Molecular mechanisms of hydrotropism in seedling roots of Arabidopsis thaliana (Brassicaceae)

Not so hidden anymore: Advances and challenges in understanding root growth under water deficits

Limited water availability is a major environmental factor constraining plant development and crop yields. One of the prominent adaptations of plants to water deficits is the maintenance of root growth that enables sustained access to soil water. Despite early recognition of the adaptive significance of root growth maintenance under water deficits, progress in understanding has been hampered by the inherent complexity of root systems and their interactions with the soil environment. We highlight selected milestones in the understanding of root growth responses to water deficits, with emphasis on founding studies that have shaped current knowledge and set the stage for further investigation. We revisit the concept of integrated biophysical and metabolic regulation of plant growth and use this framework to review central growth-regulatory processes occurring within root growth zones under water stress at subcellular to organ scales. Key topics include the primary processes of modifications of cell wall-yielding properties and osmotic adjustment, as well as regulatory roles of abscisic acid and its interactions with other hormones. We include consideration of long-recognized responses for which detailed mechanistic understanding has been elusive until recently, for example hydrotropism, and identify gaps in knowledge, ongoing challenges, and opportunities for future research.

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.

Involvement of Dynamic Adjustment of ABA, Proline and Sugar Levels in Rhizomes in Effective Acclimation of Solidago gigantea to Contrasting Weather and Soil Conditions in the Country of Invasion

Giant goldenrod (Solidago gigantea Aiton) is one of the most invasive plant species occurring in Europe. Since little is known about the molecular mechanisms contributing to its invasiveness, we examined the natural dynamics of the content of rhizome compounds, which can be crucial for plant resistance and adaptation to environmental stress. We focused on rhizomes because they are the main vector of giant goldenrod dispersion in invaded lands. Water-soluble sugars, proline, and abscisic acid (ABA) were quantified in rhizomes, as well as ABA in the rhizosphere from three different but geographically close natural locations in Poland (50°04'11.3″ N, 19°50'40.2″ E) under extreme light, thermal, and soil conditions, in early spring, late summer, and late autumn. The genetic diversity of plants between locations was checked using the random amplified polymorphic DNA (RAPD) markers. Sugar and proline content was assayed spectrophotometrically, and abscisic acid (ABA) with the ELISA immunomethod. It can be assumed that the accumulation of sugars in giant goldenrod rhizomes facilitated the process of plant adaptation to adverse environmental conditions (high temperature and/or water scarcity) caused by extreme weather in summer and autumn. The same was true for high levels of proline and ABA in summer. On the other hand, the lowering of proline and ABA in autumn did not confirm the previous assumptions about their synthesis in rhizomes during the acquisition of frost resistance by giant goldenrod. However, in the location with intensive sunlight and most extreme soil conditions, a constant amount of ABA in rhizomes was noticed as well as its exudation into the rhizosphere. This research indicates that soluble sugars, proline, and ABA alterations in rhizomes can participate in the mechanism of acclimation of S. gigantea to specific soil and meteorological conditions in the country of invasion irrespective of plant genetic variation.

The link between hydrotropism and phototropism in Arabidopsis roots

This article comments on:

Pang L, Kobayashi A, Atsumi Y, Miyazawa Y, Fujii N, Dietrich D, Bennett MJ, Takahashi H. 2023. MIZU-KUSSEI1 (MIZ1) and GNOM/MIZ2 control not only positive hydrotropism but also phototropism in Arabidopsis roots. Journal of Experimental Botany 74, 5026–5038.

MIZU-KUSSEI1 (MIZ1) and GNOM/MIZ2 control not only positive hydrotropism but also phototropism in Arabidopsis roots

In response to unilateral blue light illumination, roots of some plant species such as Arabidopsis thaliana exhibit negative phototropism (bending away from light), which is important for light avoidance in nature. MIZU-KUSSEI1 (MIZ1) and GNOM/MIZ2 are essential for positive hydrotropism (i.e. in the presence of a moisture gradient, root bending towards greater water availability). Intriguingly, mutations in these genes also cause a substantial reduction in phototropism. Here, we examined whether the same tissue-specific sites of expression required for MIZ1- and GNOM/MIZ2-regulated hydrotropism in Arabidopsis roots are also required for phototropism. The attenuated phototropic response of miz1 roots was completely restored when a functional MIZ1-green fluorescent protein (GFP) fusion was expressed in the cortex of the root elongation zone but not in other tissues such as root cap, meristem, epidermis, or endodermis. The hydrotropic defect and reduced phototropism of miz2 roots were restored by GNOM/MIZ2 expression in either the epidermis, cortex, or stele, but not in the root cap or endodermis. Thus, the sites in root tissues that are involved in the regulation of MIZ1- and GNOM/MIZ2-dependent hydrotropism also regulate phototropism. These results suggest that MIZ1- and GNOM/MIZ2-mediated pathways are, at least in part, shared by hydrotropic and phototropic responses in Arabidopsis roots.

