Plasma Membrane-Associated Ca2+-Binding Protein PCaP1 is Involved in Root Hydrotropism of Arabidopsis thaliana

Hydrotropism mechanisms and their interplay with gravitropism

Plants partly optimize their water recruitment from the growth medium by directing root growth toward a moisture source, a phenomenon termed hydrotropism. The default mechanism of downward growth, termed gravitropism, often functions to counteract hydrotropism when the water-potential gradient deviates from the gravity vector. This review addresses the identity of the root sites in which hydrotropism-regulating factors function to attenuate gravitropism and the interplay between these various factors. In this context, the function of hormones, including auxin, abscisic acid, and cytokinins, as well as secondary messengers, calcium ions, and reactive oxygen species in the conflict between these two opposing tropisms is discussed. We have assembled the available data on the effects of various chemicals and genetic backgrounds on both gravitropism and hydrotropism, to provide an up-to-date perspective on the interactions that dictate the orientation of root tip growth. We specify the relevant open questions for future research. Broadening our understanding of root mechanisms of water recruitment holds great potential for providing advanced approaches and technologies that can improve crop plant performance under less-than-optimal conditions, in light of predicted frequent and prolonged drought periods due to global climate change.

Role of Abscisic Acid, Reactive Oxygen Species, and Ca2+ Signaling in Hydrotropism-Drought Avoidance-Associated Response of Roots

Plant roots exert hydrotropism in response to moisture gradients to avoid drought stress. The regulatory mechanism underlying hydrotropism involves novel regulators such as MIZ1 and GNOM/MIZ2 as well as abscisic acid (ABA), reactive oxygen species (ROS), and Ca2+ signaling. ABA, ROS, and Ca2+ signaling are also involved in plant responses to drought stress. Although the mechanism of moisture gradient perception remains largely unknown, the sensory apparatus has been reported to reside in the root elongation zone rather than in the root cap. In Arabidopsis roots, hydrotropism is mediated by the action of MIZ1 and ABA in the cortex of the elongation zone, the accumulation of ROS at the root curvature, and the variation in the cytosolic Ca2+ concentration in the entire root tip including the root cap and stele of the elongation zone. Moreover, root exposure to moisture gradients has been proposed to cause asymmetric ABA distribution or Ca2+ signaling, leading to the induction of the hydrotropic response. A comprehensive and detailed analysis of hydrotropism regulators and their signaling network in relation to the tissues required for their function is apparently crucial for understanding the mechanisms unique to root hydrotropism. Here, referring to studies on plant responses to drought stress, we summarize the recent findings relating to the role of ABA, ROS, and Ca2+ signaling in hydrotropism, discuss their functional sites and plausible networks, and raise some questions that need to be answered in future studies.

How plants sense and respond to osmotic stress

Drought is one of the most serious abiotic stresses to land plants. Plants sense and respond to drought stress to survive under water deficiency. Scientists have studied how plants sense drought stress, or osmotic stress caused by drought, ever since Charles Darwin, and gradually obtained clues about osmotic stress sensing and signaling in plants. Osmotic stress is a physical stimulus that triggers many physiological changes at the cellular level, including changes in turgor, cell wall stiffness and integrity, membrane tension, and cell fluid volume, and plants may sense some of these stimuli and trigger downstream responses. In this review, we emphasized water potential and movements in organisms, compared putative signal inputs in cell wall-containing and cell wall-free organisms, prospected how plants sense changes in turgor, membrane tension, and cell fluid volume under osmotic stress according to advances in plants, animals, yeasts, and bacteria, summarized multilevel biochemical and physiological signal outputs, such as plasma membrane nanodomain formation, membrane water permeability, root hydrotropism, root halotropism, Casparian strip and suberin lamellae, and finally proposed a hypothesis that osmotic stress responses are likely to be a cocktail of signaling mediated by multiple osmosensors. We also discussed the core scientific questions, provided perspective about the future directions in this field, and highlighted the importance of robust and smart root systems and efficient source-sink allocations for generating future high-yield stress-resistant crops and plants.

Understanding role of roots in plant response to drought: Way forward to climate-resilient crops

Drought stress leads to a significant amount of agricultural crop loss. Thus, with changing climatic conditions, it is important to develop resilience measures in agricultural systems against drought stress. Roots play a crucial role in regulating plant development under drought stress. In this review, we have summarized the studies on the role of roots and root-mediated plant responses. We have also discussed the importance of root system architecture (RSA) and the various structural and anatomical changes that it undergoes to increase survival and productivity under drought. Various genes, transcription factors, and quantitative trait loci involved in regulating root growth and development are also discussed. A summarization of various instruments and software that can be used for high-throughput phenotyping in the field is also provided in this review. More comprehensive studies are required to help build a detailed understanding of RSA and associated traits for breeding drought-resilient cultivars.

