The TGN/EE SNARE protein SYP61 and the ubiquitin ligase ATL31 cooperatively regulate plant responses to carbon/nitrogen conditions in Arabidopsis.

Ubiquitination is a post-translational modification involving the reversible attachment of the small protein ubiquitin to a target protein. Ubiquitination is involved in numerous cellular processes, including the membrane trafficking of cargo proteins. However, the ubiquitination of the trafficking machinery components and their involvement in environmental responses are not well understood. Here, we report that the Arabidopsis thaliana trans-Golgi network/early endosome localized SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) protein SYP61 interacts with the transmembrane ubiquitin ligase ATL31, a key regulator of resistance to disrupted carbon (C)/nitrogen/(N)-nutrient conditions. SYP61 is a key component of membrane trafficking in Arabidopsis. The subcellular localization of ATL31 was disrupted in knockdown mutants of SYP61, and the insensitivity of ATL31-overexpressing plants to high C/low N-stress was repressed in these mutants, suggesting that SYP61 and ATL31 cooperatively function in plant responses to nutrient stress. SYP61 is ubiquitinated in plants, and its ubiquitination level is upregulated under low C/high N-nutrient conditions. These findings provide important insights into the ubiquitin signaling and membrane trafficking machinery in plants.

The mRNA-binding proteome of a critical phase transition during Arabidopsis seed germination.

Arabidopsis thaliana seed germination is marked by extensive translational control at two critical phase transitions. The first transition refers to the start of hydration, the hydration translational shift. The second shift, the germination translational shift (GTS) is the phase between testa rupture and radicle protrusion at which the seed makes the all or nothing decision to germinate. The mechanism behind the translational regulation at these phase transitions is unknown. RNA binding proteins (RBPs) are versatile players in the post-transcriptional control of messenger RNAs (mRNAs) and as such candidates for regulating translation during seed germination. Here, we report the mRNA binding protein repertoire of seeds during the GTS. Thirty seed specific RBPs and 22 dynamic RBPs were identified during the GTS, like the putative RBP Vacuolar ATPase subunit A and RBP HSP101. Several stress granule markers were identified in this study, which suggests that seeds are prepared to quickly adapt the translation of specific mRNAs in response to changes in environmental conditions during the GTS. Taken together this study provides a detailed insight into the world of RBPs during seed germination and their possible regulatory role during this developmentally regulated process.

External Mechanical Cues Reveal a Katanin-Independent Mechanism behind Auxin-Mediated Tissue Bending in Plants.

Tissue folding is a central building block of plant and animal morphogenesis. In dicotyledonous plants, hypocotyl folds to form hooks after seedling germination that protects their aerial stem cell niche during emergence from soil. Auxin response factors and auxin transport are reported to play a key role in this process. Here, we show that the microtubule-severing enzyme katanin contributes to hook formation. However, by exposing hypocotyls to external mechanical cues mimicking the natural soil environment, we reveal that auxin response factors ARF7/ARF19, auxin influx carriers, and katanin are dispensable for apical hook formation, indicating that these factors primarily play the role of catalyzers of tissue bending in the absence of external mechanical cues. Instead, our results reveal the key roles of the non-canonical TMK-mediated auxin pathway, PIN efflux carriers, and cellulose microfibrils as components of the core pathway behind hook formation in the presence or absence of external mechanical cues.

Interplay between Cell Wall and Auxin Mediates the Control of Differential Cell Elongation during Apical Hook Development.

Differential growth plays a crucial role during morphogenesis [1-3]. In plants, development occurs within mechanically connected tissues, and local differences in cell expansion lead to deformations at the organ level, such as buckling or bending [4, 5]. During early seedling development, bending of hypocotyl by differential cell elongation results in apical hook structure that protects the shoot apical meristem from being damaged during emergence from the soil [6, 7]. Plant hormones participate in apical hook development, but not how they mechanistically drive differential growth [8]. Here, we present evidence of interplay between hormonal signals and cell wall in auxin-mediated differential cell elongation using apical hook development as an experimental model. Using genetic and cell biological approaches, we show that xyloglucan (a major primary cell wall component) mediates asymmetric mechanical properties of epidermal cells required for hook development. The xxt1 xxt2 mutant, deficient in xyloglucan [9], displays severe defects in differential cell elongation and hook development. Analysis of xxt1 xxt2 mutant reveals a link between cell wall and transcriptional control of auxin transporters PINFORMEDs (PINs) and AUX1 crucial for establishing the auxin response maxima required for preferential repression of elongation of the cells on the inner side of the hook. Genetic evidence identifies auxin response factor ARF2 as a negative regulator acting downstream of xyloglucan-dependent control of hook development and transcriptional control of polar auxin transport. Our results reveal a crucial feedback process between the cell wall and transcriptional control of polar auxin transport, underlying auxin-dependent control of differential cell elongation in plants.

A seaweed's response to a warming world.

For plants, climate change comes with more challenging facets than just increasing temperature. While terrestrial forests are suffering from erratic rainfall, drought and wildfires, marine vegetation is under a different kind of threat. Rapidly melting polar ice caps are causing a surge of freshwater in the seas, lowering the salinity near coastlines. For marine plants adapted to grow in seawater, hyposalinity can be a serious detriment to growth. To assess the possible impact of climate change on marine flora, Li et al. (2019) explored the physiological and transcriptomic response of the kelp Saccharina latissima to increased temperature and hyposaline conditions.

A step closer toward making many from the one.