Wheat ABA Receptor TaPYL5 Constitutes a Signaling Module with Its Downstream Partners TaPP2C53/TaSnRK2.1/TaABI1 to Modulate Plant Drought Response

Abscisic acid receptors (ABR) play crucial roles in transducing the ABA signaling initiated by osmotic stresses, which has a significant impact on plant acclimation to drought by modulating stress-related defensive physiological processes. We characterized TaPYL5, a member of the ABR family in wheat (Triticum aestivum), as a mediator of drought stress adaptation in plants. The signals derived from the fusion of TaPYL5-GFP suggest that the TaPYL5 protein was directed to various subcellular locations, namely stomata, plasma membrane, and nucleus. Drought stress significantly upregulated the TaPYL5 transcripts in roots and leaves. The biological roles of ABA and drought responsive cis-elements, specifically ABRE and recognition sites MYB, in mediating gene transcription under drought conditions were confirmed by histochemical GUS staining analysis for plants harbouring a truncated TaPYL5 promoter. Yeast two-hybrid and BiFC assays indicated that TaPYL5 interacted with TaPP2C53, a clade A member of phosphatase (PP2C), and the latter with TaSnRK2.1, a kinase member of the SnRK2 family, implying the formation of an ABA core signaling module TaPYL5/TaPP2C53/TaSnRK2.1. TaABI1, an ABA responsive transcription factor, proved to be a component of the ABA signaling pathway, as evidenced by its interaction with TaSnRK2.1. Transgene analysis of TaPYL5 and its module partners, as well as TaABI1, revealed that they have an effect on plant drought responses. TaPYL5 and TaSnRK2.1 positively regulated plant drought acclimation, whereas TaPP2C53 and TaABI1 negatively regulated it. This coincided with the osmotic stress-related physiology shown in their transgenic lines, such as stomata movement, osmolytes biosynthesis, and antioxidant enzyme function. TaPYL5 significantly altered the transcription of numerous genes involved in biological processes related to drought defense. Our findings suggest that TaPYL5 is one of the most important regulators in plant drought tolerance and a valuable target for engineering drought-tolerant cultivars in wheat.

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.

Plant Gravitropism: From Mechanistic Insights into Plant Function on Earth to Plants Colonizing Other Worlds

Gravitropism, the growth of roots and shoots toward or away from the direction of gravity, has been studied for centuries. Such studies have not only led to a better understanding of the gravitropic process itself, but also paved new paths leading to deeper mechanistic insights into a wide range of research areas. These include hormone biology, cell signal transduction, regulation of gene expression, plant evolution, and plant interactions with a variety of environmental stimuli. In addition to contributions to basic knowledge about how plants function, there is accumulating evidence that gravitropism confers adaptive advantages to crops, particularly under marginal agricultural soils. Therefore, gravitropism is emerging as a breeding target for enhancing agricultural productivity. Moreover, research on gravitropism has spawned several studies on plant growth in microgravity that have enabled researchers to uncouple the effects of gravity from other tropisms. Although rapid progress on understanding gravitropism witnessed during the past decade continues to be driven by traditional molecular, physiological, and cell biological tools, these tools have been enriched by technological innovations in next-generation omics platforms and microgravity analog facilities. In this chapter, we review the field of gravitropism by highlighting recent landmark studies that have provided unique insights into this classic research topic while also discussing potential contributions to agriculture on Earth and beyond.

Inter-tissue and inter-organ signaling in drought stress response and phenotyping of drought tolerance

Plant response to drought stress includes systems for intracellular regulation of gene expression and signaling, as well as inter-tissue and inter-organ signaling, which helps entire plants acquire stress resistance. Plants sense water-deficit conditions both via the stomata of leaves and roots, and transfer water-deficit signals from roots to shoots via inter-organ signaling. Abscisic acid is an important phytohormone involved in the drought stress response and adaptation, and is synthesized mainly in vascular tissues and guard cells of leaves. In leaves, stress-induced abscisic acid is distributed to various tissues by transporters, which activates stomatal closure and expression of stress-related genes to acquire drought stress resistance. Moreover, the stepwise stress response at the whole-plant level is important for proper understanding of the physiological response to drought conditions. Drought stress is sensed by multiple types of sensors as molecular patterns of abiotic stress signals, which are transmitted via separate parallel signaling networks to induce downstream responses, including stomatal closure and synthesis of stress-related proteins and metabolites. Peptide molecules play important roles in the inter-organ signaling of dehydration from roots to shoots, as well as signaling of osmotic changes and reactive oxygen species/Ca²⁺ . In this review, we have summarized recent advances in research on complex plant drought stress responses, focusing on inter-tissue signaling in leaves and inter-organ signaling from roots to shoots. We have discussed the mechanisms via which drought stress adaptations and resistance are acquired at the whole-plant level, and have proposed the importance of quantitative phenotyping for measuring plant growth under drought conditions.

LAZY1-LIKE-mediated gravity signaling pathway in root gravitropic set-point angle control

Gravity signaling components contribute to the control of root gravitropic set-point angle through protein polarization relay within columella.

Evidence for root adaptation to a spatially discontinuous water availability in the absence of external water potential gradients

We hereby show that root systems adapt to a spatially discontinuous pattern of water availability even when the gradients of water potential across them are vanishingly small. A paper microfluidic approach allowed us to expose the entire root system of Brassica rapa plants to a square array of water sources, separated by dry areas. Gradients in the concentration of water vapor across the root system were as small as 10^-4⋅mM⋅m^-1 (∼4 orders of magnitude smaller than in conventional hydrotropism assays). Despite such minuscule gradients (which greatly limit the possible influence of the well-understood gradient-driven hydrotropic response), our results show that 1) individual roots as well as the root system as a whole adapt to the pattern of water availability to maximize access to water, and that 2) this adaptation increases as water sources become more rare. These results suggest that either plant roots are more sensitive to water gradients than humanmade water sensors by 3-5 orders of magnitude, or they might have developed, like other organisms, mechanisms for water foraging that allow them to find water in the absence of an external gradient in water potential.

