Bipartite phosphoinositide-dependent modulation of auxin signaling during xylem differentiation in Arabidopsis thaliana roots

Ectopic assembly of an auxin efflux control machinery shifts developmental trajectories

Polar auxin transport in the Arabidopsis (Arabidopsis thaliana) root tip maintains high auxin levels around the stem cell niche that gradually decrease in dividing cells but increase again once they transition toward differentiation. Protophloem differentiates earlier than other proximal tissues and employs a unique auxin "canalization" machinery that is thought to balance auxin efflux with retention. It consists of a proposed activator of PIN-FORMED (PIN) auxin efflux carriers, the cAMP-, cGMP- and Calcium-dependent (AGC) kinase PROTEIN KINASE ASSOCIATED WITH BRX (PAX); its inhibitor, BREVIS RADIX (BRX); and PHOSPHATIDYLINOSITOL-4-PHOSPHATE-5-KINASE (PIP5K) enzymes, which promote polar PAX and BRX localization. Because of a dynamic PAX-BRX-PIP5K interplay, the net cellular output of this machinery remains unclear. In this study, we deciphered the dosage-sensitive regulatory interactions among PAX, BRX, and PIP5K by their ectopic expression in developing xylem vessels. The data suggest that the dominant collective output of the PAX-BRX-PIP5K module is a localized reduction in PIN abundance. This requires PAX-stimulated clathrin-mediated PIN endocytosis upon site-specific phosphorylation, which distinguishes PAX from other AGC kinases. An ectopic assembly of the PAX-BRX-PIP5K module is sufficient to cause cellular auxin retention and affects root growth vigor by accelerating the trajectory of xylem vessel development. Our data thus provide direct evidence that local manipulation of auxin efflux alters the timing of cellular differentiation in the root.

Quantification of xylem connection defects during lateral root development in Arabidopsis

The cellular and molecular mechanisms underlying the establishment of vascular connections between primary and lateral roots have recently gained significant attention. Here, I present a protocol to visualize and quantify xylem connection defects during lateral root development. I describe steps for employing stains to infer whether the defects observed in xylem bridges are associated with alterations in the xylem differentiation program, including programmed cell death and secondary cell wall deposition. For complete details on the use and execution of this protocol, please refer to Blanco-Touriñán et al. (2023) and Ursache et al. (2018).1,2.

The primary root procambium contributes to lateral root formation through its impact on xylem connection

The postembryonic formation of lateral roots (LRs) starts in internal root tissue, the pericycle. An important question of LR development is how the connection of the primary root vasculature with that of the emerging LR is established and whether the pericycle and/or other cell types direct this process. Here, using clonal analysis and time-lapse experiments, we show that both the procambium and pericycle of the primary root (PR) affect the LR vascular connectivity in a coordinated manner. We show that during LR formation, procambial derivates switch their identity and become precursors of xylem cells. These cells, together with the pericycle-origin xylem, participate in the formation of what we call a "xylem bridge" (XB), which establishes the xylem connection between the PR and the nascent LR. If the parental protoxylem cell fails to differentiate, XB is still sometimes formed but via a connection with metaxylem cells, highlighting that this process has some plasticity. Using mutant analyses, we show that the early specification of XB cells is determined by CLASS III HOMEODOMAIN-LEUCINE ZIPPER (HD-ZIP III) transcription factors (TFs). Subsequent XB cell differentiation is marked by the deposition of secondary cell walls (SCWs) in spiral and reticulate/scalariform patterns, which is dependent on the VASCULAR-RELATED NAC-DOMAIN (VND) TFs. XB elements were also observed in Solanum lycopersicum, suggesting that this mechanism may be more widely conserved in plants. Together, our results suggest that plants maintain vascular procambium activity, which safeguards the functionality of newly established lateral organs by assuring the continuity of the xylem strands throughout the root system.

Phosphoinositides in plant-pathogen interaction: trends and perspectives

Phosphoinositides are important regulatory membrane lipids, with a role in plant development and cellular function. Emerging evidence indicates that phosphoinositides play crucial roles in plant defence and are also utilized by pathogens for infection. In this review, we highlight the role of phosphoinositides in plant-pathogen interaction and the implication of this remarkable convergence in the battle against plant diseases.