Oscillating Aquaporin Phosphorylation and 14-3-3 Proteins Mediate the Circadian Regulation of Leaf Hydraulics

Modulation of morphogenesis and metabolism by plant cell biomechanics: from model plants to traditional herbs

The quality of traditional herbs depends on organ morphogenesis and the accumulation of active pharmaceutical ingredients. While recent research highlights the significance of cell mechanobiology in model plant morphogenesis, our understanding of mechanical signal initiation and transduction in traditional herbs remains incomplete. Recent studies reveal a close correlation between cell wall (CW) biosynthesis and active ingredient production, yet the role of cell mechanics in balancing morphogenesis and secondary metabolism is often overlooked. This review explores how the cell wall, plasma membrane, cytoskeleton, and vacuole collaborate to regulate cell mechanics and respond to mechanical changes. We propose CW biosynthesis as a hub in connecting cell mechanics with secondary metabolism and emphasize that understanding the relationship between mechanical remodeling and secondary metabolism could provide new insights into plant cell mechanobiology and the breeding of high-quality herbs.

The Big, Mysterious World of Plant 14-3-3 Proteins

14-3-3 is a family of small regulatory proteins found exclusively in eukaryotic organisms. They selectively bind to phosphorylated molecules of partner proteins and regulate their functions. 14-3-3 proteins were first characterized in the mammalian brain approximately 60 years ago and then found in plants, 30 years later. The multifunctionality of 14-3-3 proteins is exemplified by their involvement in coordination of protein kinase cascades in animal brain and regulation of flowering, growth, metabolism, and immunity in plants. Despite extensive studies of this diverse and complex world of plant 14-3-3 proteins, our understanding of functions of these enigmatic molecules is fragmentary and unsystematic. The results of studies are often contradictory and many questions remain unanswered, including biochemical properties of 14-3-3 isoforms, structure of protein-protein complexes, and direct mechanisms by which 14-3-3 proteins influence the functions of their partners in plants. Although many plant genes coding for 14-3-3 proteins have been identified, the isoforms for in vivo and in vitro studies are often selected at random. This rather limited approach is partly due to an exceptionally large number and variety of 14-3-3 homologs in plants and erroneous a priori assumptions on the equivalence of certain isoforms. The accumulated results provide an extensive but rather fragmentary picture, which poses serious challenges for making global generalizations. This review is aimed to demonstrate the diversity and scope of studies of the functions of plant 14-3-3 proteins, as well as to identify areas that require further systematic investigation and close scientific attention.

Advances in the Structure, Function, and Regulatory Mechanism of Plant Plasma Membrane Intrinsic Proteins

Plasma membrane intrinsic proteins (PIPs), as members of the aquaporin (AQPs) family, can transport not only water but also urea, CO2, H2O2, metal ions, and trace elements. They are crucial for maintaining water balance, substance transport, and responding to various stresses. This article delves into the structure, function, response mechanism, molecular mechanism, and regulatory mechanism of PIPs as a result of biological and abiotic stresses. It also summarizes current research trends surrounding PIPs and highlights potential research directions for further exploration. The aim is to assist researchers in related fields in gaining a more comprehensive understanding and precise insight into the advancements in PIP research.

Mechanistic Insights into the Function of 14-3-3 Proteins as Negative Regulators of Brassinosteroid Signaling in Arabidopsis

Brassinosteroids (BRs) are vital plant steroid hormones sensed at the cell surface by a membrane signaling complex comprising the receptor kinase BRI1 and a SERK family co-receptor kinase. Activation of this complex lead to dissociation of the inhibitor protein BKI1 from the receptor and to differential phosphorylation of BZR1/BES1 transcription factors by the glycogen synthase kinase 3 protein BIN2. Many phosphoproteins of the BR signaling pathway, including BRI1, SERKs, BKI1 and BZR1/BES1 can associate with 14-3-3 proteins. In this study, we use quantitative ligand binding assays to define the minimal 14-3-3 binding sites in the N-terminal lobe of the BRI1 kinase domain, in BKI1, and in BZR1 from Arabidopsis thaliana. All three motifs require to be phosphorylated to specifically bind 14-3-3s with mid- to low-micromolar affinity. BR signaling components display minimal isoform preference within the 14-3-3 non-ε subgroup. 14-3-3λ and 14-3-3 ω isoform complex crystal structures reveal that BKI1 and BZR1 bind as canonical type II 14-3-3 linear motifs. Disruption of key amino acids in the phosphopeptide binding site through mutation impairs the interaction of 14-3-3λ with all three linear motifs. Notably, quadruple loss-of-function mutants from the non-ε group exhibit gain-of-function BR signaling phenotypes, suggesting a role for 14-3-3 proteins as overall negative regulators of the BR pathway. Collectively, our work provides further mechanistic and genetic evidence for the regulatory role of 14-3-3 proteins at various stages of the BR signaling cascade.

Unsaturation in the air spaces of leaves and its implications

Modern plant physiological theory stipulates that the resistance to water movement from plants to the atmosphere is overwhelmingly dominated by stomata. This conception necessitates a corollary assumption-that the air spaces in leaves must be nearly saturated with water vapour; that is, with a relative humidity that does not decline materially below unity. As this idea became progressively engrained in scientific discourse and textbooks over the last century, observations inconsistent with this corollary assumption were occasionally reported. Yet, evidence of unsaturation gained little traction, with acceptance of the prevailing framework motivated by three considerations: (1) leaf water potentials measured by either thermocouple psychrometry or the Scholander pressure chamber are largely consistent with the framework; (2) being able to assume near saturation of intercellular air spaces was transformational to leaf gas exchange analysis; and (3) there has been no obvious mechanism to explain a variable, liquid-phase resistance in the leaf mesophyll. Here, we review the evidence that refutes the assumption of universal, near saturation of air spaces in leaves. Refining the prevailing paradigm with respect to this assumption provides opportunities for identifying and developing mechanisms for increased plant productivity in the face of increasing evaporative demand imposed by global climate change.