Zoom-in to molecular mechanisms underlying root growth and function under heterogeneous soil environment and abiotic stresses

MAIN CONCLUSION

The review describes tissue-specific and non-cell autonomous molecular responses regulating the root system architecture and function in plants. Phenotypic plasticity of roots relies on specific molecular and tissue specific responses towards local and microscale heterogeneity in edaphic factors. Unlike gravitropism, hydrotropism in Arabidopsis is regulated by MIZU KUSSIE1 (MIZ1)-dependent asymmetric distribution of cytokinin and activation of Arabidopsis response regulators, ARR16 and ARR17 on the lower water potential side of the root leading to higher cell division and root bending. The cortex specific role of Abscisic acid (ABA)-activated SNF1-related protein kinase 2.2 (SnRK2.2) and MIZ1 in elongation zone is emerging for hydrotropic curvature. Halotropism involves clathrin-mediated internalization of PIN FORMED 2 (PIN2) proteins at the side facing higher salt concentration in the root tip, and ABA-activated SnRK2.6 mediated phosphorylation of cortical microtubule-associated protein Spiral2-like (SP2L) in the root transition zone, which results in anisotropic cell expansion and root bending away from higher salt. In hydropatterning, Indole-3-acetic acid 3 (IAA3) interacts with SUMOylated-ARF7 (Auxin response factor 7) and prevents expression of Lateral organ boundaries-domain 16 (LBD16) in air-side of the root, while on wet side of the root, IAA3 cannot repress the non-SUMOylated-ARF7 thereby leading to LBD16 expression and lateral root development. In root vasculature, ABA induces expression of microRNA165/microRNA166 in endodermis, which moves into the stele to target class III Homeodomain leucine zipper protein (HD-ZIP III) mRNA in non-cell autonomous manner. The bidirectional gradient of microRNA165/6 and HD-ZIP III mRNA regulates xylem patterning under stress. Understanding the tissue specific molecular mechanisms regulating the root responses under heterogeneous and stress environments will help in designing climate-resilient crops.

Role of myristoylation in modulating PCaP1 interaction with calmodulin

Plasma membrane-associated Cation-binding Protein 1 (PCaP1) belongs to the plant-unique DREPP protein family with largely unknown biological functions but ascertained roles in plant development and calcium (Ca2+) signaling. PCaP1 is anchored to the plasma membrane via N-myristoylation and a polybasic cluster, and its N-terminal region can bind Ca2+/calmodulin (CaM). However, the molecular determinants of PCaP1-Ca2+-CaM interaction and the functional impact of myristoylation in the complex formation and Ca2+ sensitivity of CaM remained to be elucidated. Herein, we investigated the direct interaction between Arabidopsis PCaP1 (AtPCaP1) and CaM1 (AtCaM1) using both myristoylated and non-myristoylated peptides corresponding to the N-terminal region of AtPCaP1. ITC analysis showed that AtCaM1 forms a high affinity 1:1 complex with AtPCaP1 peptides and the interaction is strictly Ca2+-dependent. Spectroscopic and kinetic Ca2+ binding studies showed that the myristoylated peptide dramatically increased the Ca2+-binding affinity of AtCaM1 and slowed the Ca2+ dissociation rates from both the C- and N-lobes, thus suggesting that the myristoylation modulates the mechanism of AtPCaP1 recognition by AtCaM1. Furthermore, NMR and CD spectroscopy revealed that the structure of both the N- and C-lobes of Ca2+-AtCaM1 changes markedly in the presence of the myristoylated AtPCaP1 peptide, which assumes a helical structure in the final complex. Overall, our results indicate that AtPCaP1 biological function is strictly related to the presence of multiple ligands, i.e., the myristoyl moiety, Ca2+ ions and AtCaM1 and only a full characterization of their equilibria will allow for a complete molecular understanding of the putative role of PCaP1 as signal protein.

The BR signaling pathway regulates primary root development and drought stress response by suppressing the expression of PLT1 and PLT2 in Arabidopsis thaliana

Introduction

With the warming global climate, drought stress has become an important abiotic stress factor limiting plant growth and crop yield. As the most rapidly drought-sensing organs of plants, roots undergo a series of changes to enhance their ability to absorb water, but the molecular mechanism is unclear.