Long-distance stress and developmental signals associated with abscisic acid signaling in environmental responses

Flowering plants consist of highly differentiated organs, including roots, leaves, shoots and flowers, which have specific roles: root system for water and nutrient uptake, leaves for photosynthesis and gas exchange and reproductive organs for seed production. The communication between organs through the vascular system, by which water, nutrient and signaling molecules are transported, is essential for coordinated growth and development of the whole plant, particularly under adverse conditions. Here, we highlight recent progress in understanding how signaling pathways of plant hormones are associated with long-distance stress and developmental signals, with particular focus on environmental stress responses. In addition to the root-to-shoot peptide signal that induces abscisic acid accumulation in leaves under drought stress conditions, we summarize the diverse stress-responsive peptide signals reported to date to play a role in environmental responses.

Smart Root Search (SRS): A Novel Nature-Inspired Search Algorithm

In this paper, a novel heuristic search algorithm called Smart Root Search (SRS) is proposed. SRS employs intelligent foraging behavior of immature, mature and hair roots of plants to explore and exploit the problem search space simultaneously. SRS divides the search space into several subspaces. It thereupon utilizes the branching and drought operations to focus on richer areas of promising subspaces while extraneous ones are not thoroughly ignored. To achieve this, the smart reactions of the SRS model are designed to act based on analyzing the heterogeneous conditions of various sections of different search spaces. In order to evaluate the performance of the SRS, it was tested on a set of known unimodal and multimodal test functions. The results were then compared with those obtained using genetic algorithms, particle swarm optimization, differential evolution and imperialist competitive algorithms and then analyzed statistically. The results demonstrated that the SRS outperformed comparative algorithms for 92% and 82% of the investigated unimodal and multimodal test functions, respectively. Therefore, the SRS is a promising nature-inspired optimization algorithm.

Morpho-Physiology and Antioxidant Enzyme Activities of Transgenic Rice Plant Overexpressing ABP57 under Reproductive Stage Drought Condition

MR219 transgenic rice line which overexpressed an auxin-binding protein (ABP57) and its wild-type cultivar, MR219, were screened under well-watered (WW) and drought stress (DS) conditions at the early reproductive stage. This study was conducted with the standard planting distance and under a normal environment to assess the yield advantages based on the field conditions. The aim of this study was to understand the response of these rice genotypes towards DS at morpho-physiological, biochemical, and agronomical levels. It was found that the DS had affected all these levels of the genotypes studied; however, the transgenic plant showed a higher number of tillers, flag leaf area, biomass, relative water content, total chlorophyll content, and antioxidative defense mechanism than the MR219 under DS. Compared to its wild-type, the transgenic plant showed an increased leaf photosynthetic rate by 7% under WW and 11% under DS. The transgenic plant also showed higher yields than MR219 under the WW (10%) and DS (59%). The results propose that drought tolerance is significantly improved in the MR219 transgenic rice line. It may develop a new opportunity for the drought-tolerant rice breeding programme via overexpression of ABP57.

Root architecture and hydraulics converge for acclimation to changing water availability

Because of intense transpiration and growth, the needs of plants for water can be immense. Yet water in the soil is most often heterogeneous if not scarce due to more and more frequent and intense drought episodes. The converse context, flooding, is often associated with marked oxygen deficiency and can also challenge the plant water status. Under our feet, roots achieve an incredible challenge to meet the water demand of the plant's aerial parts under such dramatically different environmental conditions. For this, they continuously explore the soil, building a highly complex, branched architecture. On shorter time scales, roots keep adjusting their water transport capacity (their so-called hydraulics) locally or globally. While the mechanisms that directly underlie root growth and development as well as tissue hydraulics are being uncovered, the signalling mechanisms that govern their local and systemic adjustments as a function of water availability remain largely unknown. A comprehensive understanding of root architecture and hydraulics as a whole (in other terms, root hydraulic architecture) is needed to apprehend the strategies used by plants to optimize water uptake and possibly improve crops regarding this crucial trait.

Hydrotropism in the primary roots of maize

Recent studies mainly in Arabidopsis have renewed interest and discussion in some of the key issues in hydrotropism of roots, such as the site of water sensing and the involvement of auxin. We examined hydrotropism in maize (Zea mays) primary roots. We determined the site of water sensing along the root using a nonintrusive method. Kinematic analysis was conducted to investigate spatial root elongation during hydrotropic response. Indole-3-acetic acid (IAA) and other hormones were quantified using LC-MS/MS. The transcriptome was analyzed using RNA sequencing. Main results: The very tip of the root is the most sensitive to the hydrostimulant. Hydrotropic bending involves coordinated adjustment of spatial cell elongation and cell flux. IAA redistribution occurred in maize roots, preceding hydrotropic bending. The redistribution is caused by a reduction of IAA content on the side facing a hydrostimulant, resulting in a higher IAA content on the dry side. Transcriptomic analysis of the elongation zone prior to bending identified IAA response and lignin synthesis/wall cross-linking as some of the key processes occurring during the early stages of hydrotropic response. We conclude that maize roots differ from Arabidopsis in the location of hydrostimulant sensing and the involvement of IAA redistribution.

All Roads Lead to Auxin: Post-translational Regulation of Auxin Transport by Multiple Hormonal Pathways

Auxin is a key hormonal regulator, that governs plant growth and development in concert with other hormonal pathways. The unique feature of auxin is its polar, cell-to-cell transport that leads to the formation of local auxin maxima and gradients, which coordinate initiation and patterning of plant organs. The molecular machinery mediating polar auxin transport is one of the important points of interaction with other hormones. Multiple hormonal pathways converge at the regulation of auxin transport and form a regulatory network that integrates various developmental and environmental inputs to steer plant development. In this review, we discuss recent advances in understanding the mechanisms that underlie regulation of polar auxin transport by multiple hormonal pathways. Specifically, we focus on the post-translational mechanisms that contribute to fine-tuning of the abundance and polarity of auxin transporters at the plasma membrane and thereby enable rapid modification of the auxin flow to coordinate plant growth and development.