Tubulin participates in establishing protoxylem vessel reinforcement patterns and hydraulic conductivity in maize

Water transportation to developing tissues relies on the structure and function of plant xylem cells. Plant microtubules govern the direction of cellulose microfibrils and guide secondary cell wall formation and morphogenesis. However, the relevance of microtubule-determined xylem wall thickening patterns in plant hydraulic conductivity remains unclear. In the present study, we identified a maize (Zea mays) semi-dominant mutant, designated drought-overly-sensitive1 (ZmDos1), the upper leaves of which wilted even when exposed to well-watered conditions during growth; the wilting phenotype was aggravated by increased temperatures and decreased humidity. Protoxylem vessels in the stem and leaves of the mutant showed altered thickening patterns of the secondary cell wall (from annular to spiral), decreased inner diameters, and limited water transport efficiency. The causal mutation for this phenotype was found to be a G-to-A mutation in the maize gene α-tubulin4, resulting in a single amino acid substitution at position 196 (E196K). Ectopic expression of the mutant α-tubulin4 in Arabidopsis (Arabidopsis thaliana) changed the orientation of microtubule arrays, suggesting a determinant role of this gene in microtubule assembly and secondary cell wall thickening. Our findings suggest that the spiral wall thickenings triggered by the α-tubulin mutation are stretched during organ elongation, causing a smaller inner diameter of the protoxylem vessels and affecting water transport in maize. This study underscores the importance of tubulin-mediated protoxylem wall thickening in regulating plant hydraulics, improves our understanding of the relationships between protoxylem structural features and functions, and offers candidate genes for the genetic enhancement of maize.

Two aquaporins, PIP1;1 and PIP2;1, mediate the uptake of neonicotinoid pesticides in plants

Neonicotinoids (NEOs), a large class of organic compounds, are a type of commonly used pesticide for crop protection. Their uptake and accumulation in plants are prerequisites for their intra- and intercellular movements, transformation, and function. Understanding the molecular mechanisms that underpin NEO uptake by plants is crucial for effective application, which remains elusive. Here, we demonstrate that NEOs enter plant cells primarily through the transmembrane symplastic pathway and accumulate mainly in the cytosol. Two plasma membrane intrinsic proteins discovered in Brassica rapa, BraPIP1;1 and BraPIP2;1, were found to encode aquaporins (AQPs) that are highly permeable to NEOs in different plant species and facilitate NEO subcellular diffusion and accumulation. Their conserved transport function was further demonstrated in Xenopus laevis oocyte and yeast assays. BraPIP1;1 and BraPIP2;1 gene knockouts and interaction assays suggested that their proteins can form functional heterotetramers. Assessment of the potential of mean force indicated a negative correlation between NEO uptake and the energy barrier of BraPIP1;1 channels. This study shows that AQPs transport organic compounds with greater osmolarity than previously thought, providing new insight into the molecular mechanisms of organic compound uptake and facilitating innovations in systemic pesticides.

Characterization of SIPs-type aquaporins and their roles in response to environmental cues in rice (Oryza sativa L.)

BACKGROUND

Aquaporins (AQPs) facilitate water diffusion across biological membranes and are involved in all phases of growth and development. Small and basic intrinsic proteins (SIPs) belong to the fourth subfamily of the plant AQPs. Although SIPs are widely present in higher plants, reports on SIPs are limited. Rice is one of the major food crops in the world, and water use is an important factor affecting rice growth and development; therefore, this study aimed to provide information relevant to the function and environmental response of the rice SIP gene family.

RESULTS

The rice (Oryza sativa L. japonica) genome encodes two SIP-like genes, OsSIP1 and OsSIP2, whose products are predominantly located in the endoplasmic reticulum (ER) membrane but transient localization to the plasma membrane is not excluded. Heterologous expression in a yeast aquaglyceroporin-mutant fps1Δ showed that both OsSIP1 and OsSIP2 made the cell more sensitive to KCl, sorbitol and H2O2, indicating facilitated permeation of water and hydrogen peroxide. In addition, the yeast cells expressing OsSIP2 were unable to efflux the toxic methylamine taken up by the endogenous MEP permeases, but OsSIP1 showed subtle permeability to methylamine, suggesting that OsSIP1 may have a wider conducting pore than OsSIP2. Expression profiling in different rice tissues or organs revealed that OsSIP1 was expressed in all tissues tested, whereas OsSIP2 was preferentially expressed in anthers and weakly expressed in other tissues. Consistent with this, histochemical staining of tissues expressing the promoter-β-glucuronidase fusion genes revealed their tissue-specific expression profile. In rice seedlings, both OsSIPs were upregulated to varied levels under different stress conditions, including osmotic shock, high salinity, unfavorable temperature, redox challenge and pathogen attack, as well as by hormonal treatments such as GA, ABA, MeJA, SA. However, a reduced expression of both OsSIPs was observed under dehydration treatment.

CONCLUSIONS

Our results suggest that SIP-like aquaporins are not restricted to the ER membrane and are likely to be involved in unique membrane functions in substrate transport, growth and development, and environmental response.

Illuminating plant water dynamics: the role of light in leaf hydraulic regulation

Light intensity and quality influence photosynthesis directly but also have an indirect effect by increasing stomatal apertures and enhancing gas exchange. Consequently, in areas such as the upper canopy, a high water demand for transpiration and temperature regulation is created. This paper explores how light intensity and the natural high Blue-Light (BL) : Red-Light (RL) ratio in these areas, is important for controlling leaf hydraulic conductance (Kleaf ) by BL signal transduction, increasing water permeability in cells surrounding the vascular tissue, in supporting the enormous water demands. Conversely, shaded inner-canopy areas receive less radiation, have lower water and cooling demands, and exhibit reduced Kleaf due to diminished intensity and BL induction. Intriguingly, shaded leaves display higher water-use efficiency (compared with upper-canopy) due to decreased transpiration and cooling requirements while the presence of RL supports photosynthesis.

Genome-wide molecular evolution analysis of the GRF and GIF gene families in Plantae (Archaeplastida)

BACKGROUND

Plant growth-regulating factors (GRFs) and GRF-interacting factors (GIFs) interact with each other and collectively have important regulatory roles in plant growth, development, and stress responses. Therefore, it is of great significance to explore the systematic evolution of GRF and GIF gene families. However, our knowledge and understanding of the role of GRF and GIF genes during plant evolution has been fragmentary.

RESULTS

In this study, a large number of genomic and transcriptomic datasets of algae, mosses, ferns, gymnosperms and angiosperms were used to systematically analyze the evolution of GRF and GIF genes during the evolution of plants. The results showed that GRF gene first appeared in the charophyte Klebsormidium nitens, whereas the GIF genes originated relatively early, and these two gene families were mainly expanded by segmental duplication events after plant terrestrialization. During the process of evolution, the protein sequences and functions of GRF and GIF family genes are relatively conservative. As cooperative partner, GRF and GIF genes contain the similar types of cis-acting elements in their promoter regions, which enables them to have similar transcriptional response patterns, and both show higher levels of expression in reproductive organs and tissues and organs with strong capacity for cell division. Based on protein-protein interaction analysis and verification, we found that the GRF-GIF protein partnership began to be established in pteridophytes and is highly conserved across different terrestrial plants.

