Xixi Zhang

In plant development, receptor kinases are often active in disparate cell types, with each requiring vastly different signaling outputs. The ERECTA (ER) receptor kinase and its homologs Erecta-Like1 (ERL1) and ERL2 exemplify this pleiotropy. In Arabidopsis , they influence stomatal patterning, shoot meristem function, and vascular cambium activity, among other functions. Such diverse functionality raises the question of how ER signaling can specify distinct cell behaviors. One key mechanism occurs via cell-type specific interactions with coreceptors, ligands, or other proteins that modulate signaling. However, little is known about ER interactors in the vascular cambium, a bifacial stem cell niche that generates phloem and xylem. Combinatorial mutations between ER , ERL1, and ERL2 and receptor kinases of a second family, Phloem Intercalated with Xylem ( PXY) , PXY-LIKE1 ( PXL1) , and PXL2 , show severe cambial defects, but the mechanism underpinning these phenotypes is not known. Here, we show that PXY and PXL proteins form protein complexes with ER family members. In genetic analyses, plant lines in which PXY signaling was constitutively active had dramatic phenotypic changes that required the presence of ER or ERL2. Our results demonstrate that PXY signaling mediated cambium regulation in part depends on ER signaling and explains ER function in the cambium. Because the cambium produces xylem, which constitutes the wood in vascular plants, our findings position PXY–ER complexes at the center of the accumulation of this versatile biomaterial and essential carbon sink.

The vascular cambium contains bifacial stem cells producing secondary phloem in one and secondary xylem to the opposite direction. In Arabidopsis roots, these stem cells are first established adjacent to primary xylem cells suggesting that undifferentiated primary xylem cells function as stem-cell organizers. Here, we show that auxin transport-guided auxin accumulation in the cells with primary xylem identity (PXI cells) within the root apical meristem defines the vascular cambial stem-cell organizer already during the primary development. Ablation of PXI cells or transient interference of auxin transport resulted in respecification of stem cells within the vascular bundle in a scattered manner. Similarly, ablation of cells with secondary xylem identity (SXI cells) in the mature vascular cambium resulted in the regeneration of SXI cells from stem cells within the same radial cell files. Reciprocally, ablation of stem cells in mature vascular cambium led to respecification of new stem cells from adjacent SXI cells. Thus, in response to injury, xylem identity cells and stem cells form an interchangeable unit, which we propose to define as a stem cell niche. Successful respecification of stem cells required functional auxin transport and presence of undifferentiated xylem cells. Overall, our findings reveal that undifferentiated secondary xylem functions as the stem cell reservoir and highlight the key role of auxin transport-mediated auxin accumulation in stem cell positioning during cambium development and regeneration.

Wood constitutes the largest reservoir of terrestrial biomass. Composed of xylem, it arises from one side of the vascular cambium, a bifacial stem cell niche that also produces phloem on the opposing side. It is currently unknown which molecular factors endow cambium stem cell identity. Here we show that TRACHEARY ELEMENT DIFFERENTIATION INHIBITORY FACTOR (TDIF) ligand–activated PHLOEM INTERCALATED WITH XYLEM (PXY) receptors promote the expression of CAMBIUM-EXPRESSED AINTEGUMENTA-LIKE (CAIL) transcription factors to define cambium stem cell identity in the Arabidopsis root. By sequestrating the phloem-originated TDIF, xylem-expressed PXY confines the TDIF signaling front, resulting in the activation of CAIL expression and stem cell identity in only a narrow domain. Our findings show how signals emanating from cells on opposing sides ensure robust yet dynamically adjustable positioning of a bifacial stem cell layer.

Summary: Secondary xylem and phloem originate from a lateral meristem called the vascular cambium that consists of one to several layers of meristematic cells. Recent lineage tracing studies have shown that only one of the cambial cells in each radial cell file functions as the stem cell, capable of producing both secondary xylem and phloem. Here, we first review how phytohormones and signalling peptides regulate vascular cambium formation and activity. We then propose how the stem cell concept, familiar from apical meristems, could be applied to cambium studies. Finally, we discuss how this concept could set the basis for future research.