Cloning and characterization of a gene encoding MIZ1, a domain of unknown function protein and its role in salt and drought stress in rice

Dwindling fresh water resources and climate change poses serious threats to rice production. Roots play crucial role in sensing water gradient and directing growth of the plant towards water through a mechanism called hydrotropism. Since very little information is available on root hydrotropism in major food crops, this study was carried out to clone and characterize an ortholog of Arabidopsis MIZU-KUSSEI1 (MIZ1) from rice. Contrasting rice genotypes for drought and salt tolerance were selected based on phenotyping for root traits. Nagina 22 and CR-262-4 were identified as most tolerant and Pusa Sugandh 5 and Pusa Basmati 1121 were identified as most susceptible varieties for both drought and salt stresses. Allele mining of MIZ1 in these varieties identified a 12 bp Indel but did not show specific allelic association with stress tolerance. Analysis of allelic variation of OsMIZ1 in 3024 rice genotypes of 3K genome lines using Rice SNP-Seek database revealed 49 InDels. Alleles with the 12 bp deletions were significantly prevalent in indica group as compared to that of japonica group. Real-time RT-PCR analysis revealed that OsMIZ1 expression levels were upregulated significantly in tolerant cv. Nagina 22 and CR-262-4 under osmotic stress, while under salt stress, it was significantly upregulated only in CR-262-4 but maintained in Nagina 22 under salt stress. However, in the roots of susceptible genotypes, OsMIZ1 expression decreased under both the stresses. These results highlight the possible involvement of OsMIZ1 in drought and salt stress tolerance in rice. Furthermore, expression studies using publically available resources showed that enhanced expression of OsMIZ1 is regulated in response to disease infections, mineral deficiency, and heavy metal stresses and is also expressed in reproductive tissues in addition to roots. These findings indicate potential involvement of MIZ1 in developmental and stress response processes in rice.

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.

Developmental Responses to Water and Salinity in Root Systems

Roots provide the primary mechanism that plants use to absorb water and nutrients from their environment. These functions are dependent on developmental mechanisms that direct root growth and branching into regions of soil where these resources are relatively abundant. Water is the most limiting factor for plant growth, and its availability is determined by the weather, soil structure, and salinity. In this review, we define the developmental pathways that regulate the direction of growth and branching pattern of the root system, which together determine the expanse of soil from which a plant can access water. The ability of plants to regulate development in response to the spatial distribution of water is a focus of many recent studies and provides a model for understanding how biological systems utilize positional cues to affect signaling and morphogenesis. A better understanding of these processes will inform approaches to improve crop water use efficiency to more sustainably feed a growing population.

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

Root hydrotropism is an essential growth response to water potential gradients in plants. To understand the mechanism, fundamental elements such as MIZU-KUSSEI 1 (MIZ1) have been investigated extensively. We investigated the physiological role of a plasma membrane-associated cation-binding protein (PCaP1) and examined the effect of PCaP1 loss-of-function mutations on root hydrotropism. pcap1 knockout mutants showed a defect in root bending as a hydrotropic response, although gravitropism was normal in pcap1 mutants. When pcap1 seedlings were treated with abscisic acid, a negative regulator of gravitropism, the seedlings showed normal gravitropism. The hydrotropism defect in pcap1 mutants was clearly rescued by introducing the genomic sequence of PCaP1 with an endodermis-specific promoter. Analysis of PCaP1-greenfluorescent protein-expressing roots by confocal laser scanning microscopy revealed that PCaP1 was stably associated with the plasma membrane in most cells, but in the cytoplasm of endodermal cells at the bending region. Furthermore, we prepared a transgenic line overexpressing MIZ1 on the pcap1 background and found that the pcap1 hydrotropism defect was rescued. Our results indicate that PCaP1 in the endodermal cells of the root elongation zone is involved in the hydrotropic response. We suggest that PCaP1 contributes to hydrotropism through a MIZ1-independent pathway or as one of the upstream components that transduce water potential signals to MIZ1.

A method for rapid analysis of the root hydrotropic response in Arabidopsis thaliana

The system for analyzing the hydrotropic curvature with a moisture gradient in wild-type Arabidopsis roots was modified. Optimal conditions were determined for detecting a hydrotropic curvature of 90° just after 4 h of stimulation. This system only requires 15 ml of a solution of K2CO3 with a density of 1.48 g·ml-1 to generate a rapid moisture gradient inside a square Petri dish without decreasing root growth. In this, the root growth rate observed in hydrostimulated wild-type and miz1 mutant, utilized as a negative control, increases sixfold compared with those roots examined using the former method.

Autophagy mediates hydrotropic response in Arabidopsis thaliana roots

This work shows that autophagy plays a key role in the hydrotropic curvature of Arabidopsis thaliana roots. An analysis of GFP-ATG8a transgenic plants showed that autophagosomes accumulated in the root curvature 2 h after the transfer of seedlings to Normal Medium-Water Stress Medium (NM-WSM). Autophagy flux was required for root bending. Remarkably, several atg mutants did not show hydrotropic curvature in NM-WSM or the splitting-agar system. Hyper, an H₂O₂ sensor showed that H₂O₂ preferentially accumulated in the root curvature at a similar rate as the autophagosomes did during hydrotropic response. Peroxidase and ROBH activity inhibition affected, negatively or positively root curvature. This data suggested H₂O₂ balance was required for root bending. Malondialdehyde, a metabolite used as an indicator of oxidative stress, accumulated at the same rate during the development of the curvature in NM-WSM. These results suggest that autophagy is required for the hydrotropic response in NM-WSM. We discuss the possible regulatory role of H₂O₂ on autophagy during the hydrotropic response that might relieve oxidative stress provoked by water stress. NM-WSM is water stress system suitable for studying hydrotropic responses on a short-term basis.