CONCLUSIONS

These results provide a foundation for further exploration of the molecular evolution and biological functions of GRF and GIF genes.

Protein kinase SnRK2.4 is a key regulator of aquaporins and root hydraulics in Arabidopsis

Soil water uptake by roots is a key component of plant water homeostasis contributing to plant growth and survival under ever-changing environmental conditions. The water transport capacity of roots (root hydraulic conductivity; Lpr ) is mostly contributed by finely regulated Plasma membrane Intrinsic Protein (PIP) aquaporins. In this study, we used natural variation of Arabidopsis for the identification of quantitative trait loci (QTLs) contributing to Lpr . Using recombinant lines from a biparental cross (Cvi-0 x Col-0), we show that the gene encoding class 2 Sucrose-Non-Fermenting Protein kinase 2.4 (SnRK2.4) in Col-0 contributes to >30% of Lpr by enhancing aquaporin-dependent water transport. At variance with the inactive and possibly unstable Cvi-0 SnRK2.4 form, the Col-0 form interacts with and phosphorylates the prototypal PIP2;1 aquaporin at Ser121 and stimulates its water transport activity upon coexpression in Xenopus oocytes and yeast cells. Activation of PIP2;1 by Col-0 SnRK2.4 in yeast also requires its protein kinase activity and can be counteracted by clade A Protein Phosphatases 2C. SnRK2.4 shows all hallmarks to be part of core abscisic acid (ABA) signaling modules. Yet, long-term (>3 h) inhibition of Lpr by ABA possibly involves a SnRK2.4-independent inhibition of PIP2;1. SnRK2.4 also promotes stomatal aperture and ABA-induced inhibition of primary root growth. The study identifies a key component of Lpr and sheds new light on the functional overlap and specificity of SnRK2.4 with respect to other ABA-dependent or independent SnRK2s.

Circadian regulation of metabolism across photosynthetic organisms

Circadian regulation produces a biological measure of time within cells. The daily cycle in the availability of light for photosynthesis causes dramatic changes in biochemical processes in photosynthetic organisms, with the circadian clock having crucial roles in adaptation to these fluctuating conditions. Correct alignment between the circadian clock and environmental day-night cycles maximizes plant productivity through its regulation of metabolism. Therefore, the processes that integrate circadian regulation with metabolism are key to understanding how the circadian clock contributes to plant productivity. This forms an important part of exploiting knowledge of circadian regulation to enhance sustainable crop production. Here, we examine the roles of circadian regulation in metabolic processes in source and sink organ structures of Arabidopsis. We also evaluate possible roles for circadian regulation in root exudation processes that deposit carbon into the soil, and the nature of the rhythmic interactions between plants and their associated microbial communities. Finally, we examine shared and differing aspects of the circadian regulation of metabolism between Arabidopsis and other model photosynthetic organisms, and between circadian control of metabolism in photosynthetic and non-photosynthetic organisms. This synthesis identifies a variety of future research topics, including a focus on metabolic processes that underlie biotic interactions within ecosystems.

Exploring aquaporin functions during changes in leaf water potential

Maintenance of optimal leaf tissue humidity is important for plant productivity and food security. Leaf humidity is influenced by soil and atmospheric water availability, by transpiration and by the coordination of water flux across cell membranes throughout the plant. Flux of water and solutes across plant cell membranes is influenced by the function of aquaporin proteins. Plants have numerous aquaporin proteins required for a multitude of physiological roles in various plant tissues and the membrane flux contribution of each aquaporin can be regulated by changes in protein abundance, gating, localisation, post-translational modifications, protein:protein interactions and aquaporin stoichiometry. Resolving which aquaporins are candidates for influencing leaf humidity and determining how their regulation impacts changes in leaf cell solute flux and leaf cavity humidity is challenging. This challenge involves resolving the dynamics of the cell membrane aquaporin abundance, aquaporin sub-cellular localisation and location-specific post-translational regulation of aquaporins in membranes of leaf cells during plant responses to changes in water availability and determining the influence of cell signalling on aquaporin permeability to a range of relevant solutes, as well as determining aquaporin influence on cell signalling. Here we review recent developments, current challenges and suggest open opportunities for assessing the role of aquaporins in leaf substomatal cavity humidity regulation.

Why is FERONIA pleiotropic?

The plant cell wall has many roles: structure, hydraulics, signalling and immunity. Monitoring its status is therefore essential for plant life. Among many candidate cell wall sensors, FERONIA, a member of the Catharanthus roseus receptor-like kinase-1-like kinase (CrRLK1L) family, has received considerable attention, notably because of its numerous interactors and its implication in many biological pathways. Conversely, such an analytical dissection may blur its core function. Here we revisit the array of feronia phenotypes as an attempt to identify a unifying feature behind the plethora of biological and biochemical functions. We propose that the contribution of FERONIA in monitoring turgor-dependent cell wall tension may explain its pleiotropy.

A 14-3-3 protein positively regulates rice salt tolerance by stabilizing phospholipase C1

The phosphatidylinositol-specific phospholipase Cs (PI-PLCs) catalyze the hydrolysis of phosphatidylinositols, which play crucial roles in signaling transduction during plant development and stress response. However, the regulation of PI-PLC is still poorly understood. A previous study showed that a rice PI-PLC, OsPLC1, was essential to rice salt tolerance. Here, we identified a 14-3-3 protein, OsGF14b, as an interaction partner of OsPLC1. Similar to OsPLC1, OsGF14b also positively regulates rice salt tolerance, and their interaction can be promoted by NaCl stress. OsGF14b also positively regulated the hydrolysis activity of OsPLC1, and is essential to NaCl-induced activation of rice PI-PLCs. We further discovered that OsPLC1 was degraded via ubiquitin-proteasome pathway, and OsGF14b could inhibit the ubiquitination of OsPLC1 to protect OsPLC1 from degradation. Under salt stress, the OsPLC1 protein level in osgf14b was lower than the corresponding value of WT, whereas overexpression of OsGF14b results in a significant increase of OsPLC1 stability. Taken together, we propose that OsGF14b can interact with OsPLC1 and promote its activity and stability, thereby improving rice salt tolerance. This study provides novel insights into the important roles of 14-3-3 proteins in regulating protein stability and function in response to salt stress.

