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Significance Contamination of water and foods with arsenic (As) poses a threat to millions of people worldwide. Because the rice grain is the major source of As intake, reducing the transfer of As from soil to the grain is a pressing public health issue. We found that a member of the Oryza sativa C-type ATP-binding cassette transporter (OsABCC) family, OsABCC1, detoxifies As and reduces the amount of As in the rice grain. OsABCC1 in the upper nodes of rice plants restricts the distribution of As to the grain by sequestering it in the vacuoles of the phloem companion cells of diffuse vascular bundles directly connected to the grain. Our work suggests a strategy for limiting As accumulation in rice grains and thereby reducing human As exposure. Arsenic (As) is a chronic poison that causes severe skin lesions and cancer. Rice ( Oryza sativa L.) is a major dietary source of As; therefore, reducing As accumulation in the rice grain and thereby diminishing the amount of As that enters the food chain is of critical importance. Here, we report that a member of the Oryza sativa C-type ATP-binding cassette (ABC) transporter (OsABCC) family, OsABCC1, is involved in the detoxification and reduction of As in rice grains. We found that OsABCC1 was expressed in many organs, including the roots, leaves, nodes, peduncle, and rachis. Expression was not affected when plants were exposed to low levels of As but was up-regulated in response to high levels of As. In both the basal nodes and upper nodes, which are connected to the panicle, OsABCC1 was localized to the phloem region of vascular bundles. Furthermore, OsABCC1 was localized to the tonoplast and conferred phytochelatin-dependent As resistance in yeast. Knockout of OsABCC1 in rice resulted in decreased tolerance to As, but did not affect cadmium toxicity. At the reproductive growth stage, the As content was higher in the nodes and in other tissues of wild-type rice than in those of OsABCC1 knockout mutants, but was significantly lower in the grain. Taken together, our results indicate that OsABCC1 limits As transport to the grains by sequestering As in the vacuoles of the phloem companion cells of the nodes in rice.

Apical growth in plants initiates upon seed germination, whereas radial growth is primed only during early ontogenesis in procambium cells and activated later by the vascular cambium 1 . Although it is not known how radial growth is organized and regulated in plants, this system resembles the developmental competence observed in some animal systems, in which pre-existing patterns of developmental potential are established early on 2 , 3 . Here we show that in Arabidopsis the initiation of radial growth occurs around early protophloem-sieve-element cell files of the root procambial tissue. In this domain, cytokinin signalling promotes the expression of a pair of mobile transcription factors—PHLOEM EARLY DOF 1 (PEAR1) and PHLOEM EARLY DOF 2 (PEAR2)—and their four homologues (DOF6, TMO6, OBP2 and HCA2), which we collectively name PEAR proteins. The PEAR proteins form a short-range concentration gradient that peaks at protophloem sieve elements, and activates gene expression that promotes radial growth. The expression and function of PEAR proteins are antagonized by the HD-ZIP III proteins, well-known polarity transcription factors 4 —the expression of which is concentrated in the more-internal domain of radially non-dividing procambial cells by the function of auxin, and mobile miR165 and miR166 microRNAs. The PEAR proteins locally promote transcription of their inhibitory HD-ZIP III genes, and thereby establish a negative-feedback loop that forms a robust boundary that demarks the zone of cell division. Taken together, our data establish that during root procambial development there exists a network in which a module that links PEAR and HD-ZIP III transcription factors integrates spatial information of the hormonal domains and miRNA gradients to provide adjacent zones of dividing and more-quiescent cells, which forms a foundation for further radial growth. Radial growth in the roots of Arabidopsis , which is mediated by gene expression activated by the mobile PEAR1 and PEAR2 transcription factors, is initiated around protophloem-sieve-element cell files of procambial tissue.

Land plants thrive in soils showing vastly different properties and environmental stresses 1 . Root systems can adapt to contrasting soil conditions and stresses, yet how their responses are programmed at the individual cell scale remains unclear. Using single-cell RNA sequencing and spatial transcriptomic approaches, we showed major expression changes in outer root cell types when comparing the single-cell transcriptomes of rice roots grown in gel versus soil conditions. These tissue-specific transcriptional responses are related to nutrient homeostasis, cell wall integrity and defence in response to heterogeneous soil versus homogeneous gel growth conditions. We also demonstrate how the model soil stress, termed compaction, triggers expression changes in cell wall remodelling and barrier formation in outer and inner root tissues, regulated by abscisic acid released from phloem cells. Our study reveals how root tissues communicate and adapt to contrasting soil conditions at single-cell resolution. Single-cell RNA sequencing and spatial transcriptomic approaches reveal major expression changes in outer root cell types when grown in soil versus gel conditions, and also uncover how root tissues communicate and adapt to contrasting soil conditions at single-cell resolution.