Hydrotropism: how roots search for water

Fresh water is an increasingly scarce resource for agriculture. Plant roots mediate water uptake from the soil and have developed a number of adaptive traits such as hydrotropism to aid water foraging. Hydrotropism modifies root growth to respond to a water potential gradient in soil and grow towards areas with a higher moisture content. Abscisic acid (ABA) and a small number of genes, including those encoding ABA signal transducers, MIZ2/GNOM, and the hydrotropism-specific MIZ1, are known to be necessary for the response in Arabidopsis thaliana, whereas the role of auxin in hydrotropism appears to vary depending on the plant species. This review will describe recent progress characterizing the hormonal regulation of hydrotropism. Recent advances in identifying the sites of hydrotropic perception and response, together with its interaction with gravitropism, will also be discussed. Finally, I will describe putative mechanisms for perception of the water potential gradient and a potential role for hydrotropism in acclimatizing plants to drought conditions.

Synergy between root hydrotropic response and root biomass in maize (Zea mays L.) enhances drought avoidance

Roots of higher plants change their growth direction in response to moisture, avoiding drought and gaining maximum advantage for development. This response is termed hydrotropism. There have been few studies of root hydrotropism in grasses, particularly in maize. Our goal was to test whether an enhanced hydrotropic response of maize roots correlates with a better adaptation to drought and partial/lateral irrigation in field studies. We developed a laboratory bioassay for testing hydrotropic response in primary roots of 47 maize elite DTMA (Drought Tolerant Maize for Africa) hybrids. After phenotyping these hybrids in the laboratory, selected lines were tested in the field. Three robust and three weak hybrids were evaluated employing three irrigation procedures: normal irrigation, partial lateral irrigation and drought. Hybrids with a robust hydrotropic response showed growth and developmental patterns, under drought and partial lateral irrigation, that differed from weak hydrotropic responders. A correlation between root crown biomass and grain yield in hybrids with robust hydrotropic response was detected. Hybrids with robust hydrotropic response showed earlier female flowering whereas several root system traits, such as projected root area, median width, maximum width, skeleton width, skeleton nodes, average tip diameter, rooting depth skeleton, thinner aboveground crown roots, as well as stem diameter, were considerably higher than in weak hydrotropic responders in the three irrigation procedures utilized. These results demonstrate the benefit of intensive phenotyping of hydrotropism in primary roots since maize plants that display a robust hydrotropic response grew better under drought and partial lateral irrigation, indicating that a selection for robust hydrotropism might be a promising breeding strategy to improve drought avoidance in maize.

Physiological and transcriptome analysis of heteromorphic leaves and hydrophilic roots in response to soil drying in desert Populus euphratica

Populus euphratica Olivier, which has been considered as a tree model for the study of higher plant response to abiotic stresses, survive in the desert ecosystem characterized by extreme drought stress. To survive in the harsh environmental condition the plant species have developed some plasticity such as the development of heteromorphic leaves and well-developed roots system. We investigated the physiological and molecular mechanisms enabling this species to cope with severe stress caused by drought. The heterophylly, evolved from linear to toothed-ovate shape, showed the significant difference in cuticle thickness, stomata densities, and sizes. Physiological parameters, SOD, POD, PPO, CAT activity, free proline, soluble protein and MDA contents fluctuated in response to soil drying. Gene expression profile of roots monitored at control and 4 moisture gradients regimes showed the up-regulation of 124, 130, 126 and 162 and down-regulation of 138, 251, 314, 168 DEGs, respectively. Xyloglucan endotransglucosylase/ hydrolase gene (XET) up-regulated at different moisture gradients, was cloned and expressed in tobacco. The XET promoter sequence harbors the drought signaling responsive cis-elements. The promoter expression activity varies in different organs. Over-expression and knocked down transgenic tobacco plant analysis confirmed the role of XET gene in roots growth and drought resistance.

Gravitropism interferes with hydrotropism via counteracting auxin dynamics in cucumber roots: clinorotation and spaceflight experiments

Roots of land plants show gravitropism and hydrotropism in response to gravity and moisture gradients, respectively, for controlling their growth orientation. Gravitropism interferes with hydrotropism, although the mechanistic aspects are poorly understood. Here, we differentiated hydrotropism from gravitropism in cucumber roots by conducting clinorotation and spaceflight experiments. We also compared mechanisms regulating hydrotropism and auxin-regulated gravitropism. Clinorotated or microgravity (μG)-grown cucumber seedling roots hydrotropically bent toward wet substrate in the presence of moisture gradients, but they grew straight in the direction of normal gravitational force at the Earth's surface (1G) on the ground or centrifuge-generated 1G in space. The roots appeared to become hydrotropically more sensitive to moisture gradients under μG conditions in space. Auxin transport inhibitors significantly reduced the hydrotropic response of clinorotated seedling roots. The auxin efflux protein CsPIN5 was differentially expressed in roots of both clinorotated and μG-grown seedlings; with higher expression in the high-humidity (concave) side than the low-humidity (convex) side of hydrotropically responding roots. Our results suggest that roots become hydrotropically sensitive in μG, and CsPIN5-mediated auxin transport has an important role in inducing root hydrotropism. Thus, hydrotropic and gravitropic responses in cucumber roots may compete via differential auxin dynamics established in response to moisture gradients and gravity.