Arabidopsis histone H3 lysine 9 methyltransferases KYP/SUVH5/6 are involved in leaf development by interacting with AS1-AS2 to repress KNAT1 and KNAT2

The Arabidopsis H3K9 methyltransferases KRYPTONITE/SUPPRESSOR OF VARIEGATION 3-9 HOMOLOG 4 (KYP/SUVH4), SUVH5 and SUVH6 are redundantly involved in silencing of transposable elements (TEs). Our recent study indicated that KYP/SUVH5/6 can directly interact with the histone deacetylase HDA6 to synergistically regulate TE expression. However, the function of KYP/SUVH5/6 in plant development is still unclear. The transcriptional factors ASYMMETRIC LEAVES1 (AS1) and AS2 form a transcription complex, which is involved in leaf development by repressing the homeobox genes KNOTTED-LIKE FROM ARABIDOPSIS THALIANA 1 (KNAT1) and KNAT2. In this study, we found that KYP and SUVH5/6 directly interact with AS1-AS2 to repress KNAT1 and KNAT2 by altering histone H3 acetylation and H3K9 dimethylation levels. In addition, KYP can directly target the promoters of KNAT1 and KNAT2, and the binding of KYP depends on AS1. Furthermore, the genome-wide occupancy profile of KYP indicated that KYP is enriched in the promoter regions of coding genes, and the binding of KYP is positively correlated with that of AS1 and HDA6. Together, these results indicate that Arabidopsis H3K9 methyltransferases KYP/SUVH5/6 are involved in leaf development by interacting with AS1-AS2 to alter histone H3 acetylation and H3K9 dimethylation from KNAT1 and KNAT2 loci.

Abscisic acid modulates neighbor proximity-induced leaf hyponasty in Arabidopsis

Leaves of shade-avoiding plants such as Arabidopsis (Arabidopsis thaliana) change their growth pattern and position in response to low red to far-red ratios (LRFRs) encountered in dense plant communities. Under LRFR, transcription factors of the phytochrome-interacting factor (PIF) family are derepressed. PIFs induce auxin production, which is required for promoting leaf hyponasty, thereby favoring access to unfiltered sunlight. Abscisic acid (ABA) has also been implicated in the control of leaf hyponasty, with gene expression patterns suggesting that LRFR regulates the ABA response. Here, we show that LRFR leads to a rapid increase in ABA levels in leaves. Changes in ABA levels depend on PIFs, which regulate the expression of genes encoding isoforms of the enzyme catalyzing a rate-limiting step in ABA biosynthesis. Interestingly, ABA biosynthesis and signaling mutants have more erect leaves than wild-type Arabidopsis under white light but respond less to LRFR. Consistent with this, ABA application decreases leaf angle under white light; however, this response is inhibited under LRFR. Tissue-specific interference with ABA signaling indicates that an ABA response is required in different cell types for LRFR-induced hyponasty. Collectively, our data indicate that LRFR triggers rapid PIF-mediated ABA production. ABA plays a different role in controlling hyponasty under white light than under LRFR. Moreover, ABA exerts its activity in multiple cell types to control leaf position.

Burning questions for a warming and changing world: 15 unknowns in plant abiotic stress

We present unresolved questions in plant abiotic stress biology as posed by 15 research groups with expertise spanning eco-physiology to cell and molecular biology. Common themes of these questions include the need to better understand how plants detect water availability, temperature, salinity, and rising carbon dioxide (CO2) levels; how environmental signals interface with endogenous signaling and development (e.g. circadian clock and flowering time); and how this integrated signaling controls downstream responses (e.g. stomatal regulation, proline metabolism, and growth versus defense balance). The plasma membrane comes up frequently as a site of key signaling and transport events (e.g. mechanosensing and lipid-derived signaling, aquaporins). Adaptation to water extremes and rising CO2 affects hydraulic architecture and transpiration, as well as root and shoot growth and morphology, in ways not fully understood. Environmental adaptation involves tradeoffs that limit ecological distribution and crop resilience in the face of changing and increasingly unpredictable environments. Exploration of plant diversity within and among species can help us know which of these tradeoffs represent fundamental limits and which ones can be circumvented by bringing new trait combinations together. Better defining what constitutes beneficial stress resistance in different contexts and making connections between genes and phenotypes, and between laboratory and field observations, are overarching challenges.

Nitrate reductase enzymes in alga Chattonella subsalsa are regulated by environmental cues at the translational and post-translational levels

Nitrate reductase (NR) catalyzes the rate-limiting step in nitrate assimilation. Plant and algal NRs have a highly conserved domain architecture but differ in regulation. In plants, NR activity is regulated by reversible phosphorylation and subsequent binding of 14-3-3 proteins at a conserved serine residue. Algal NRs typically lack 14-3-3 binding motifs, which have only recently been identified in a few algal species. Previous research indicates that the alga, Chattonella subsalsa, possesses a novel NR, NR2-2/2HbN (NR2), which incorporates a 2/2 hemoglobin domain. A second NR (NR3) in C. subsalsa lacks the cytochrome b5 (heme-Fe) domain but includes a putative binding motif for 14-3-3 proteins. The expression of NR2 and NR3 genes indicates that NR2 transcript abundance was regulated by light, nitrogen source, and temperature, while NR3 transcript levels were only regulated by light. Here, we measured total NR activity in C. subsalsa and the potential for regulation of NR activity by putative 14-3-3 binding proteins. Results indicate that NR activity in C. subsalsa was regulated by light, nitrogen source, and temperature at the translational level. NR activity was also regulated by endogenous rhythm and temperature at the post-translational level, supporting the hypothesis that NR3 is regulated by 14-3-3 binding proteins. Together with a previous report describing the regulation of NR gene expression in C. subsalsa, results suggest that C. subsalsa responds to environmental conditions by differential regulation of NRs at transcriptional, translational, and post-translational levels. This flexibility may provide a competitive advantage for this species in the environment. To date, this is the first report which provides evidence for the potential post-translational regulation of NR by 14-3-3 proteins in algal species and suggests that regulatory mechanisms for NR activity may be shared between plants and some algal species.

Phosphorylation of a wheat aquaporin at two sites enhances both plant growth and defense

Eukaryotic aquaporins share the characteristic of functional multiplicity in transporting distinct substrates and regulating various processes, but the underlying molecular basis for this is largely unknown. Here, we report that the wheat (Triticum aestivum) aquaporin TaPIP2;10 undergoes phosphorylation to promote photosynthesis and productivity and to confer innate immunity against pathogens and a generalist aphid pest. In response to elevated atmospheric CO₂ concentrations, TaPIP2;10 is phosphorylated at the serine residue S280 and thereafter transports CO₂ into wheat cells, resulting in enhanced photosynthesis and increased grain yield. In response to apoplastic H₂O₂ induced by pathogen or insect attacks, TaPIP2;10 is phosphorylated at S121 and this phosphorylated form transports H₂O₂ into the cytoplasm, where H₂O₂ intensifies host defenses, restricting further attacks. Wheat resistance and grain yield could be simultaneously increased by TaPIP2;10 overexpression or by expressing a TaPIP2;10 phosphomimic with aspartic acid substitutions at S121 and S280, thereby improving both crop productivity and immunity.

