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Casparian strips are ring-like cell-wall modifications in the root endodermis of vascular plants. Their presence generates a paracellular barrier, analogous to animal tight junctions, that is thought to be crucial for selective nutrient uptake, exclusion of pathogens, and many other processes. Despite their importance, the chemical nature of Casparian strips has remained a matter of debate, confounding further molecular analysis. Suberin, lignin, lignin-like polymers, or both, have been claimed to make up Casparian strips. Here we show that, in Arabidopsis, suberin is produced much too late to take part in Casparian strip formation. In addition, we have generated plants devoid of any detectable suberin, which still establish functional Casparian strips. In contrast, manipulating lignin biosynthesis abrogates Casparian strip formation. Finally, monolignol feeding and lignin-specific chemical analysis indicates the presence of archetypal lignin in Casparian strips. Our findings establish the chemical nature of the primary root-diffusion barrier in Arabidopsis and enable a mechanistic dissection of the formation of Casparian strips, which are an independent way of generating tight junctions in eukaryotes.

Pectin is an abundant cell wall polysaccharide with essential roles in various biological processes. The structural diversity of pectins, along with the numerous combinations of the enzymes responsible for pectin biosynthesis and modification, plays key roles in ensuring the specificity and plasticity of cell wall remodeling in different cell types and under different environmental conditions. This review focuses on recent progress in understanding various aspects of pectin, from its biosynthetic and modification processes to its biological roles in different cell types. In particular, we describe recent findings that cell wall modifications serve not only as final outputs of internally determined pathways, but also as key components of intercellular communication, with pectin as a major contributor to this process. The comprehensive view of the diverse roles of pectin presented here provides an important basis for understanding how cell wall-enclosed plant cells develop, differentiate, and interact.

The identification of PILS proteins, putative auxin transport facilitators, suggests that intracellular auxin transport might be evolutionarily older than directional, cell-to-cell PIN-dependent auxin transport, and highlights the developmental importance of intracellular auxin transport. Keeping auxin in its place Auxin is a key hormone regulating plant growth and development. Directed transport of auxin between cells is mediated by auxin-efflux carriers called PINs. Here, Jürgen Kleine-Vehn and colleagues identify a previously unknown auxin-transporter family called PIN-LIKES (PILS). These transporters, located in the endoplasmic reticulum, regulate intracellular auxin accumulation and metabolism and maintain cellular auxin homeostasis. This finding suggests that compartmentalization of auxin is important for developmental processes. The phytohormone auxin acts as a prominent signal, providing, by its local accumulation or depletion in selected cells, a spatial and temporal reference for changes in the developmental program 1 , 2 , 3 , 4 , 5 , 6 , 7 . The distribution of auxin depends on both auxin metabolism (biosynthesis, conjugation and degradation) 8 , 9 , 10 and cellular auxin transport 11 , 12 , 13 , 14 , 15 . We identified in silico a novel putative auxin transport facilitator family, called PIN-LIKES (PILS). Here we illustrate that PILS proteins are required for auxin-dependent regulation of plant growth by determining the cellular sensitivity to auxin. PILS proteins regulate intracellular auxin accumulation at the endoplasmic reticulum and thus auxin availability for nuclear auxin signalling. PILS activity affects the level of endogenous auxin indole-3-acetic acid (IAA), presumably via intracellular accumulation and metabolism. Our findings reveal that the transport machinery to compartmentalize auxin within the cell is of an unexpected molecular complexity and demonstrate this compartmentalization to be functionally important for a number of developmental processes.

Rapid stomatal closure is essential for water conservation in plants and is thus critical for survival under water deficiency. To close stomata rapidly, guard cells reduce their volume by converting a large central vacuole into a highly convoluted structure. However, the molecular mechanisms underlying this change are poorly understood. In this study, we used pH-indicator dyes to demonstrate that vacuolar convolution is accompanied by acidification of the vacuole in fava bean (Vicia faba) guard cells during abscisic acid (ABA)—induced stomatal closure. Vacuolar acidification is necessary for the rapid stomatal closure induced by ABA, since a double mutant of the vacuolar H + -ATPase vha-a2 vha-a3 and vacuolar H + -PPase mutant vhp1 showed delayed stomatal closure. Furthermore, we provide evidence for the critical role of phosphatidylinositol 3,5-bisphosphate [PtdIns(3,5)P 2 ] in changes in pH and morphology of the vacuole. Single and double Arabidopsis thaliana null mutants of phosphatidylinositol 3-phosphate 5-kinases (PI3P5Ks) exhibited slow stomatal closure upon ABA treatment compared with the wild type. Moreover, an inhibitor of PI3P5K reduced vacuolar acidification and convolution and delayed stomatal closure in response to ABA. Taken together, these results suggest that rapid ABA-induced stomatal closure requires PtdIns(3,5)P 2 , which is essential for vacuolar acidification and convolution.