Strigolactones (SLs) are carotenoid-derived plant hormones that control shoot branching and communications between host plants and symbiotic fungi or root parasitic plants. Extensive studies have identified the key components participating in SL biosynthesis and signalling, whereas the catabolism or deactivation of endogenous SLs in planta remains largely unknown. Here, we report that the Arabidopsis carboxylesterase 15 (AtCXE15) and its orthologues function as efficient hydrolases of SLs. We show that overexpression of AtCXE15 promotes shoot branching by dampening SL-inhibited axillary bud outgrowth. We further demonstrate that AtCXE15 could bind and efficiently hydrolyse SLs both in vitro and in planta. We also provide evidence that AtCXE15 is capable of catalysing hydrolysis of diverse SL analogues and that such CXE15-dependent catabolism of SLs is evolutionarily conserved in seed plants. These results disclose a catalytic mechanism underlying homoeostatic regulation of SLs in plants, which also provides a rational approach to spatial-temporally manipulate the endogenous SLs and thus architecture of crops and ornamental plants.

Cell and tissue polarization is fundamental for plant growth and morphogenesis. The polar, cellular localization ofArabidopsisPIN-FORMED (PIN) proteins is crucial for their function in directional auxin transport. The clustering of PIN polar cargoes within the plasma membrane has been proposed to be important for the maintenance of their polar distribution. However, the more detailed features of PIN clusters and the cellular requirements of cargo clustering remain unclear. Here, we characterized PIN clusters in detail by means of multiple advanced microscopy and quantification methods, such as 3D quantitative imaging or freeze-fracture replica labeling. The size and aggregation types of PIN clusters were determined by electron microscopy at the nanometer level at different polar domains and at different developmental stages, revealing a strong preference for clustering at the polar domains. Pharmacological and genetic studies revealed that PIN clusters depend on phosphoinositol pathways, cytoskeletal structures and specific cell-wall components as well as connections between the cell wall and the plasma membrane. This study identifies the role of different cellular processes and structures in polar cargo clustering and provides initial mechanistic insight into the maintenance of polarity in plants and other systems.

Flowering plants display the highest diversity among plant species and have notably shaped terrestrial landscapes. Nonetheless, the evolutionary origin of their unprecedented morphological complexity remains largely an enigma. Here, we show that the coevolution of cis-regulatory and coding regions of PIN-FORMED (PIN) auxin transporters confined their expression to certain cell types and directed their subcellular localization to particular cell sides, which together enabled dynamic auxin gradients across tissues critical to the complex architecture of flowering plants. Extensive intraspecies and interspecies genetic complementation experiments with PINs from green alga up to flowering plant lineages showed that PIN genes underwent three subsequent, critical evolutionary innovations and thus acquired a triple function to regulate the development of three essential components of the flowering plant Arabidopsis: shoot/root, inflorescence, and floral organ. Our work highlights the critical role of functional innovations within the PIN gene family as essential prerequisites for the origin of flowering plants.

Directional intercellular transport of the phytohormone auxin mediated by PIN-FORMED (PIN) efflux carriers has essential roles in both coordinating patterning processes and integrating multiple external cues by rapidly redirecting auxin fluxes. PIN activity is therefore regulated by multiple internal and external cues, for which the underlying molecular mechanisms are not fully elucidated. Here, we demonstrate that 3′-PHOSPHOINOSITIDE-DEPENDENT PROTEIN KINASE1 (PDK1), which is conserved in plants and mammals, functions as a molecular hub that perceives upstream lipid signalling and modulates downstream substrate activity through phosphorylation. Using genetic analysis, we show that the loss-of-function Arabidopsis pdk1.1 pdk1.2 mutant exhibits a plethora of abnormalities in organogenesis and growth due to defective polar auxin transport. Further cellular and biochemical analyses reveal that PDK1 phosphorylates D6 protein kinase, a well-known upstream activator of PIN proteins. We uncover a lipid-dependent phosphorylation cascade that connects membrane-composition-based cellular signalling with plant growth and patterning by regulating morphogenetic auxin fluxes.

ALA3 and related flippases are important interactors and regulators of ADP ribosylation factor GTPase Exchange Factors regulating the polarity and trafficking o