Calcium-dependent protein kinase 16 phosphorylates and activates the aquaporin PIP2;2 to regulate reversible flower opening in Gentiana scabra

Flower opening is important for successful pollination in many plant species, and some species repeatedly open and close their flowers. This is thought to be due to turgor pressure changes caused by water influx/efflux, which depends on osmotic oscillations in the cells. In some ornamental plants, water-transporting aquaporins, also known as plasma membrane intrinsic proteins (PIPs), may play an important role in flower opening. However, the molecular mechanism(s) involved in corolla movement are largely unknown. Gentian (Gentiana spp.) flowers undergo reversible movement in response to temperature and light stimuli; using gentian as a model, we showed that the Gentiana scabra aquaporins GsPIP2;2 and GsPIP2;7 regulate repeated flower opening. In particular, phosphorylation of a C-terminal serine residue of GsPIP2;2 is important for its transport activity and relates closely to the flower re-opening rate. Furthermore, GsPIP2;2 is phosphorylated and activated by the calcium (Ca2+)-dependent protein kinase GsCPK16, which is activated by elevated cytosolic Ca2+ levels in response to temperature and light stimuli. We propose that GsCPK16-dependent phosphorylation and activation of GsPIP2;2 regulate gentian flower re-opening, with stimulus-induced Ca2+ signals acting as triggers.

Out of the blue: Phototropins of the leaf vascular bundle sheath mediate the regulation of leaf hydraulic conductance by blue light

The Arabidopsis (Arabidopsis thaliana) leaf veins bundle-sheath cells (BSCs)-a selective barrier to water and solutes entering the mesophyll-increase the leaf radial hydraulic conductance (Kleaf) by acidifying the xylem sap by their plasma membrane H+-ATPase,  AHA2. Based on this and on the BSCs' expression of phototropins PHOT1 and PHOT2, and the known blue light (BL)-induced Kleaf increase, we hypothesized that, resembling the guard cells, BL perception by the BSCs' phots activates its H+-ATPase, which, consequently, upregulates Kleaf. Indeed, under BL, the Kleaf of the knockout mutant lines phot1-5, phot2-1, phot1-5 phot2-1, and aha2-4 was lower than that of the wild-type (WT). BSC-only-directed complementation of phot1-5 or aha2-4 by PHOT1 or AHA2, respectively, restored the BL-induced Kleaf increase. BSC-specific silencing of PHOT1 or PHOT2 prevented such Kleaf increase. A xylem-fed kinase inhibitor (tyrphostin 9) replicated this also in WT plants. White light-ineffective in the phot1-5 mutant-acidified the xylem sap (relative to darkness) in WT and in the PHOT1-complemented phot1-5. These results, supported by BL increase of BSC protoplasts' water permeability and cytosolic pH and their hyperpolarization by BL, identify the BSCs as a second phot-controlled water conductance element in leaves, in series with stomatal conductance. Through both, BL regulates the leaf water balance.

Leaf and Branch Hydraulic Plasticity of Two Light-Demanding Broadleaved Tree Species Differing in Water-Use Strategy

Global climate change creates new environmental scenarios and selective pressures; thus, a better understanding of the plasticity of plant functional traits is needed to predict how plant species will respond to shifts in climate. Among the important functional traits for plants are their hydraulic properties which ultimately determine their photosynthetic capacity, growth rate, and survival in a changing environment. In this study, the light sensitivity of leaf (KL) and branch hydraulic conductance (KB) to fast changes in irradiance, and hydraulic plasticity (PIh) was studied in two broadleaved tree species differing in water-use strategy—silver birch (Betula pendula) and hybrid aspen (Populus × wettsteinii). The KL increased by a factor of 3.5 and 1.5 from minimal values recorded in darkness to maximal values in high light conditions for birch and aspen, respectively, indicating a significantly higher PIh for birch (0.72) than for aspen leaves (0.35). KB increased 1.5-fold from dark to light conditions for both species. The high light sensitivity of KL and KB provides a regulatory mechanism to maintain a balance between transpirational demand and hydraulic supply. The plasticity of these traits increases the ability of plants to cope with a rapidly changing environment and to adapt to global climate change.

Comprehensive Effects of Flowering Locus T-Mediated Stem Growth in Tobacco

In flowering plants, Flowering locus T (FT) encodes a major florigen. It is a key flowering hormone in controlling flowering time and has a wide range of effects on plant development. Although the mechanism by which FT promotes flowering is currently clearly understood, comprehensive effects of the FT gene on plant growth have not been evaluated. Therefore, the effects of FT on vegetative growth need to be explored for a complete understanding of the molecular functions of the FT gene. In this study, the Jatropha curcas L. FT gene was overexpressed in tobacco (JcFT^OE) in order to discover multiple aspects and related mechanisms of how the FT gene affects plant development. In JcFT^OE plants, root, stem, and leaf development was strongly affected. Stem tissues were selected for further transcriptome analysis. In JcFT^OE plants, stem growth was affected because of changes in the nucleus, cytoplasm, and cell wall. In the nucleus of JcFT^OE plants, the primary effect was to weaken all aspects of DNA replication, which ultimately affected the cell cycle and cell division. The number of stem cells decreased significantly in JcFT^OE plants, which decreased the thickness and height of tobacco stems. In the cell wall of JcFT^OE plants, hemicellulose and cellulose contents increased, with the increase in hemicellulose associated with up-regulation of xylan synthase-related genes expression. In the cytoplasm of JcFT^OE plants, the primary effects were on biogenesis of ribonucleoprotein complexes, photosynthesis, carbohydrate biosynthesis, and the cytoskeleton. In addition, in the cytoplasm of JcFT^OE plants, there were changes in certain factors of the core oscillator, expression of many light-harvesting chlorophyll a/b binding proteins was down-regulated, and expression of fructose 1,6-bisphosphatase genes was up-regulated to increase starch content in tobacco stems. Changes in the xylem and phloem of JcFT^OE plants were also identified, and in particular, xylem development was affected by significant increases in expression of irregular xylem genes.