The endodermis represents the main barrier to extracellular diffusion in plant roots, and it is central to current models of plant nutrient uptake. Despite this, little is known about the genes setting up this endodermal barrier. In this study, we report the identification and characterization of a strong barrier mutant, schengen3 (sgn3). We observe a surprising ability of the mutant to maintain nutrient homeostasis, but demonstrate a major defect in maintaining sufficient levels of the macronutrient potassium. We show that SGN3/GASSHO1 is a receptor-like kinase that is necessary for localizing CASPARIAN STRIP DOMAIN PROTEINS (CASPs)—major players of endodermal differentiation—into an uninterrupted, ring-like domain. SGN3 appears to localize into a broader band, embedding growing CASP microdomains. The discovery of SGN3 strongly advances our ability to interrogate mechanisms of plant nutrient homeostasis and provides a novel actor for localized microdomain formation at the endodermal plasma membrane. Plant roots forage in the soil for minerals and water, but they must also provide a barrier that stops these nutrients leaking back out of the plant and stops microbes invading and causing disease. The endodermis—an inner layer of cells that surrounds the veins that run along the middle of a root—acts as such a barrier in young roots. Polymers that repel water are deposited between the cells in the roots of almost all vascular plants—which include ferns, conifers, and flowering plants—to form a band around the endodermis called the ‘Casparian strip’. This strip seals off the young roots and stops water moving through the gaps between plant cells, but still allows minerals, nutrients, and water to be transported through the root cells and into the plant. However, the importance of this structure has yet to be tested due to the lack of mutant plants without a Casparian strip. Pfister et al. now report that deleting the gene that encodes a protein called SCHENGEN3 in the model plant Arabidopsis thaliana causes the Casparian strip to be interrupted by irregularly sized holes. This protein is normally found at high levels in the root endodermis, where it is embedded into the cell membranes. Pfister et al. also showed that without the SCHENGEN3 protein, other proteins called CASPs—that normally mark out a stripe around the root cells where the Casparian strip will form—only accumulated in discontinuous patches. Further experiments revealed that deleting the gene for SCHENGEN3 does not cause general problems in delivering the CASP proteins to the cell membrane; instead, it specifically stops the CASP proteins from forming a single, uninterrupted stripe. Unexpectedly, disrupting the Casparian strip did not appear to hinder many of the functions of a root. The mutant plants could still take up water and nutrients, and the leaves of mutant plants had normal levels of many essential minerals—with the exception of potassium. The level of this mineral was much lower in mutant plants without the SCHENGEN3 protein. Pfister et al. suggest that in plants that lack an intact Casparian strip, potassium is continuously leaked from the root into the soil. These findings reveal that in Arabidopsis, at least, the Casparian strip might not be as important as once thought for helping the plant to take up and accumulate water and nutrients. Further work is now needed to uncover the as yet unknown backup systems that might be able to compensate for the loss of this structure.

Cellular calcium acts as a second messenger and regulates diverse developmental events and stress responses. Cytosolic calcium has long been considered as an important regulator of senescence, however, the role of Ca in plant senescence has remained elusive. Here we show that the ( ) gene, which encodes Ca -binding protein calmodulin 1, positively regulates leaf senescence in . Yellowing of leaves, accumulation of reactive oxygen species (ROS), and expression of the ( ) were significantly enhanced in overexpression plants. In contrast, abscisic acid (ABA)-triggered ROS production and stomatal closure were reduced in plants. We found a positive-feedback regulation loop among three signaling components, CaM1, RPK1, and RbohF, which physically associate with each other. RPK1 positively regulates the expression of the gene, and the CaM1 protein, in turn, up-regulates gene expression. Interestingly, the expression of was down-regulated in , and mutants. We show that CaM1 positively regulates ROS production, leaf senescence, and ABA response in .

