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Plant cells build nanofibrillar walls that are central to plant growth, morphogenesis and mechanics. Starting from simple sugars, three groups of polysaccharides, namely, cellulose, hemicelluloses and pectins, with very different physical properties are assembled by the cell to make a strong yet extensible wall. This Review describes the physics of wall growth and its regulation by cellular processes such as cellulose production by cellulose synthase, modulation of wall pH by plasma membrane H+-ATPase, wall loosening by expansin and signalling by plant hormones such as auxin and brassinosteroid. In addition, this Review discusses the nuanced roles, properties and interactions of cellulose, matrix polysaccharides and cell wall proteins and describes how wall stress and wall loosening cooperatively result in cell wall growth.Plant cells assemble a strong yet extensible primary cell wall consisting largely of polysaccharides. Emerging models of wall growth integrate physical properties such as mechanical strength and tension with cellular processes that govern wall loosening and expansion.

Economically important softwood from conifers is mainly composed of the polysaccharides cellulose, galactoglucomannan and xylan, and the phenolic polymer, lignin. The interactions between these polymers lead to wood mechanical strength and must be overcome in biorefining. Here, we use 13 C multidimensional solid-state NMR to analyse the polymer interactions in never-dried cell walls of the softwood, spruce. In contrast to some earlier softwood cell wall models, most of the xylan binds to cellulose in the two-fold screw conformation. Moreover, galactoglucomannan alters its conformation by intimately binding to the surface of cellulose microfibrils in a semi-crystalline fashion. Some galactoglucomannan and xylan bind to the same cellulose microfibrils, and lignin is associated with both of these cellulose-bound polysaccharides. We propose a model of softwood molecular architecture which explains the origin of the different cellulose environments observed in the NMR experiments. Our model will assist strategies for improving wood usage in a sustainable bioeconomy. Understanding the interactions between the constituents of the cell walls in wood is important for understanding the mechanical properties. Here, the authors report on a solid-state NMR study of never-dried softwood, noticing differences to previous reports and develop a model of softwood architecture.

The maturation of green fleshy fruit to become colourful and flavoursome is an important strategy for plant reproduction and dispersal. In tomato ( Solanum lycopersicum ) and many other species, fruit ripening is intimately linked to the biogenesis of chromoplasts, the plastids that are abundant in ripe fruit and specialized for the accumulation of carotenoid pigments. Chromoplasts develop from pre-existing chloroplasts in the fruit, but the mechanisms underlying this transition are poorly understood. Here, we reveal a role for the chloroplast-associated protein degradation (CHLORAD) proteolytic pathway in chromoplast differentiation. Knockdown of the plastid ubiquitin E3 ligase SP1, or its homologue SPL2, delays tomato fruit ripening, whereas overexpression of SP1 accelerates ripening, as judged by colour changes. We demonstrate that SP1 triggers broader effects on fruit ripening, including fruit softening, and gene expression and metabolism changes, by promoting the chloroplast-to-chromoplast transition. Moreover, we show that tomato SP1 and SPL2 regulate leaf senescence, revealing conserved functions of CHLORAD in plants. We conclude that SP1 homologues control plastid transitions during fruit ripening and leaf senescence by enabling reconfiguration of the plastid protein import machinery to effect proteome reorganization. The work highlights the critical role of chromoplasts in fruit ripening, and provides a theoretical basis for engineering crop improvements. Fruit ripening is a fine-tuned process involving wholesale changes to both the structure and metabolism of the fruit. Now, the CHLORAD proteolytic pathway is shown to regulate chromoplast development, thus altering the ripening process of tomato fruits.

ER-mitochondrial contact sites (EMCSs) are important for mitochondrial function. Here, we have identified a EMCS complex, comprising a family of uncharacterised mitochondrial outer membrane proteins, TRB1, TRB2, and the ER protein, VAP27-1. In Arabidopsis, there are three TraB family isoforms and the trb1/trb2 double mutant exhibits abnormal mitochondrial morphology, strong starch accumulation, and impaired energy metabolism, indicating that these proteins are essential for normal mitochondrial function. Moreover, TRB1 and TRB2 proteins also interact with ATG8 in order to regulate mitochondrial degradation (mitophagy). The turnover of depolarised mitochondria is significantly reduced in both trb1/trb2 and VAP27 mutants ( vap27-1,3,4,6) under mitochondrial stress conditions, with an increased population of dysfunctional mitochondria present in the cytoplasm. Consequently, plant recovery after stress is significantly perturbed, suggesting that TRB1-regulated mitophagy and ER-mitochondrial interaction are two closely related processes. Taken together, we ascribe a dual role to TraB family proteins which are component of the EMCS complex in eukaryotes, regulating both interaction of the mitochondria to the ER and mitophagy. Contact sites permit the exchange of signals and materials between organelles. Here the authors characterize a protein complex associated with an ER-mitochondrial contact site in Arabidopsis that is required for mitochondrial function and mitochondrial turnover

Morphogenesis of multicellular organs requires coordination of cellular growth. In plants, cell growth is determined by turgor pressure and the mechanical properties of the cell wall, which also glues cells together. Because plants have to integrate tissue-scale mechanical stresses arising through growth in a fixed tissue topology, they need to monitor cell wall mechanical status and adapt growth accordingly. Molecular factors have been identified, but whether cell geometry contributes to wall sensing is unknown. Here we propose that plant cell edges act as cell-wall-sensing domains during growth. We describe two Receptor-Like Proteins, RLP4 and RLP4-L1, which occupy a unique polarity domain at cell edges established through a targeted secretory transport pathway. We show that RLP4s associate with the cell wall at edges via their extracellular domain, respond to changes in cell wall mechanics and contribute to directional growth control in Arabidopsis .