Optimizing Crop Water Use for Drought and Climate Change Adaptation Requires a Multi-Scale Approach

Selection criteria that co-optimize water use efficiency and yield are needed to promote plant productivity in increasingly challenging and variable drought scenarios, particularly dryland cereals in the semi-arid tropics. Optimizing water use efficiency and yield fundamentally involves transpiration dynamics, where restriction of maximum transpiration rate helps to avoid early crop failure, while maximizing grain filling. Transpiration restriction can be regulated by multiple mechanisms and involves cross-organ coordination. This coordination involves complex feedbacks and feedforwards over time scales ranging from minutes to weeks, and from spatial scales ranging from cell membrane to crop canopy. Aquaporins have direct effect but various compensation and coordination pathways involve phenology, relative root and shoot growth, shoot architecture, root length distribution profile, as well as other architectural and anatomical aspects of plant form and function. We propose gravimetric phenotyping as an integrative, cross-scale solution to understand the dynamic, interwoven, and context-dependent coordination of transpiration regulation. The most fruitful breeding strategy is likely to be that which maintains focus on the phene of interest, namely, daily and season level transpiration dynamics. This direct selection approach is more precise than yield-based selection but sufficiently integrative to capture attenuating and complementary factors.

Hormonal and environmental signaling pathways target membrane water transport

Plant water transport and its molecular components including aquaporins are responsive, across diverse time scales, to an extremely wide array of environmental and hormonal signals. These include water deficit and abscisic acid (ABA) but also more recently identified stimuli such as peptide hormones or bacterial elicitors. The present review makes an inventory of corresponding signalling pathways. It identifies some main principles, such as the central signalling role of ROS, with a dual function of aquaporins in water and hydrogen peroxide transport, the importance of aquaporin phosphorylation that is targeted by multiple classes of protein kinases, and the emerging role of lipid signalling. More studies including systems biology approaches are now needed to comprehend how plant water transport can be adjusted in response to combined stresses.

The Aquaporin TaPIP2;10 Confers Resistance to Two Fungal Diseases in Wheat

Plants employ aquaporins (AQPs) of the plasma membrane intrinsic protein (PIP) family to import environmental substrates, thereby affecting various processes, such as the cellular responses regulated by the signaling molecule hydrogen peroxide (H₂O₂). Common wheat (Triticum aestivum) contains 24 candidate members of the PIP family, designated as TaPIP1;1 to TaPIP1;12 and TaPIP2;1 to TaPIP2;12. None of these TaPIP candidates have been characterized for substrate selectivity or defense responses in their source plant. Here, we report that T. aestivum AQP TaPIP2;10 facilitates the cellular uptake of H₂O₂ to confer resistance against powdery mildew and Fusarium head blight, two devastating fungal diseases in wheat throughout the world. In wheat, the apoplastic H₂O₂ signal is induced by fungal attack, while TaPIP2;10 is stimulated to translocate this H₂O₂ into the cytoplasm, where it activates defense responses to restrict further attack. TaPIP2;10-mediated transport of H₂O₂ is essential for pathogen-associated molecular pattern-triggered plant immunity (PTI). Typical PTI responses are induced by the fungal infection and intensified by overexpression of the TaPIP2;10 gene. TaPIP2;10 overexpression causes a 70% enhancement in wheat resistance to powdery mildew and an 86% enhancement in resistance to Fusarium head blight. By reducing the disease severities, TaPIP2;10 overexpression brings about >37% increase in wheat grain yield. These results verify the feasibility of using an immunity-relevant AQP to concomitantly improve crop productivity and immunity.

The circadian clock gene circuit controls protein and phosphoprotein rhythms in Arabidopsis thaliana

Twenty-four-hour, circadian rhythms control many eukaryotic mRNA levels, whereas the levels of their more stable proteins are not expected to reflect the RNA rhythms, emphasizing the need to test the circadian regulation of protein abundance and modification. Here we present circadian proteomic and phosphoproteomic time series from Arabidopsis thaliana plants under constant light conditions, estimating that just 0.4% of quantified proteins but a much larger proportion of quantified phospho-sites were rhythmic. Approximately half of the rhythmic phospho-sites were most phosphorylated at subjective dawn, a pattern we term the “phospho-dawn.” Members of the SnRK/CDPK family of protein kinases are candidate regulators. A CCA1-overexpressing line that disables the clock gene circuit lacked most circadian protein phosphorylation. However, the few phospho-sites that fluctuated despite CCA1-overexpression still tended to peak in abundance close to subjective dawn, suggesting that the canonical clock mechanism is necessary for most but perhaps not all protein phosphorylation rhythms. To test the potential functional relevance of our datasets, we conducted phosphomimetic experiments using the bifunctional enzyme fructose-6-phosphate-2-kinase/phosphatase (F2KP), as an example. The rhythmic phosphorylation of diverse protein targets is controlled by the clock gene circuit, implicating posttranslational mechanisms in the transmission of circadian timing information in plants.

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Circadian (phospho)proteomics time courses of plants with or without functional clock.

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Most protein abundance/phosphorylation rhythms require a transcriptional oscillator.

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The majority of rhythmic phosphosites peak around subjective dawn (“phospho-dawn”).

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A phosphorylated serine of the metabolic enzyme F2KP has functional relevance.

YWHAZ interacts with DAAM1 to promote cell migration in breast cancer

Dishevelled-associated activator of morphogenesis 1 (DAAM1) is a critical driver in facilitating metastasis in breast cancer (BrCa). However, molecular mechanisms for the regulation of DAAM1 activation are only partially elucidated. In this research, the expression levels of YWHAZ and DAAM1 were examined by immunohistochemistry (IHC) staining in BrCa tissues. The functional roles of tyrosine 3-monooxygenase/tryptophan 5-monooxygenase activation protein zeta (YWHAZ)–DAAM1 axis and their regulator microRNA-613 (miR-613) in BrCa cells and associated molecular mechanisms were demonstrated in vitro. As results, the expression levels of DAAM1 and YWHAZ were significantly upregulated in BrCa tissues compared with normal tissues and remarkably associated with poor prognosis. Besides, DAAM1 and YWHAZ were positively correlated with each other in BrCa tissues. YWHAZ interacted and colocalized with DAAM1 in BrCa cells, which was essential for DAAM1-mediated microfilament remodeling and RhoA activation. Moreover, miR-613 directly targeted both YWHAZ and DAAM1, contributing to inhibiting BrCa cells migration via blocking the complex of YWHAZ–DAAM1. To sum up, these data reveal that YWHAZ regulates DAAM1 activation, and the YWHAZ–DAAM1 complex is directly targeted by the shared post-transcriptional regulator miR-613.