Plant molecular farming (PMF), or “pharming,” leverages plant cells or whole plants as expression systems to produce recombinant proteins for pharmaceuticals and other applications. This approach has emerged as a viable alternative to traditional platforms like Escherichia coli and mammalian cell lines, offering distinct advantages such as low production costs, high protein stability, and human-like post-translational modifications. However, the reliance on terrestrial plants as bioreactors poses challenges, including competition with food crops for agricultural resources and the risk of contaminating the food supply. As a result, identifying new host platforms for efficient recombinant protein production is a critical priority for advancing PMF. In this review, we highlight duckweeds-small, fast-growing aquatic monocots in the family Lemnaceae -as a promising alternative. Duckweeds offer advantages such as rapid growth, high biomass yield, and a rich metabolic profile, making them an attractive platform for recombinant protein production. We summarize recent developments in the use of duckweeds for PMF, including advancements in tissue culture, transformation techniques, and the expanding availability of genetic resources. Finally, we discuss remaining challenges and propose future directions for establishing duckweeds as a robust host platform in synthetic biology.

The root hair is a model system for understanding plant cell tip growth. As phosphatidylinositol 3-phosphate [PtdIns(3)P] has been shown in other plant cell types to regulate factors that affect root hair growth, including reactive oxygen species (ROS) levels, cytoskeleton, and endosomal movement, we hypothesized that PtdIns(3)P is also important for root hair elongation. The enzyme that generates PtdIns(3)P, phosphatidylinositol 3-kinase (PI3K), was expressed in root hair cells of transgenic plants containing the PI3K promoter:β-glucuronidase reporter construct. To obtain genetic evidence for the role of PtdIns(3)P in root hair elongation, we attempted to isolate Arabidopsis (Arabidopsis thaliana) mutant plants that did not express the gene VPS34 encoding the PI3K enzyme. However, the homozygous mutant was lethal due to gametophytic defects, and heterozygous plants were not discernibly different from wild-type plants. Alternatively, we made transgenic plants expressing the PtdIns(3)P-binding FYVE domain in the root hair cell to block signal transduction downstream of PtdIns(3)P. These transgenic plants had shorter root hairs and a reduced hair growth rate compared with wild-type plants. In addition, LY294002, a PI3K-specific inhibitor, inhibited root hair elongation but not initiation. In LY294002-treated root hair cells, endocytosis at the stage of final fusion of the late endosomes to the tonoplast was inhibited and ROS level decreased in a dose-dependent manner. Surprisingly, the LY294002 effects on ROS and root hair elongation were similar in rhd2 mutant plants, suggesting that RHD2 was not the major ROS generator in the PtdIns(3)P-mediated root hair elongation process. Collectively, these results suggest that PtdIns(3)P is required for maintenance of the processes essential for root hair cell elongation.

Phosphatidylinositol 3-kinase has been reported to be important for normal plant growth. To characterize the role of the enzyme further, we attempted to isolate Arabidopsis (Arabidopsis thaliana) plants that do not express the gene, but we could not recover homozygous mutant plants. The progeny of VPS34/vps34 heterozygous plants, harboring a T-DNA insertion, showed a segregation ratio of 1:1:0 for wild-type, heterozygous, and homozygous mutant plants, indicating a gametophytic defect. Genetic transmission analysis showed that the abnormal segregation ratio was due to failure to transmit the mutant allele through the male gametophyte. Microscopic observation revealed that 2-fold higher proportions of pollen grains in heterozygous plants than wild-type plants were dead or showed reduced numbers of nuclei. Many mature pollen grains from the heterozygous plants contained large vacuoles even until the mature pollen stage, whereas pollen from wild-type plants contained many small vacuoles beginning from the vacuolated pollen stage, which indicated that vacuoles in many of the heterozygous mutant pollen did not undergo normal fission after the first mitotic division. Taken together, our results suggest that phosphatidylinositol 3-kinase is essential for vacuole reorganization and nuclear division during pollen development.

Phosphatidic acid (PA) level increases during various stress conditions. However, the physiological roles of this lipid in stress response remain largely unknown. In this study, we report that PA induced leaf cell death and elevated the levels of reactive oxygen species (ROS) in the whole leaf and single cells. To further elucidate the mechanism of PA-induced cell death, we then examined whether Rho-related small G protein (ROP) 2, which enhanced ROS production in an in vitro assay, is involved in PA-induced ROS production and cell death. In response to PA, transgenic leaves of Arabidopsis expressing a constitutively active rop2 mutant exhibited earlier cell death and higher levels of ROS than wild type (WT), whereas those expressing a dominant-negative rop2 mutant exhibited later cell death and lower ROS. However, in the absence of exogenous PA, no spontaneous cell death or elevated ROS was observed in constitutively active rop2 plants, suggesting that the activation of ROP GTPase alone is insufficient to activate the ROP-mediated ROS generation pathway. These results suggest that PA modulates an additional factor required for the active ROP-mediated ROS generation pathway. Therefore, PA may be an important regulator of ROP-regulated ROS generation and the cell death process during various stress and defense responses of plants.