DOQ corresponds to the previously identified ANGUSTIFOLIA gene. The genetic analysis revealed that the doq-1 mutant exhibits all additional mutant phenotypes and conversely that other an alleles show the slm phenotypes. We further provide evidence that SUB and AN physically interact and that AN is not required for subcellular localization of SUB. CONCLUSIONS: Our data suggest that AN is involved in SUB signal transduction pathways. In addition, they reveal previously unreported functions of AN in several biological processes, such as ovule development, cell morphogenesis in floral meristems, and root hair patterning. Finally, SUB and AN may directly interact at the plasma membrane to mediate SUB-dependent signaling.

Structure-function analysis of STRUBBELIG, an Arabidopsis atypical receptor-like kinase involved in tissue morphogenesis.

Tissue morphogenesis in plants requires the coordination of cellular behavior across clonally distinct histogenic layers. The underlying signaling mechanisms are presently being unraveled and are known to include the cell surface leucine-rich repeat receptor-like kinase STRUBBELIG in Arabidopsis. To understand better its mode of action an extensive structure-function analysis of STRUBBELIG was performed. The phenotypes of 20 EMS and T-DNA-induced strubbelig alleles were assessed and homology modeling was applied to rationalize their possible effects on STRUBBELIG protein structure. The analysis was complemented by phenotypic, cell biological, and pharmacological investigations of a strubbelig null allele carrying genomic rescue constructs encoding fusions between various mutated STRUBBELIG proteins and GFP. The results indicate that STRUBBELIG accepts quite some sequence variation, reveal the biological importance for the STRUBBELIG N-capping domain, and reinforce the notion that kinase activity is not essential for its function in vivo. Furthermore, individual protein domains of STRUBBELIG cannot be related to specific STRUBBELIG-dependent biological processes suggesting that process specificity is mediated by factors acting together with or downstream of STRUBBELIG. In addition, the evidence indicates that biogenesis of a functional STRUBBELIG receptor is subject to endoplasmic reticulum-mediated quality control, and that an MG132-sensitive process regulates its stability. Finally, STRUBBELIG and the receptor-like kinase gene ERECTA interact synergistically in the control of internode length. The data provide genetic and molecular insight into how STRUBBELIG regulates intercellular communication in tissue morphogenesis.

Inter-cell-layer signalling during Arabidopsis ovule development mediated by the receptor-like kinase STRUBBELIG.

Plant organs, such as ovules and flowers, arise through cellular events that are precisely co-ordinated between cells within and across clonally distinct cell layers. Receptor-like kinases are cell-surface receptors that perceive and relay intercellular information. In Arabidopsis the leucine-rich repeat receptor-like kinase STRUBBELIG (SUB) is required for integument initiation and outgrowth during ovule development, floral organ shape and the control of the cell division plane in the first subepidermal cell layer of floral meristems, among other functions. A major goal is to understand SUB-mediated signal transduction at the molecular level. Present evidence suggests that SUB affects neighbouring cells in a non-cell-autonomous fashion. In addition, our results indicate that SUB is an atypical, or kinase-dead, kinase. Forward genetics identified three genes, QUIRKY (QKY), ZERZAUST and DETORQUEO, that are thought to contribute to SUB-dependent signal transduction. QKY encodes a predicted membrane-bound protein with four cytoplasmic C(2) domains. By analogy to animal proteins with related domain topology, we speculate that QKY may be involved in Ca(2+)-dependent signalling and membrane trafficking. Studying SUB-dependent signalling will contribute to our understanding of how atypical kinases mediate signal transduction and how cells co-ordinate their behaviour to allow organs, such as ovules, to develop their three-dimensional architecture.

DETORQUEO, QUIRKY, and ZERZAUST represent novel components involved in organ development mediated by the receptor-like kinase STRUBBELIG in Arabidopsis thaliana.

Intercellular signaling plays an important role in controlling cellular behavior in apical meristems and developing organs in plants. One prominent example in Arabidopsis is the regulation of floral organ shape, ovule integument morphogenesis, the cell division plane, and root hair patterning by the leucine-rich repeat receptor-like kinase STRUBBELIG (SUB). Interestingly, kinase activity of SUB is not essential for its in vivo function, indicating that SUB may be an atypical or inactive receptor-like kinase. Since little is known about signaling by atypical receptor-like kinases, we used forward genetics to identify genes that potentially function in SUB-dependent processes and found recessive mutations in three genes that result in a sub-like phenotype. Plants with a defect in DETORQEO (DOQ), QUIRKY (QKY), and ZERZAUST (ZET) show corresponding defects in outer integument development, floral organ shape, and stem twisting. The mutants also show sub-like cellular defects in the floral meristem and in root hair patterning. Thus, SUB, DOQ, QKY, and ZET define the STRUBBELIG-LIKE MUTANT (SLM) class of genes. Molecular cloning of QKY identified a putative transmembrane protein carrying four C(2) domains, suggesting that QKY may function in membrane trafficking in a Ca(2+)-dependent fashion. Morphological analysis of single and all pair-wise double-mutant combinations indicated that SLM genes have overlapping, but also distinct, functions in plant organogenesis. This notion was supported by a systematic comparison of whole-genome transcript profiles during floral development, which molecularly defined common and distinct sets of affected processes in slm mutants. Further analysis indicated that many SLM-responsive genes have functions in cell wall biology, hormone signaling, and various stress responses. Taken together, our data suggest that DOQ, QKY, and ZET contribute to SUB-dependent organogenesis and shed light on the mechanisms, which are dependent on signaling through the atypical receptor-like kinase SUB.