Auxin transport and response requirements for root hydrotropism differ between plant species

The direction of auxin transport changes in gravistimulated roots, causing auxin accumulation in the lower side of horizontally reoriented roots. This study found that auxin was similarly involved in hydrotropism and gravitropism in rice and pea roots, but hydrotropism in Lotus japonicus roots was independent of both auxin transport and response. Application of either auxin transport inhibitors or an auxin response inhibitor decreased both hydrotropism and gravitropism in rice roots, and reduced hydrotropism in pea roots. However, Lotus roots treated with these inhibitors showed reduced gravitropism but an unaltered or an enhanced hydrotropic response. Inhibiting auxin biosynthesis substantially reduced both tropisms in rice and Lotus roots. Removing the final 0.2 mm (including the root cap) from the root tip inhibited gravitropism but not hydrotropism in rice seedling roots. These results suggested that modes of auxin involvement in hydrotropism differed between plant species. In rice roots, although auxin transport and responses were required for both gravitropism and hydrotropism, the root cap was involved in the auxin regulation of gravitropism but not hydrotropism. Hydrotropism in Lotus roots, however, may be regulated by a novel mechanism that is independent of both auxin transport and the TIR1/AFBs auxin response pathway.

Root hydrotropism is controlled via a cortex-specific growth mechanism

Plants can acclimate by using tropisms to link the direction of growth to environmental conditions. Hydrotropism allows roots to forage for water, a process known to depend on abscisic acid (ABA) but whose molecular and cellular basis remains unclear. Here we show that hydrotropism still occurs in roots after laser ablation removed the meristem and root cap. Additionally, targeted expression studies reveal that hydrotropism depends on the ABA signalling kinase SnRK2.2 and the hydrotropism-specific MIZ1, both acting specifically in elongation zone cortical cells. Conversely, hydrotropism, but not gravitropism, is inhibited by preventing differential cell-length increases in the cortex, but not in other cell types. We conclude that root tropic responses to gravity and water are driven by distinct tissue-based mechanisms. In addition, unlike its role in root gravitropism, the elongation zone performs a dual function during a hydrotropic response, both sensing a water potential gradient and subsequently undergoing differential growth.

Robust root growth in altered hydrotropic response1 (ahr1) mutant of Arabidopsis is maintained by high rate of cell production at low water potential gradient

Hydrotropism is the directional root growth response determined by water stimulus. In a water potential gradient system (WPGS) the roots of the Arabidopsis wild type have a diminished root growth compared to normal medium (NM). In contrast, the altered hydrotropic response1 (ahr1) mutant roots maintain their robust growth in the same WPGS. The aims of this work were to ascertain how ahr1 roots could sustain growth in the WPGS, with a special focus on the integration of cellular processes involved in the signaling that determines root growth during abiotic stress and their relation to hydrotropism. Cellular analysis of the root apical meristem of ahr1 mutant contrary to the wild type showed an absence of changes in the meristem length, the elongation zone length, the length of fully elongated cells, and the cell cycle duration. The robust and steady root growth of ahr1 seedlings in the WPGS is explained by the mutant capacity to maintain cell production and cell elongation at the same level as in the NM. Analysis of auxin response at a transcriptional level showed that roots of the ahr1 mutant had a lower auxin response when grown in the WPGS, compared to wild type, indicating that auxin signaling participates in attenuation of root growth under water stress conditions. Also, wild type plants exhibited a high increase in proline content while ahr1 mutants showed minimum changes in the Normal Medium→Water Stress Medium (NM→WSM), a lower water potential gradient system than the WPGS. Accordingly, in this condition, gene expression of Δ1-6 Pyrroline-5-Carboxylate Synthetase1 (P5CS1) involved in proline synthesis strongly increased in wild type but not in ahr1 seedlings. The ahr1 phenotype shows unique features since the mutant root cells continue to proliferate and grow in the presence of a progressively negative water potential gradient at a level comparable to wild type growing in the NM. As such, it represents an exceptional resource for understanding hydrotropism.

Roots Withstanding their Environment: Exploiting Root System Architecture Responses to Abiotic Stress to Improve Crop Tolerance

To face future challenges in crop production dictated by global climate changes, breeders and plant researchers collaborate to develop productive crops that are able to withstand a wide range of biotic and abiotic stresses. However, crop selection is often focused on shoot performance alone, as observation of root properties is more complex and asks for artificial and extensive phenotyping platforms. In addition, most root research focuses on development, while a direct link to the functionality of plasticity in root development for tolerance is often lacking. In this paper we review the currently known root system architecture (RSA) responses in Arabidopsis and a number of crop species to a range of abiotic stresses, including nutrient limitation, drought, salinity, flooding, and extreme temperatures. For each of these stresses, the key molecular and cellular mechanisms underlying the RSA response are highlighted. To explore the relevance for crop selection, we especially review and discuss studies linking root architectural responses to stress tolerance. This will provide a first step toward understanding the relevance of adaptive root development for a plant's response to its environment. We suggest that functional evidence on the role of root plasticity will support breeders in their efforts to include root properties in their current selection pipeline for abiotic stress tolerance, aimed to improve the robustness of crops.