Results and methods

In this study, we found that PLT1 and PLT2, two important transcription factors of root development in Arabidopsis thaliana, are involved in the plant response to drought and are inhibited by BR signaling. PLT1- and PLT2-overexpressing plants showed greater drought tolerance than wild-type plants. Furthermore, we found that BZR1 could bind to the promoter of PLT1 and inhibit its transcriptional activity in vitro and in vivo. PLT1 and PLT2 were regulated by BR signaling in root development and PLT2 could partially rescue the drought sensitivity of bes1-D. In addition, RNA-seq data analysis showed that BR-regulated root genes and PLT1/2 target genes were also regulated by drought; for example, CIPK3, RCI2A, PCaP1, PIP1;5, ERF61 were downregulated by drought and PLT1/2 but upregulated by BR treatment; AAP4, WRKY60, and AT5G19970 were downregulated by PLT1/2 but upregulated by drought and BR treatment; and RGL2 was upregulated by drought and PLT1/2 but downregulated by BR treatment.

Discussion

Our findings not only reveal the mechanism by which BR signaling coordinates root growth and drought tolerance by suppressing the expression of PLT1 and PLT2 but also elucidates the relationship between drought and root development. The current study thus provides an important theoretical basis for the improvement of crop yield under drought conditions.

The plasma membrane-associated cation-binding protein PCaP1 of Arabidopsis thaliana is a uranyl-binding protein

Uranium (U) is a naturally-occurring radionuclide that is toxic to living organisms. Given that proteins are primary targets of U(VI), their identification is an essential step towards understanding the mechanisms of radionuclide toxicity, and possibly detoxification. Here, we implemented a chromatographic strategy including immobilized metal affinity chromatography to trap protein targets of uranyl in Arabidopsis thaliana. This procedure allowed the identification of 38 uranyl-binding proteins (UraBPs) from root and shoot extracts. Among them, UraBP25, previously identified as plasma membrane-associated cation-binding protein 1 (PCaP1), was further characterized as a protein interacting in vitro with U(VI) and other metals using spectroscopic and structural approaches, and in planta through analyses of the fate of U(VI) in Arabidopsis lines with altered PCaP1 gene expression. Our results showed that recombinant PCaP1 binds U(VI) in vitro with affinity in the nM range, as well as Cu(II) and Fe(III) in high proportions, and that Ca(II) competes with U(VI) for binding. U(VI) induces PCaP1 oligomerization through binding at the monomer interface, at both the N-terminal structured domain and the C-terminal flexible region. Finally, U(VI) translocation in Arabidopsis shoots was affected in pcap1 null-mutant, suggesting a role for this protein in ion trafficking in planta.

MicroRNAs balance growth and salt stress responses in sweet sorghum

Salt stress is one of the major causes of reduced crop production, limiting agricultural development globally. Plants have evolved with complex systems to maintain the balance between growth and stress responses, where signaling pathways such as hormone signaling play key roles. Recent studies revealed that hormones are modulated by microRNAs (miRNAs). Previously, two sweet sorghum (Sorghum bicolor) inbred lines with different salt tolerance were identified: the salt-tolerant M-81E and the salt-sensitive Roma. The levels of endogenous hormones in M-81E and Roma varied differently under salt stress, showing a different balance between growth and stress responses. miRNA and degradome sequencing showed that the expression of many upstream transcription factors regulating signal transduction and hormone-responsive genes was directly induced by differentially expressed miRNAs, whose levels were very different between the two sweet sorghum lines. Furthermore, the effects of representative miRNAs on salt tolerance in sorghum were verified through a transformation system mediated by Agrobacterium rhizogenes. Also, miR-6225-5p reduced the level of Ca²⁺ in the miR-6225-5p-overexpressing line by inhibiting the expression of the Ca²⁺ uptake gene SbGLR3.1 in the root epidermis and affected salt tolerance in sorghum. This study provides evidence for miRNA-mediated growth and stress responses in sweet sorghum.

Crop Root Responses to Drought Stress: Molecular Mechanisms, Nutrient Regulations, and Interactions with Microorganisms in the Rhizosphere

Roots play important roles in determining crop development under drought. Under such conditions, the molecular mechanisms underlying key responses and interactions with the rhizosphere in crop roots remain limited compared with model species such as Arabidopsis. This article reviews the molecular mechanisms of the morphological, physiological, and metabolic responses to drought stress in typical crop roots, along with the regulation of soil nutrients and microorganisms to these responses. Firstly, we summarize how root growth and architecture are regulated by essential genes and metabolic processes under water-deficit conditions. Secondly, the functions of the fundamental plant hormone, abscisic acid, on regulating crop root growth under drought are highlighted. Moreover, we discuss how the responses of crop roots to altered water status are impacted by nutrients, and vice versa. Finally, this article explores current knowledge of the feedback between plant and soil microbial responses to drought and the manipulation of rhizosphere microbes for improving the resilience of crop production to water stress. Through these insights, we conclude that to gain a more comprehensive understanding of drought adaption mechanisms in crop roots, future studies should have a network view, linking key responses of roots with environmental factors.