The Arabidopsis receptor-like kinase STRUBBELIG mediates inter-cell-layer signaling during floral development.

In plants important questions relate to the mechanisms that control signaling between the histogenic cell layers of apical meristems and developing organs. The Arabidopsis putative atypical leucine-rich repeat receptor-like kinase STRUBBELIG (SUB) regulates amongst others floral organ shape, the plane of cell division in cells of the first subepidermal cell layer of floral meristems, ovule integument morphogenesis, and root hair patterning. Reporter assays using a functional translational fusion between SUB and EGFP indicate that SUB expression is largely confined to interior tissues in young flowers, ovules, and roots. In contrast, SUB mRNA expression can be monitored in all cell layers of those tissues. Specifically, SUB protein is not detectable in cells that show a sub mutant phenotype. Rather, SUB is detected in directly neighbouring cells in flower and ovule primordia, or in cells that are separated from mutant cells by two cell diameters in the root. Inhibitor studies corroborate a posttranscriptional regulation of SUB. Phenotypic analysis of sub-1 plants expressing a SUB:EGFP gene under the control of tissue and epidermis-specific promoters support the notion that SUB-dependent signal transduction relies on the production of secondary intercellular signals. The combined results indicate that SUB acts in a non-cell-autonomous fashion, functions in a radial inside-out signaling process, and mediates cell morphogenesis and cell fate across clonally distinct cell layers in floral primordia, developing ovules, and root meristems.

Cell wall modifications in Arabidopsis plants with altered alpha-L-arabinofuranosidase activity.

Although cell wall remodeling is an essential feature of plant growth and development, the underlying molecular mechanisms are poorly understood. This work describes the characterization of Arabidopsis (Arabidopsis thaliana) plants with altered expression of ARAF1, a bifunctional alpha-L-arabinofuranosidase/beta-D-xylosidase (At3g10740) belonging to family 51 glycosyl-hydrolases. ARAF1 was localized in several cell types in the vascular system of roots and stems, including xylem vessels and parenchyma cells surrounding the vessels, the cambium, and the phloem. araf1 T-DNA insertional mutants showed no visible phenotype, whereas transgenic plants that overexpressed ARAF1 exhibited a delay in inflorescence emergence and altered stem architecture. Although global monosaccharide analysis indicated only slight differences in cell wall composition in both mutant and overexpressing lines, immunolocalization experiments using anti-arabinan (LM6) and anti-xylan (LM10) antibodies indicated cell type-specific alterations in cell wall structure. In araf1 mutants, an increase in LM6 signal intensity was observed in the phloem, cambium, and xylem parenchyma in stems and roots, largely coinciding with ARAF1 expression sites. The ectopic overexpression of ARAF1 resulted in an increase in LM10 labeling in the secondary walls of interfascicular fibers and xylem vessels. The combined ARAF1 gene expression and immunolocalization studies suggest that arabinan-containing pectins are potential in vivo substrates of ARAF1 in Arabidopsis.

STRUBBELIG defines a receptor kinase-mediated signaling pathway regulating organ development in Arabidopsis.

An open question remains as to what coordinates cell behavior during organogenesis, permitting organs to reach their appropriate size and shape. The Arabidopsis gene STRUBBELIG (SUB) defines a receptor-mediated signaling pathway in plants. SUB encodes a putative leucine-rich repeat transmembrane receptor-like kinase. The mutant sub phenotype suggests that SUB affects the formation and shape of several organs by influencing cell morphogenesis, the orientation of the division plane, and cell proliferation. Mutational analysis suggests that the kinase domain is important for SUB function. Biochemical assays using bacterially expressed fusion proteins indicate that the SUB kinase domain lacks enzymatic phosphotransfer activity. Furthermore, transgenes encoding WT and different mutant variants of SUB were tested for their ability to rescue the mutant sub phenotype. These genetic data also indicate that SUB carries a catalytically inactive kinase domain. The SUB receptor-like kinase may therefore signal in an atypical fashion.

Two alpha-L-arabinofuranosidase genes in Arabidopsis thaliana are differentially expressed during vegetative growth and flower development.

Glycosyl hydrolases are important mediators of plant cell wall modification during plant development. These enzymes catalyse the hydrolytic release of specific sugars, such as L-arabinose, from the polysaccharide-rich cell wall matrix. The cloning and expression analysis of two genes, AtASD1 and AtASD2, encoding putative alpha-L-arabinofuranosidases in Arabidopsis thaliana are reported here. AtASD1 and AtASD2 identities were assigned on the basis of homology to plant and microbial family 51 glycoside hydrolases. Using RT-PCR, RNA gel blot analysis and reporter gene expression analysis, AtASD1 and AtASD2 were shown to have different developmental expression profiles. High levels of AtASD1 promoter activity are present in multiple tissues during vegetative and reproductive growth. AtASD1 expression is particularly intense in zones of cell proliferation, the vascular system, developing and regressing floral tissues, and floral abscission zones. By comparison, AtASD2 expression is limited to the vasculature of older root tissue and to some floral organs and floral abscission zones.