PIP aquaporin pH sensing is regulated by the length and charge of the C-terminal region

Plant PIP aquaporins play a central role in controlling plant water status. The current structural model for PIP pH-gating states that the main pH sensor is located in loopD and that all the mobile cytosolic elements participate in a complex interaction network that ensures the closed structure. However, the precise participation of the last part of the C-terminal domain (CT) in PIP pH gating remains unknown. This last part has not been resolved in PIP crystal structures and is a key difference between PIP1 and PIP2 paralogues. Here, by a combined experimental and computational approach, we provide data about the role of CT in pH gating of Beta vulgaris PIP. We demonstrate that the length of CT and the positive charge located among its last residues modulate the pH at which the open/closed transition occurs. We also postulate a molecular-based mechanism for the differential pH sensing in PIP homo- or heterotetramers by performing atomistic molecular dynamics simulations (MDS) on complete models of PIP tetramers. Our findings show that the last part of CT can affect the environment of loopD pH sensors in the closed state. Results presented herein contribute to the understanding of how the characteristics of CT in PIP channels play a crucial role in determining the pH at which water transport through these channels is blocked, highlighting the relevance of the differentially conserved very last residues in PIP1 and PIP2 paralogues.

Citrus NIP5;1 aquaporin regulates cell membrane water permeability and alters PIPs plasma membrane localization

The ER or donut-like structures localized aquaporin NIP5;1, which interacts with PIPs and alters their localization from plasma membrane to donut-like structures, regulates water permeability. NOD26-like intrinsic proteins (NIPs) play important roles in nutrient uptake and response to various stresses. However, there have been few studies of their functions in water transportation in citrus. Here, we demonstrate the functions of a novel citrus NIP aquaporin (CsNIP5;1) via multiple physiological and biochemical experiments. CsNIP5;1 showed high water permeability when expressed in Xenopus laevis oocytes and yeast. However, subcellular localization assays showed that this protein was localized in the endoplasmic reticulum (ER) or donut-like structures in citrus callus and tobacco leaf. Meanwhile, overexpression of CsNIP5;1 led to a reduction in the water permeability of citrus callus. Protein-protein interaction experiments and subcellular localization assays further revealed that CsNIP5;1 physically interacted with PIPs (CsPIP1;1 and AtPIP2;1), which altered their subcellular localization from the plasma membrane to donut-like structures. Together, CsNIP5;1 was identified as a good water channel when expressed in oocytes and yeast. Meanwhile, CsNIP5;1 participated in the regulation of water permeability of citrus callus, which may be associated with CsNIP5;1-induced re-localization of water channels PIPs. In summary, these results provide new insights into the regulatory mechanism of AQPs-mediated water diffusion.

Adaptable and Multifunctional Ion-Conducting Aquaporins

Aquaporins function as water and neutral solute channels, signaling hubs, disease virulence factors, and metabolon components. We consider plant aquaporins that transport ions compared to some animal counterparts. These are candidates for important, as yet unidentified, cation and anion channels in plasma, tonoplast, and symbiotic membranes. For those individual isoforms that transport ions, water, and gases, the permeability spans 12 orders of magnitude. This requires tight regulation of selectivity via protein interactions and posttranslational modifications. A phosphorylation-dependent switch between ion and water permeation in AtPIP2;1 might be explained by coupling between the gates of the four monomer water channels and the central pore of the tetramer. We consider the potential for coupling between ion and water fluxes that could form the basis of an electroosmotic transducer. A grand challenge in understanding the roles of ion transporting aquaporins is their multifunctional modes that are dependent on location, stress, time, and development.

Plant transporters involved in combating boron toxicity: beyond 3D structures

Membrane transporters control the movement and distribution of solutes, including the disposal or compartmentation of toxic substances that accumulate in plants under adverse environmental conditions. In this minireview, in the light of the approaching 100th anniversary of unveiling the significance of boron to plants (K. Warington, 1923; Ann. Bot.37, 629) we discuss the current state of the knowledge on boron transport systems that plants utilise to combat boron toxicity. These transport proteins include: (i) nodulin-26-like intrinsic protein-types of aquaporins, and (ii) anionic efflux (borate) solute carriers. We describe the recent progress made on the structure–function relationships of these transport proteins and point out that this progress is integral to quantitative considerations of the transporter's roles in tissue boron homeostasis. Newly acquired knowledge at the molecular level has informed on the transport mechanics and conformational states of boron transport systems that can explain their impact on cell biology and whole plant physiology. We expect that this information will form the basis for engineering transporters with optimised features to alleviate boron toxicity tolerance in plants exposed to suboptimal soil conditions for sustained food production.

Identification and Expression Analyses of the Special 14-3-3 Gene Family in Papaya and its Involvement in Fruit Development, Ripening, and Abiotic Stress Responses

Plant 14-3-3 proteins play key roles in regulating growth, development, and stress responses. However, little is known about this gene family in papaya (Carica papaya L.). We characterized eight 14-3-3 genes from the papaya genome and designed them as CpGRF1-8. Based on phylogenetic, conserved motif, and gene structure analyses, papaya CpGRFs were divided into ε and non-ε groups. Expression analysis showed differential and class-specific transcription patterns in different organs. Quantitative real-time polymerase chain reaction analysis showed that most CpGRFs had large changes in expression during fruit development and ripening. This indicated that the CpGRFs were involved in regulating fruit development and ripening. Significant expression changes occurred after cold, salt, and drought treatments in papaya seedlings, indicating that CpGRFs were also involved in signaling responses to abiotic stress. These results provide a transcription profile of 14-3-3 genes in organs, during fruit development and ripening and in response to stress. Some highly expressed, fruit-specific, and stress-responsive candidate CpGRFs will be identified for further genetic improvement of papayas.

The extra-vascular water pathway regulates dynamic leaf hydraulic decline and recovery in Populus nigra

Leaf hydraulic conductance (K_leaf ) is highly dynamic and typically responds to changes in water status and irradiance. However, the relative contribution of vascular (K_x ) and extra-vascular (K_ox ) water pathways to K_leaf changes in response to water potential decline and recovery in function of light conditions remains poorly investigated. We investigated the dynamic responses of leaf hydraulics in Populus nigra L. by measuring K_leaf , K_x , and K_ox changes under drought and upon recovery. Measurements were done at both low and high irradiance (LI and HI, respectively). K_leaf increased and became more vulnerable to dehydration under HI conditions than LI, due to marked changes of K_ox . After re-watering, K_leaf recovered in parallel with K_ox recovery, but K_leaf response to irradiance remained inhibited. Strong correlations between K_leaf and drought-induced membrane damage demonstrated the relevance of the cell-to-cell water pathway in driving the dynamic responses of K_leaf under drought and recovery. Our findings highlight the importance of coordination between water and light availability in modulating the overall K_leaf response to environmental conditions.