Auxin influences plant development through several distinct concentration-dependent effects 1 . In the Arabidopsis root tip, polar auxin transport by PIN-FORMED (PIN) proteins creates a local auxin accumulation that is required for the maintenance of the stem-cell niche 2 – 4 . Proximally, stem-cell daughter cells divide repeatedly before they eventually differentiate. This developmental gradient is accompanied by a gradual decrease in auxin levels as cells divide, and subsequently by a gradual increase as the cells differentiate 5 , 6 . However, the timing of differentiation is not uniform across cell files. For instance, developing protophloem sieve elements (PPSEs) differentiate as neighbouring cells still divide. Here we show that PPSE differentiation involves local steepening of the post-meristematic auxin gradient. BREVIS RADIX (BRX) and PROTEIN KINASE ASSOCIATED WITH BRX (PAX) are interacting plasma-membrane-associated, polarly localized proteins that co-localize with PIN proteins at the rootward end of developing PPSEs. Both brx and pax mutants display impaired PPSE differentiation. Similar to other AGC-family kinases, PAX activates PIN-mediated auxin efflux, whereas BRX strongly dampens this stimulation. Efficient BRX plasma-membrane localization depends on PAX, but auxin negatively regulates BRX plasma-membrane association and promotes PAX activity. Thus, our data support a model in which BRX and PAX are elements of a molecular rheostat that modulates auxin flux through developing PPSEs, thereby timing PPSE differentiation. The coordinated action of BREVIS RADIX (BRX) and PROTEIN KINASE ASSOCIATED WITH BRX (PAX) regulates auxin flux during the development of protophloem sieve elements in the root meristem of Arabidopsis .

A major question in plant biology concerns the specification and functional differentiation of cell types. This is in the context of constraints imposed by networks of cell walls that both adhere cells and contribute to the form and function of developing organs. Here, we report the identification of a glycan epitope that is specific to phloem sieve element cell walls in several systems. A monoclonal antibody, designated LM26, binds to the cell wall of phloem sieve elements in stems of Arabidopsis (Arabidopsis thaliana), Miscanthus × giganteus, and notably sugar beet (Beta vulgaris) roots where phloem identification is an important factor for the study of phloem unloading of Suc. Using microarrays of synthetic oligosaccharides, the LM26 epitope has been identified as a 𝛽-1,6-galactosyl substitution of 𝛽-1,4-galactan requiring more than three backbone residues for optimized recognition. This branched galactan structure has previously been identified in garlic (Allium sativum) bulbs in which the LM26 epitope is widespread throughout most cell walls including those of phloem cells. Garlic bulb cell wall material has been used to confirm the association of the LM26 epitope with cell wall pectic rhamnogalacturonan-I polysaccharides. In the phloem tissues of grass stems, the LM26 epitope has a complementary pattern to that of the LM5 linear 𝛽-1,4-galactan epitope, which is detected only in companion cell walls. Mechanical probing of transverse sections of M. x giganteus stems and leaves by atomic force microscopy indicates that phloem sieve element cell walls have a lower indentation modulus (indicative of higher elasticity) than companion cell walls.

Cell differentiation is a complex process involving multiple steps, from initial cell fate specification to final differentiation. Procambial/cambial cells, which act as vascular stem cells, differentiate into both xylem and phloem cells during vascular development. Recent studies have identified regulatory cascades for xylem differentiation. However, the molecular mechanism underlying phloem differentiation is largely unexplored due to technical challenges. Here, we established an ectopic induction system for phloem differentiation named Vascular Cell Induction Culture System Using Arabidopsis Leaves (VISUAL). Our results verified similarities between VISUAL-induced Arabidopsis thaliana phloem cells and in vivo sieve elements. We performed network analysis using transcriptome data with VISUAL to dissect the processes underlying phloem differentiation, eventually identifying a factor involved in the regulation of the master transcription factor gene APL. Thus, our culture system opens up new avenues not only for genetic studies of phloem differentiation, but also for future investigations of multidirectional differentiation from vascular stem cells.