MDP25 mediates the fine-tuning of microtubule organization in response to salt stress

Microtubules are dynamic cytoskeleton structures playing fundamental roles in plant responses to salt stress. The precise mechanisms by which microtubule organization is regulated under salt stress are largely unknown. Here, we report that Arabidopsis thaliana MICROTUBULE-DESTABILIZING PROTEIN 25 (MDP25; also known as PLASMA MEMBRANE-ASSOCIATED CATION-BINDING PROTEIN 1 (PCaP1)) helps regulate microtubule organization. Under salt treatment, elevated cytosolic Ca²⁺ concentration caused MDP25 to partially dissociate from the plasma membrane, promoting microtubule depolymerization. When Ca²⁺ signaling was blocked by BAPTA-AM or LaCl₃ , microtubule depolymerization in wild-type and MDP25-overexpressing cells was slower, while there was no obvious change in mdp25 cells. Knockout of MDP25 improved microtubule reassembly and was conducive to microtubule integrity under long-term salt treatment and microtubule recovery after salt stress. Moreover, mdp25 seedlings exhibited a higher survival rate under salt stress. The presence microtubule-disrupting reagent oryzalin or microtubule-stabilizing reagent paclitaxel differentially affected the survival rates of different genotypes under salt stress. MDP25 promoted microtubule instability by affecting the catastrophe and rescue frequencies, shrinkage rate and time in pause phase at the microtubule plus-end and the depolymerization rate at the microtubule minus-end. These findings reveal a role for MDP25 in regulating microtubule organization under salt treatment by affecting microtubule dynamics.

Size and activity of the root meristem: A key for drought resistance and a key model of drought-related signaling

Plants make many adjustments to their growth and development in response to even small changes in water availability. Under such conditions, root elongation can be actively restricted by stress-related signaling mechanisms. Here we look at how the Arabidopsis thaliana root meristem can be affected by moderate water limitation (low water potential, ψ_w ). Recent characterization of the clade E Growth-Regulating (EGR) protein phosphatases and Microtubule Associated Stress Protein 1 (MASP1) provides an example of how active restriction of root meristem size allows the plant to downregulate root elongation during low ψ_w stress. EGR2 protein accumulation in cortex cells of the transition zone at the distal end of the root meristem illustrates how the balance of cell division versus cell expansion signals at this critical location can determine meristem size and root elongation during low ψ_w . These characteristics of EGRs also raise the question of whether they may also be involved in hydrotropism, and, more broadly, whether hydrotropism is a distinct response or a specific manifestation of more general mechanisms used to adjust root growth under moderate severity low ψ_w whether or not a gradient of water availability is present. These questions, as well as a better understanding of how specific cell layers (cortex and endodermis) seem to have an outsized role in growth regulation and better understanding the roles of plasma membrane-based signaling and polar-localized proteins in the regulation of root meristem size and cell division activity are key to elucidating the cellular mechanisms that determine root growth behavior during soil drying.

Root plasticity under abiotic stress

Abiotic stresses increasingly threaten existing ecological and agricultural systems across the globe. Plant roots perceive these stresses in the soil and adapt their architecture accordingly. This review provides insights into recent discoveries showing the importance of root system architecture (RSA) and plasticity for the survival and development of plants under heat, cold, drought, salt, and flooding stress. In addition, we review the molecular regulation and hormonal pathways involved in controlling RSA plasticity, main root growth, branching and lateral root growth, root hair development, and formation of adventitious roots. Several stresses affect root anatomy by causing aerenchyma formation, lignin and suberin deposition, and Casparian strip modulation. Roots can also actively grow toward favorable soil conditions and avoid environments detrimental to their development. Recent advances in understanding the cellular mechanisms behind these different root tropisms are discussed. Understanding root plasticity will be instrumental for the development of crops that are resilient in the face of abiotic stress.