Root Architecture Diversity and Meristem Dynamics in Different Populations of Arabidopsis thaliana

Arabidopsis thaliana has been an excellent model system for molecular genetic approaches to development and physiology. More recently, the potential of studying various accessions collected from diverse habitats has been started to exploit. Col-0 has been the best-studied accession but we now know that several traits show significant divergences among them. In this work, we focused in the root that has become a key system for development. We studied root architecture and growth dynamics of 12 Arabidopsis accessions. Our data reveal a wide variability in root architecture and root length among accessions. We also found variability in the root apical meristem (RAM), explained mainly by cell size at the RAM transition domain and possibly by peculiar forms of organization at the stem cell niche in some accessions. Contrary to Col-0 reports, in some accessions the RAM size not always explains the variations in the root length; indicating that elongated cell size could be more relevant in the determination of root length than the RAM size itself. This study contributes to investigations dealing with understanding the molecular and cellular basis of phenotypic variation, the role of plasticity on adaptation, and the developmental mechanisms that may restrict phenotypic variation in response to contrasting environmental conditions.

The holistic rhizosphere: integrating zones, processes, and semantics in the soil influenced by roots

Despite often being conceptualized as a thin layer of soil around roots, the rhizosphere is actually a dynamic system of interacting processes. Hiltner originally defined the rhizosphere as the soil influenced by plant roots. However, soil physicists, chemists, microbiologists, and plant physiologists have studied the rhizosphere independently, and therefore conceptualized the rhizosphere in different ways and using contrasting terminology. Rather than research-specific conceptions of the rhizosphere, the authors propose a holistic rhizosphere encapsulating the following components: microbial community gradients, macroorganisms, mucigel, volumes of soil structure modification, and depletion or accumulation zones of nutrients, water, root exudates, volatiles, and gases. These rhizosphere components are the result of dynamic processes and understanding the integration of these processes will be necessary for future contributions to rhizosphere science based upon interdisciplinary collaborations. In this review, current knowledge of the rhizosphere is synthesized using this holistic perspective with a focus on integrating traditionally separated rhizosphere studies. The temporal dynamics of rhizosphere activities will also be considered, from annual fine root turnover to diurnal fluctuations of water and nutrient uptake. The latest empirical and computational methods are discussed in the context of rhizosphere integration. Clarification of rhizosphere semantics, a holistic model of the rhizosphere, examples of integration of rhizosphere studies across disciplines, and review of the latest rhizosphere methods will empower rhizosphere scientists from different disciplines to engage in the interdisciplinary collaborations needed to break new ground in truly understanding the rhizosphere and to apply this knowledge for practical guidance.

Environmental Control of Root System Biology

The plant root system traverses one of the most complex environments on earth. Understanding how roots support plant life on land requires knowing how soil properties affect the availability of nutrients and water and how roots manipulate the soil environment to optimize acquisition of these resources. Imaging of roots in soil allows the integrated analysis and modeling of environmental interactions occurring at micro- to macroscales. Advances in phenotyping of root systems is driving innovation in cross-platform-compatible methods for data analysis. Root systems acclimate to the environment through architectural changes that act at the root-type level as well as through tissue-specific changes that affect the metabolic needs of the root and the efficiency of nutrient uptake. A molecular understanding of the signaling mechanisms that guide local and systemic signaling is providing insight into the regulatory logic of environmental responses and has identified points where crosstalk between pathways occurs.

The root cap: a short story of life and death

Over 130 years ago, Charles Darwin recognized that sensory functions in the root tip influence directional root growth. Modern plant biology has unravelled that many of the functions that Darwin attributed to the root tip are actually accomplished by a particular organ-the root cap. The root cap surrounds and protects the meristematic stem cells at the growing root tip. Due to this vanguard position, the root cap is predisposed to receive and transmit environmental information to the root proper. In contrast to other plant organs, the root cap shows a rapid turnover of short-lived cells regulated by an intricate balance of cell generation, differentiation, and degeneration. Thanks to these particular features, the root cap is an excellent developmental model system, in which generation, differentiation, and degeneration of cells can be investigated in a conveniently compact spatial and temporal frame. In this review, we give an overview of the current knowledge and concepts of root cap biology, focusing on the model plant Arabidopsis thaliana.

The divining root: moisture-driven responses of roots at the micro- and macro-scale

Water is fundamental to plant life, but the mechanisms by which plant roots sense and respond to variations in water availability in the soil are poorly understood. Many studies of responses to water deficit have focused on large-scale effects of this stress, but have overlooked responses at the sub-organ or cellular level that give rise to emergent whole-plant phenotypes. We have recently discovered hydropatterning, an adaptive environmental response in which roots position new lateral branches according to the spatial distribution of available water across the circumferential axis. This discovery illustrates that roots are capable of sensing and responding to water availability at spatial scales far lower than those normally studied for such processes. This review will explore how roots respond to water availability with an emphasis on what is currently known at different spatial scales. Beginning at the micro-scale, there is a discussion of water physiology at the cellular level and proposed sensory mechanisms cells use to detect osmotic status. The implications of these principles are then explored in the context of cell and organ growth under non-stress and water-deficit conditions. Following this, several adaptive responses employed by roots to tailor their functionality to the local moisture environment are discussed, including patterning of lateral root development and generation of hydraulic barriers to limit water loss. We speculate that these micro-scale responses are necessary for optimal functionality of the root system in a heterogeneous moisture environment, allowing for efficient water uptake with minimal water loss during periods of drought.

Shoot and root branch growth angle control-the wonderfulness of lateralness

The overall shape of plants, the space they occupy above and below ground, is determined principally by the number, length, and angle of their lateral branches. The function of these shoot and root branches is to hold leaves and other organs to the sun, and below ground, to provide anchorage and facilitate the uptake of water and nutrients. While in some respects lateral roots and shoots can be considered mere iterations of the primary root-shoot axis, in others there are fundamental differences in their biology, perhaps most conspicuously in the regulation their angle of growth. Here we discuss recent advances in the understanding of the control of branch growth angle, one of the most important but least understood components of the wonderful diversity of plant form observed throughout nature.