Stems that develop secondary vascular tissue (i.e. xylem and phloem derived from the vascular cambium) have unique demands on transport owing to their mass and longevity. Transport of water and assimilates must occur over long distances, while the increasing physical separation of xylem and phloem requires radial transport. Developing secondary tissue is itself a strong sink positioned between xylem and phloem along the entire length of the stem, and the integrity of these transport tissues must be maintained and protected for years if not decades. Parenchyma cells form an interconnected three-dimensional lattice throughout secondary xylem and phloem and perform critical roles in all of these tasks, yet our understanding of their physiology, the nature of their symplasmic connections, and their activity at the symplast–apoplast interface is very limited. This review highlights key historical work as well as current research on the structure and function of parenchyma in secondary vascular tissue in the hopes of spurring renewed interest in this area, which has important implications for whole-plant transport processes and resource partitioning.

The plant vascular system, composed of xylem and phloem, evolved to connect plant organs and transport various molecules between them. During the post‐embryonic growth, these conductive tissues constitutively form from cells that are derived from a lateral meristem, commonly called procambium and cambium. Procambium/cambium contains pluripotent stem cells and provides a microenvironment that maintains the stem cell population. Because vascular plants continue to form new tissues and organs throughout their life cycle, the formation and maintenance of stem cells are crucial for plant growth and development. In this decade, there has been considerable progress in understanding the molecular control of the organization and maintenance of stem cells in vascular plants. Noticeable advance has been made in elucidating the role of transcription factors and major plant hormones in stem cell maintenance and vascular tissue differentiation. These studies suggest the shared regulatory mechanisms among various types of plant stem cell pools. In this review, we focus on two aspects of stem cell function in the vascular cambium, cell proliferation and cell differentiation. Helariutta and colleagues review recent advances in our understanding of plant stem cell organization and maintenance and its regulation by transcription factors and plant hormones.

The phloem performs essential systemic functions in tracheophytes, yet little is known about its molecular genetic specification. Here we show that application of the peptide ligand CLAVATA3/EMBRYO SURROUNDING REGION 45 (CLE45) specifically inhibits specification of protophloem in Arabidopsis roots by locking the sieve element precursor cell in its preceding developmental state. CLE45 treatment, as well as viable transgenic expression of a weak CLE45G6T variant, interferes not only with commitment to sieve element fate but also with the formative sieve element precursor cell division that creates protophloem and metaphloem cell files. However, the absence of this division appears to be a secondary effect of discontinuous sieve element files and subsequent systemically reduced auxin signaling in the root meristem. In the absence of the formative sieve element precursor cell division, metaphloem identity is seemingly adopted by the normally procambial cell file instead, pointing to possibly independent positional cues for metaphloem formation. The protophloem formation and differentiation defects in brevis radix (brx) and octopus (ops) mutants are similar to those observed in transgenic seedlings with increased CLE45 activity and can be rescued by loss of function of a putative CLE45 receptor, BARELY ANY MERISTEM 3 (BAM3). Conversely, a dominant gain-of-function ops allele or mild OPS dosage increase suppresses brx defects and confers CLE45 resistance. Thus, our data suggest that delicate quantitative interplay between the opposing activities of BAM3-mediated CLE45 signals and OPS-dependent signals determines cellular commitment to protophloem sieve element fate, with OPS acting as a positive, quantitative master regulator of phloem fate.

MicroRNA156 (miR156) functions in maintaining the juvenile phase in plants. However, the mobility of this microRNA has not been demonstrated. So far, only three microRNAs, miR399, miR395, and miR172, have been shown to be mobile. We demonstrate here that miR156 is a potential graft-transmissible signal that affects plant architecture and tuberization in potato (Solanum tuberosum). Under tuber-noninductive (long-day) conditions, miR156 shows higher abundance in leaves and stems, whereas an increase in abundance of miR156 has been observed in stolons under tuber-inductive (short-day) conditions, indicative of a photoperiodic control. Detection of miR156 in phloem cells of wild-type plants and mobility assays in heterografts suggest that miR156 is a graft-transmissible signal. This movement was correlated with changes in leaf morphology and longer trichomes in leaves. Overexpression of miR156 in potato caused a drastic phenotype resulting in altered plant architecture and reduced tuber yield. miR156 overexpression plants also exhibited altered levels of cytokinin and strigolactone along with increased levels of LONELY GUY1 and StCyclin D3.1 transcripts as compared with wild-type plants. RNA ligase-mediated rapid amplification of complementary DNA ends analysis validated SQUAMOSA PROMOTER BINDING-LIKE3 (StSPL3), StSPL6, StSPL9, StSPL13, and StLIGULELESS1 as targets of miR156. Gel-shift assays indicate the regulation of miR172 by miR156 through StSPL9. miR156-resistant SPL9 overexpression lines exhibited increased miR172 levels under a short-day photoperiod, supporting miR172 regulation via the miR156-SPL9 module. Overall, our results strongly suggest that miR156 is a phloem-mobile signal regulating potato development.