The energy sensor AMP-activated protein kinase (AMPK) can activate autophagy when cellular energy production becomes compromised. However, the degree to which nutrient sensing impinges on the autophagosome closure remains unknown. Here, we provide the mechanism underlying a plant unique protein FREE1, upon autophagy-induced SnRK1α1-mediated phosphorylation, functions as a linkage between ATG conjugation system and ESCRT machinery to regulate the autophagosome closure upon nutrient deprivation. Using high-resolution microscopy, 3D-electron tomography, and protease protection assay, we showed that unclosed autophagosomes accumulated in free1 mutants. Proteomic, cellular and biochemical analysis revealed the mechanistic connection between FREE1 and the ATG conjugation system/ESCRT-III complex in regulating autophagosome closure. Mass spectrometry analysis showed that the evolutionary conserved plant energy sensor SnRK1α1 phosphorylates FREE1 and recruits it to the autophagosomes to promote closure. Mutagenesis of the phosphorylation site on FREE1 caused the autophagosome closure failure. Our findings unveil how cellular energy sensing pathways regulate autophagosome closure to maintain cellular homeostasis. Nutrient sensing impinges on the autophagosome biogenesis. Here, the authors present evidence that plant energy sensing regulates the autophagosome closure by modulating the phosphorylation status and localization of the ESCRT machinery.

Properly patterned deposition of cell wall polymers is prerequisite for the morphogenesis of plant cells. A cortical microtubule array guides the two-dimensional pattern of cell wall deposition. Yet, the mechanism underlying the three-dimensional patterning of cell wall deposition is poorly understood. In metaxylem vessels, cell wall arches are formed over numerous pit membranes, forming highly organized three-dimensional cell wall structures. Here, we show that the microtubule-associated proteins, MAP70-5 and MAP70-1, regulate arch development. The map70-1 map70-5 plants formed oblique arches in an abnormal orientation in pits. Microtubules fit the aperture of developing arches in wild-type cells, whereas microtubules in map70-1 map70-5 cells extended over the boundaries of pit arches. MAP70 caused the bending and bundling of microtubules. These results suggest that MAP70 confines microtubules within the pit apertures by altering the physical properties of microtubules, thereby directing the growth of pit arches in the proper orientation. This study provides clues to understanding how plants develop three-dimensional structure of cell walls. In plant metaxylem, three-dimensional cell wall arches are formed over pit membranes. Here, the authors show that the microtubule-associated proteins, MAP70-5 and MAP70-1, confine microtubules within the pit aperture and direct growth of pit arches in the proper orientation.

ZEITLUPE (ZTL), a photoreceptor with E3 ubiquitin ligase activity, communicates end-of-day light conditions to the plant circadian clock. It still remains unclear how ZTL protein accumulates in the light but does not destabilize target proteins before dusk. Two deubiquitylating enzymes, UBIQUITIN-SPECIFIC PROTEASE 12 and 13 (UBP12 and UBP13), which regulate clock period and protein ubiquitylation in a manner opposite to ZTL, associate with the ZTL protein complex. Here we demonstrate that the ZTL interacting partner, GIGANTEA (GI), recruits UBP12 and UBP13 to the ZTL photoreceptor complex. We show that loss of UBP12 and UBP13 reduces ZTL and GI protein levels through a post-transcriptional mechanism. Furthermore, a ZTL target protein is unable to accumulate to normal levels in ubp mutants. This demonstrates that the ZTL photoreceptor complex contains both ubiquitin-conjugating and -deconjugating enzymes, and that these two opposing enzyme types are necessary for circadian clock pacing. This shows that deubiquitylating enzymes are a core element of circadian clocks, conserved from plants to animals. The daily accumulation of the ZEITLUPE (ZTL) photoreceptor/E3 ubiquitin ligase relies on a light-dependent interaction with GIGANTEA (GI). Here the authors show that GI recruits two deubiquitylases to help stabilize the ZTL-GI complex during the day and likely counterbalance the activity of ZTL.

Patterned cell wall deposition is crucial for cell shapes and functions. In Arabidopsis xylem vessels, ROP11 GTPase locally inhibits cell wall deposition through microtubule disassembly, inducing pits in cell walls. Here, we show that an additional ROP signaling pathway promotes cell wall growth at pit boundaries. Two proteins, Boundary of ROP domain1 (BDR1) and Wallin (WAL), localize to pit boundaries and regulate cell wall growth. WAL interacts with F-actin and promotes actin assembly at pit boundaries while BDR1 is a ROP effector. BDR1 interacts with WAL, suggesting that WAL could be recruited to the plasma membrane by a ROP-dependent mechanism. These results demonstrate that BDR1 and WAL mediate a ROP-actin pathway that shapes pit boundaries. The study reveals a distinct machinery in which two closely associated ROP pathways oppositely regulate cell wall deposition patterns for the establishment of tiny but highly specialized cell wall domains. Cell wall pits allow movement of water between xylem vessels and are formed via Rho-GTPase mediated signaling that leads to local microtubule disassembly. Here, Sugiyama et al. show that an additional Rho-GTPase pathway controls cell wall deposition and actin dynamics to form pit boundaries.