The plasma membrane-associated Ca2+ -binding protein, PCaP1, is required for oligogalacturonide and flagellin-induced priming and immunity

Early signalling events in response to elicitation include reversible protein phosphorylation and re-localization of plasma membrane (PM) proteins. Oligogalacturonides (OGs) are a class of damage-associated molecular patterns (DAMPs) that act as endogenous signals to activate the plant immune response. Previous data on early phosphoproteome changes in Arabidopsis thaliana upon OG perception uncovered the immune-related phospho-regulation of several membrane proteins, among which PCaP1, a PM-anchored protein with actin filament-severing activity, was chosen for its potential involvement in OG- and flagellin-triggered responses. Here, we demonstrate that PCaP1 is required for late, but not early, responses induced by OGs and flagellin. Moreover, pcap1 mutants, unlike the wild type, are impaired in the recovery of full responsiveness to a second treatment with OGs performed 24 h after the first one. Localization studies on PCaP1 upon OG treatment in plants expressing a functional PCaP1-GFP fusion under the control of PCaP1 promoter revealed fluorescence on the PM, organized in densely packed punctate structures, previously reported as microdomains. Fluorescence was found to be associated also with endocytic vesicles, the number of which rapidly increased after OG treatment, suggesting both an endocytic turnover of PCaP1 for maintaining its homeostasis at the PM and an OG-induced endocytosis.

Histone Demethylases Coordinate the Antagonistic Interaction Between Abscisic Acid and Brassinosteroid Signaling in Arabidopsis

Abscisic acid (ABA) interacts antagonistically with brassinosteroids (BRs) to control plant growth and development in response to stress. The response to environmental cues includes hormonal control via epigenetic regulation of gene expression. However, the details of the ABA-BR crosstalk remain largely unknown. Here, we show that JUMONJI-C domain containing histone demethylases (JMJs) coordinate the antagonistic interaction between ABA and BR signaling pathways during the post-germination stage in Arabidopsis. BR blocks ABA-mediated seedling arrest through repression of JMJ30. JMJs remove the repressive histone marks from the BRASSINAZOLE RESISTANT1 (BZR1) locus for its activation to balance ABA and BR signaling pathways. JMJs and BZR1 co-regulate genes encoding three membrane proteins, a regulator of vacuole morphology, and two lipid-transfer proteins, each of which play a different role in transport. BZR1 also regulates stimuli-related target genes in a JMJ-independent pathway. Our findings suggest that the histone demethylases integrate ABA and BR signals, leading to changes in growth program after germination.

Molecular mechanisms mediating root hydrotropism: what we have observed since the rediscovery of hydrotropism

Roots display directional growth toward moisture in response to a water potential gradient. Root hydrotropism is thought to facilitate plant adaptation to continuously changing water availability. Hydrotropism has not been as extensively studied as gravitropism. However, comparisons of hydrotropic and gravitropic responses identified mechanisms that are unique to hydrotropism. Regulatory mechanisms underlying the hydrotropic response appear to differ among different species. We recently performed molecular and genetic analyses of root hydrotropism in Arabidopsis thaliana. In this review, we summarize the current knowledge of specific mechanisms mediating root hydrotropism in several plant species.

Root Tropisms: Investigations on Earth and in Space to Unravel Plant Growth Direction

Root tropisms are important responses of plants, allowing them to adapt their growth direction. Research on plant tropisms is indispensable for future space programs that envisage plant-based life support systems for long-term missions and planet colonization. Root tropisms encompass responses toward or away from different environmental stimuli, with an underexplored level of mechanistic divergence. Research into signaling events that coordinate tropistic responses is complicated by the consistent coincidence of various environmental stimuli, often interacting via shared signaling mechanisms. On Earth the major determinant of root growth direction is the gravitational vector, acting through gravitropism and overruling most other tropistic responses to environmental stimuli. Critical advancements in the understanding of root tropisms have been achieved nullifying the gravitropic dominance with experiments performed in the microgravity environment. In this review, we summarize current knowledge on root tropisms to different environmental stimuli. We highlight that the term tropism must be used with care, because it can be easily confused with a change in root growth direction due to asymmetrical damage to the root, as can occur in apparent chemotropism, electrotropism, and magnetotropism. Clearly, the use of Arabidopsis thaliana as a model for tropism research contributed much to our understanding of the underlying regulatory processes and signaling events. However, pronounced differences in tropisms exist among species, and we argue that these should be further investigated to get a more comprehensive view of the signaling pathways and sensors. Finally, we point out that the Cholodny-Went theory of asymmetric auxin distribution remains to be the central and unifying tropistic mechanism after 100 years. Nevertheless, it becomes increasingly clear that the theory is not applicable to all root tropistic responses, and we propose further research to unravel commonalities and differences in the molecular and physiological processes orchestrating root tropisms.