Somatic embryogenesis (SE) is a key technique used in plant biotechnology. The complex molecular changes associated with SE are uncharacterized in many crop species, and therefore, logically, formulating the culture conditions that induce these changes is difficult. In a study published in this issue of Physiologia Plantarum, Marimuthu et al. (2019) performed a proteomic study to characterize the molecular reprogramming during SE of an elite banana cultivar. Based on the results, they could customize culture conditions for optimal SE efficiency in several cultivars.

Strawberries under salt stress: ALA and ROS to the rescue.

Generating salt-tolerant plants that can cope with increasing soil salinity is a major goal of crop-breeding programs worldwide. Together with genetic approaches, research efforts are focusing on finding chemical modulators of salt tolerance. The exogenous application of 5-aminolevulinic acid (ALA) has been shown to improve salt tolerance in diverse crop species, but its mechanism of action is not properly understood. Wu et al. (2019) report that ALA treatment enhances reactive oxygen species (ROS) production in the roots of salt-stressed strawberry plants. Activation of several key ion transporters downstream to the ROS signal helps to sequester the toxic Na+ ions in the roots and protects the shoots against salt stress.

Finding the fragrance genes of wintersweet.

Fragrant flowers emit a complex mixture of volatile organic compounds (VOCs) that we perceive as pleasant. While we know the chemical nature of these volatiles, the molecular traits that regulate their biosynthesis are poorly understood. In this issue, Tian et al. (2019) compare the transcriptomic and proteomic profiles of a scented genotype of wintersweet (Chimonanthus praecox) with a non-scented one. By correlating the differential gene expression profile with the observed differences in VOC profiles, they attempt to identify the genes that regulate the fragrance of wintersweet.

Rho-of-plant activated root hair formation requires Arabidopsis YIP4a/b gene function.

Root hairs are protrusions from root epidermal cells with crucial roles in plant soil interactions. Although much is known about patterning, polarity and tip growth of root hairs, contributions of membrane trafficking to hair initiation remain poorly understood. Here, we demonstrate that the trans-Golgi network-localized YPT-INTERACTING PROTEIN 4a and YPT-INTERACTING PROTEIN 4b (YIP4a/b) contribute to activation and plasma membrane accumulation of Rho-of-plant (ROP) small GTPases during hair initiation, identifying YIP4a/b as central trafficking components in ROP-dependent root hair formation.

Lazy rice in space: gravity regulates helical movement in plants.

Circumnutation, the helical movement of organs, has been observed in diverse species of land plants. Whether circumnutation arises purely from internal growth oscillations or as a response to exogenous forces such as gravity is a subject of active debate. By observing rice seedlings grown under microgravity at the International Space Station (ISS) and analyzing the agravitropic lazy1 mutant, Kobayashi et al. (2019) propose gravity as the causal force that regulates circumnutation of rice coleoptiles.

Bananas tackling drought and heat - with DREBs and more.

With the changing climate, crops are facing mounting threats from multiple abiotic stresses, and studies that assess the response of plants to combinations, rather than to individual, abiotic stresses are becoming increasingly relevant. Bananas are one of the most globally important and popular food crops and their production is threatened by increasing heat and diminishing rainfall in tropical and subtropical regions. In pursuit of effective stress management strategies, Jangale et al. (2019) look into the physiological and molecular responses of banana plants to combined heat and drought stresses.

Parasitic worms hijack key plant protein to build their nest.

Parasitic nematode worms infect a variety of crop plants worldwide. Roots infected by these worms start to look rather unsavory - with knot like tumors (galls) developing all over them. At the core of each gall, a worm matures and lays its eggs. Olmo et al. (2018) looked into the developmental reprogramming that leads to gall formation and found an Arabidopsis protein to be a necessary component in this process.

Vesicular trafficking and salinity responses in plants.

Research spanning three decades has demonstrated that vesicles pinch off from the plasma membrane and traffic through the cytoplasm of plant cells, much as previously reported in animal cells. Although the well-conserved clathrin-mediated mechanism of endocytosis has been well characterized, relatively little is known about clathrin-independent pathways in plants. Modulation of endocytosis by both physical stimuli and chemical ligands has been reported in plants. Here, we review the effect of salinity-one of the most deleterious environmental assaults-on endocytosis and intracellular trafficking.

Salt-induced remodeling of spatially restricted clathrin-independent endocytic pathways in Arabidopsis root.

Endocytosis is a ubiquitous cellular process that is characterized well in animal cells in culture but poorly across intact, functioning tissue. Here, we analyze endocytosis throughout the Arabidopsis thaliana root using three classes of probes: a lipophilic dye, tagged transmembrane proteins, and a lipid-anchored protein. We observe a stratified distribution of endocytic processes. A clathrin-dependent endocytic pathway that internalizes transmembrane proteins functions in all cell layers, while a sterol-dependent, clathrin-independent pathway that takes up lipid and lipid-anchored proteins but not transmembrane proteins is restricted to the epidermal layer. Saline stress induces a third pathway that is clathrin-independent, nondiscriminatory in its choice of cargo, and operates across all layers of the root. Concomitantly, small acidic compartments in inner cell layers expand to form larger vacuole-like structures. Plants lacking function of the Rab-GEF (guanine nucleotide exchange factor) VPS9a (vacuolar protein sorting 9A) neither induce the third endocytic pathway nor expand the vacuolar system in response to salt stress. The plants are also hypersensitive to salt. Thus, saline stress reconfigures clathrin-independent endocytosis and remodels endomembrane systems, forming large vacuoles in the inner cell layers, both processes correlated by the requirement of VPS9a activity.