The Major Intrinsic Protein Family and Their Function Under Salt-Stress in Peanut

Peanut (Arachis hypogaea) is an important oil crop cultivated across the world. Abiotic stresses are the major constraint factors that defect its yield, especially in the rainfed peanut cultivation areas. Aquaporins are proteins that form a large family of more than 30 members in higher plants and play key roles in plant water balance under abiotic stress conditions. To comprehensively understand the functions of aquaporins in peanut, we identified their family genome-wide and characterized the phylogenetics, gene structure, and the conserved motif of the selective filter. In total, 64 aquaporin isoforms were identified, the NIPs were firstly categorized into NIP1s and NIP2s groups based on the phylogenetic analysis and the selective filter structure classification system. Further, we analyzed the gene expression pattern under the salt-stress conditions and found that a TIP3 member is strongly induced by salt stress, which in turn contributed to improved seed germination under salt stress when expressed in Arabidopsis. Our study thus provides comprehensive profiles on the MIP superfamily and their expression and function under salt-stress conditions. We believe that our findings will facilitate the better understanding of the roles of aquaporins in peanuts under salt salt-stress conditions.

A Cytoplasmic Receptor-like Kinase Contributes to Salinity Tolerance

Receptor-like cytoplasmic kinases (RLCKs) are receptor kinases that lack extracellular ligand-binding domains and have emerged as a major class of signaling proteins that regulate plant cellular activities in response to biotic/abiotic stresses and endogenous extracellular signaling molecules. We have identified a rice RLCK (OsRLCK311) that was significantly higher in transgenic pSARK-IPT rice (Oryza sativa) that exhibited enhanced growth under saline conditions. Overexpression of OsRLCK311 full-length protein (RLCK311FL) and the C-terminus of OsRLCK311 (ΔN) in Arabidopsis confirmed its role in salinity tolerance, both in seedlings and mature plants. Protein interaction assays indicated that OsRLCK311 and ΔN interacted in-vivo with the plasma membrane AQP AtPIP2;1. The RLCK311-PIP2;1 binding led to alterations in the stomata response to ABA, which was characterized by more open stomata of transgenic plants. Moreover, OsRLCK311-ΔN effect in mediating enhanced plant growth under saline conditions was also observed in the perennial grass Brachypodium sylvaticum, confirming its role in both dicots and monocots species. Lastly, OsRLCK311 interacted with the rice OsPIP2;1. We suggest that the rice OsRLCK311 play a role in regulating the plant growth response under saline conditions via the regulation of the stomata response to stress. This role seems to be independent of the RLCK311 kinase activity, since the overexpression of the RLCK311 C-terminus (ΔN), which lacks the kinase full domain, has a similar phenotype to RLCK311FL.

Thermal imaging as a noninvasive technique for analyzing circadian rhythms in plants

Endogenous (˜24 circadian) rhythms control an enormously diverse range of processes in plants and are, increasingly, the target of studies aimed at understanding plant performance. Although in the previous few decades most plant circadian research has focused on Arabidopsis, there is a pressing need for low-cost, high-throughput tools for analyzing rhythms in a wider variety of species. The present contribution investigates using circadian temperature oscillations as a novel marker for assaying plant circadian rhythms. A thermal imaging platform was set up to measure diel and circadian rhythms in different plant species, in wild-type and circadian mutant plants, and in leaves and flowers. Results from the thermal imaging technique were compared with those from other established circadian assay techniques. All of the dicot and monocot species examined showed robust circadian rhythms of leaf surface temperature; the effects of circadian mutations on thermocycles were similar to those reported using other techniques. In Petunia × atkinsiana plants circadian oscillations were observed in both leaves and flowers. Thermal imaging is an extremely useful technique for analyzing circadian rhythms in plants. It is predicted that the ability to make very high temporal resolution measurements may facilitate the discovery of novel aspects of circadian control.

Salicylic acid-mediated plasmodesmal closure via Remorin-dependent lipid organization

Plasmodesmata (PD) create cytoplasmic and membrane continuities between adjacent cells to facilitate cell–cell communication and virus movement. Plant cells have evolved diverse mechanisms to regulate PD plasticity against plant pathogens, including the accumulation of the defense hormone, salicylic acid (SA). However, the mechanism of how this occurs is not well understood. Here, we uncover a mechanism by which SA triggers Remorin-dependent membrane lipid nanodomain assembly, leading to enhancement of the liquid-ordered phase. The higher-ordered lipids, which are particularly enriched at PD membrane, decreased PD membrane plasticity, and thus restricted PD opening and impeded virus spreading. Our findings address a knowledge gap in plant defense mechanisms at the membrane level that rely on SA-controlled lipid order and PD closure.

Plasmodesmata (PD) are plant-specific membrane-lined channels that create cytoplasmic and membrane continuities between adjacent cells, thereby facilitating cell–cell communication and virus movement. Plant cells have evolved diverse mechanisms to regulate PD plasticity in response to numerous environmental stimuli. In particular, during defense against plant pathogens, the defense hormone, salicylic acid (SA), plays a crucial role in the regulation of PD permeability in a callose-dependent manner. Here, we uncover a mechanism by which plants restrict the spreading of virus and PD cargoes using SA signaling by increasing lipid order and closure of PD. We showed that exogenous SA application triggered the compartmentalization of lipid raft nanodomains through a modulation of the lipid raft-regulatory protein, Remorin (REM). Genetic studies, superresolution imaging, and transmission electron microscopy observation together demonstrated that Arabidopsis REM1.2 and REM1.3 are crucial for plasma membrane nanodomain assembly to control PD aperture and functionality. In addition, we also found that a 14-3-3 epsilon protein modulates REM clustering and membrane nanodomain compartmentalization through its direct interaction with REM proteins. This study unveils a molecular mechanism by which the key plant defense hormone, SA, triggers membrane lipid nanodomain reorganization, thereby regulating PD closure to impede virus spreading.

Revisiting the Basal Role of ABA - Roles Outside of Stress

The physiological roles of abscisic acid (ABA) as a stress hormone in plant responses to water shortage, including stomatal regulation and gene expression, have been well documented. However, less attention has been paid to the function of basal ABA synthesized under well-watered conditions in recent studies. In this review, we summarize progress in the understanding of how low concentrations of ABA are perceived at the molecular level and how its signaling affects plant metabolism and growth under nonstressed conditions. We also discuss the dual nature of ABA in acting as an inhibitor and activator of plant growth and development.