Spaceflight exploration in plant gravitational biology

Before there was access to space, all experiments on plant tropisms were conducted upon the background of gravity. The gravity vector could be disrupted, such as with clinorotation and random positioning machines, and by manipulating incident angles of root growth with respect to gravity, such as with Darwin's plants on slanted plates, but gravity could not be removed from the experimental equation. Access to microgravity through spaceflight has opened new doors to plant research. Here we provide an overview of some of the methodologies of conducting plant research in the unique spaceflight environment.

Root growth movements: waving and skewing

Roots anchor a plant in the soil, acquire nutrition and respond to environmental cues. Roots perform these functions using intricate movements and a variety of pathways have been implicated in mediating their growth patterns. These include endogenous genetic factors, perception of multiple environmental stimuli, signaling pathways interacting with hormonal dynamics and cellular processes of rapid cell elongation. In this review we attempt to consolidate our understanding of two specific types of root movements, waving and skewing, that arise on the surface of growth media, and how they are regulated by various genes and factors. These include crucial factors that are part of a complex nexus of processes including polar auxin transport and cytoskeletal dynamics. This knowledge can be extrapolated in the future for engineering plants with root architecture better suited for different soil and growth conditions such as abiotic stresses or even extended spaceflight. Technological innovations and interdisciplinary approaches promise to allow the tracking of root movements on a much finer scale, thus helping to expedite the discovery of more nodes in the regulation of root waving and skewing and movement in general.

GNOM regulates root hydrotropism and phototropism independently of PIN-mediated auxin transport

Plant roots exhibit tropisms in response to gravity, unilateral light and moisture gradients. During gravitropism, an auxin gradient is established by PIN auxin transporters, leading to asymmetric growth. GNOM, a guanine nucleotide exchange factor of ARF GTPase (ARF-GEF), regulates PIN localization by regulating subcellular trafficking of PINs. Therefore, GNOM is important for gravitropism. We previously isolated mizu-kussei2 (miz2), which lacks hydrotropic responses; MIZ2 is allelic to GNOM. Since PIN proteins are not required for root hydrotropism in Arabidopsis, the role of GNOM in root hydrotropism should differ from that in gravitropism. To examine this possibility, we conducted genetic analysis of gnom(miz2) and gnom trans-heterozygotes. The mutant gnom(miz2), which lacks hydrotropic responses, was partially recovered by gnom(emb30-1), which lacks GEF activity, but not by gnom(B4049), which lacks heterotypic domain interactions. Furthermore, the phototropic response of gnom trans-heterozygotes differed from that of the pin2 mutant allele eir1-1. Moreover, defects in the polarities of PIN2 and auxin distribution in a severe gnom mutant were recovered by gnom(miz2). Therefore, an unknown GNOM-mediated vesicle trafficking system may mediate root hydrotropism and phototropism independently of PIN trafficking.

MIZ1-regulated hydrotropism functions in the growth and survival of Arabidopsis thaliana under natural conditions

BACKGROUND AND AIMS

Root hydrotropism is a response to water-potential gradients that makes roots bend towards areas of higher water potential. The gene MIZU-KUSSEI1 (MIZ1) that is essential for hydrotropism in Arabidopsis roots has previously been identified. However, the role of root hydrotropism in plant growth and survival under natural conditions has not yet been proven. This study assessed how hydrotropic response contributes to drought avoidance in nature.

METHODS

An experimental system was established for the study of Arabidopsis hydrotropism in soil. Characteristics of hydrotropism were analysed by comparing the responses of the miz1 mutant, transgenic plants overexpressing MIZ1 (MIZ1OE) and wild-type plants.

KEY RESULTS

Wild-type plants developed root systems in regions with higher water potential, whereas the roots of miz1 mutant plants did not show a similar response. This pattern of root distribution induced by hydrotropism was more pronounced in MIZ1OE plants than in wild-type plants. In addition, shoot biomass and the number of plants that survived under drought conditions were much greater in MIZ1OE plants.

CONCLUSIONS

These results show that hydrotropism plays an important role in root system development in soil and contributes to drought avoidance, which results in a greater yield and plant survival under water-limited conditions. The results also show that MIZ1 overexpression can be used for improving plant productivity in arid areas.

Abscisic Acid synthesis and response

Abscisic acid (ABA) is one of the "classical" plant hormones, i.e. discovered at least 50 years ago, that regulates many aspects of plant growth and development. This chapter reviews our current understanding of ABA synthesis, metabolism, transport, and signal transduction, emphasizing knowledge gained from studies of Arabidopsis. A combination of genetic, molecular and biochemical studies has identified nearly all of the enzymes involved in ABA metabolism, almost 200 loci regulating ABA response, and thousands of genes regulated by ABA in various contexts. Some of these regulators are implicated in cross-talk with other developmental, environmental or hormonal signals. Specific details of the ABA signaling mechanisms vary among tissues or developmental stages; these are discussed in the context of ABA effects on seed maturation, germination, seedling growth, vegetative stress responses, stomatal regulation, pathogen response, flowering, and senescence.

Plant tropisms: from Darwin to the International Space Station

Plant tropisms play a fundamental role in shaping the growth form of plants, and these fascinating movements are the focus of this thematic issue of the American Journal of Botany. The issue includes 16 reviews of the current literature and eight original manuscripts written by a diverse group of international experts in their respective fields. This special issue emphasizes tropistic responses to three fundamental stimuli governing plant growth: water, light, and gravity. We hope this issue will inform the current generation and inspire the next generation of plant biologists.