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

Modulation of Root Hydrotropism and Recovery From Drought by MIZ1-like Genes in Tomato

Drought limits crop performance worldwide. Plant roots' ability to grow toward moisture, termed hydrotropism, is considered one strategy for optimizing water recruitment from the growth medium. Based on the sequence of the hydrotropism-indispensable MIZ1 protein in Arabidopsis thaliana, we identify hydrotropism and drought-responsive genes in tomato. We utilized CRISPR/Cas9 genome-editing technology for targeted mutagenesis of three hydrotropism-associated loci (MIZ1-like) in tomato (Solanum lycopersicum). We show that the three tomato MIZ1-like genes are drought-responsive and two of them are hydrostimulation-responsive. Examination of the root hydrotropic response of triple and double mutants indicated the gene SlMIZ1-1 as indispensable for tomato root hydrotropism. Moreover, expression of the SlMIZ1-1 gene in the Arabidopsis miz1 mutant effectively complemented the lost MIZ1 functionality, including root hydrotropic bending and generation of hydrotropic Ca2+ signals. Transcriptome analysis of hydrostimulated tomato root tips under control gravity and continuous clinorotation conditions was performed to identify gravitropism- and hydrotropism-responsive genes. This analysis suggested the involvement of ethylene and ABA signalling in modulating the interplay between hydrotropism and gravitropism. Unveiling the molecular mechanisms that underlie hydrotropism and drought response holds great potential for improving crop performance under limiting water availability due to global climate changes.

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.

ABA is required for differential cell wall acidification associated with root hydrotropic bending in tomato

Hydrotropism is an important adaptation of plant roots to the uneven distribution of water, with current research mainly focused on Arabidopsis thaliana. To examine hydrotropism in tomato (Solanum lycopersicum) primary roots, we used RNA sequencing to determine gene expression of root tips (apical 5 mm) on dry and wet sides of hydrostimulated roots grown on agar plates. Hydrostimulation enhances cell division and expansion on the dry side compared with the wet side of the root tip. In hydrostimulated roots, the abscisic acid (ABA) biosynthesis gene ABA4 was induced more on the dry than the wet side of root tips. The ABA biosynthesis inhibitor Fluridone and the ABA-deficient mutant notabilis (not) significantly decreased hydrotropic curvature. Wild-type, but not the ABA biosynthesis mutant not, root tips showed asymmetric H+ efflux, with greater efflux on the dry than on the wet side of root tips. Thus, ABA mediates asymmetric H+ efflux, allowing the root to bend towards the wet side to take up more water.

Recovery of root hydrotropism in miz1 mutant by eliminating root gravitropism

Mizu-kussei1 (MIZ1) plays a crucial role in root hydrotropism, but it is still unclear whether auxin-mediated gravitropism is involved in MIZ1-modulated root hydrotropism. This study aimed to investigate whether the hydrotropism of the Arabidopsis miz1 mutants could be restored through pharmacological inhibition of auxin transport or genetic modification in root gravitropism. Our findings indicate that the hydrotropic defects of miz1 mutant can be partly recovered by using an auxin transport inhibitor. Furthermore, miz1/pin2 double mutants exhibit more pronounced defects in root gravitropism compared to the wild type, while still displaying a normal hydrotropic response similar to the wild type. These results suggest that the elimination of gravitropism enables miz1 roots to become hydrotropically responsive to moisture gradients.

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.

Auxin biosynthesis, transport, and response directly attenuate hydrotropism in the latter stages to fine-tune root growth direction in Arabidopsis

Roots detect water potential gradients in the soil and orient toward moister areas, a response known as hydrotropism that aids drought avoidance. Although auxin is crucial in tropism, its polar transport is not essential for hydrotropism in Arabidopsis. Moreover, antiauxin treatments in Arabidopsis produced inconsistent outcomes: some studies indicated auxin action was necessary while others did not. In this study, we examined auxin's physiological role in hydrotropism. We found that inhibiting auxin biosynthesis or transport intensified hydrotropic bending not only in wild-type, but also in hydrotropism defective mutants, namely miz1-1 and miz2 plants. Given that miz1-1 and miz2 exhibited compromised hydrotropism even under clinorotated conditions, we infer that auxin biosynthesis and transport directly suppress hydrotropism. Additionally, tir1-10, afb1-3, and afb2-3 displayed augmented hydrotropism. We observed a significant delay in hydrotropic bending in arf7-1arf19-1, suggesting that ARF7 and ARF19 amplify hydrotropism in its early stages. To discern the functional ties of ARF7/19 with MIZ1 and MIZ2, we studied the hydrotropic phenotypes of arf7-1arf19-1miz1-1 and arf7-1arf19-1miz2. Both triple mutants had diminished early-stage hydrotropism yet showed partial but significant recovery in the later stages. Given MIZ1's role in reducing auxin levels and MIZ2's essentiality for MIZ1 functionality, we conclude that auxin inhibits hydrotropism downstream of MIZ1 in later stages to refine root bending. Furthermore, it is posited that gene expression driven by ARF7 and ARF19 is pivotal for early-stage root hydrotropism.

Genome-wide association study in two-row spring barley landraces identifies QTL associated with plantlets root system architecture traits in well-watered and osmotic stress conditions

Water availability is undoubtedly one of the most important environmental factors affecting crop production. Drought causes a gradual deprivation of water in the soil from top to deep layers and can occur at diverse stages of plant development. Roots are the first organs that perceive water deficit in soil and their adaptive development contributes to drought adaptation. Domestication has contributed to a bottleneck in genetic diversity. Wild species or landraces represent a pool of genetic diversity that has not been exploited yet in breeding program. In this study, we used a collection of 230 two-row spring barley landraces to detect phenotypic variation in root system plasticity in response to drought and to identify new quantitative trait loci (QTL) involved in root system architecture under diverse growth conditions. For this purpose, young seedlings grown for 21 days in pouches under control and osmotic-stress conditions were phenotyped and genotyped using the barley 50k iSelect SNP array, and genome-wide association studies (GWAS) were conducted using three different GWAS methods (MLM GAPIT, FarmCPU, and BLINK) to detect genotype/phenotype associations. In total, 276 significant marker-trait associations (MTAs; p-value (FDR)< 0.05) were identified for root (14 and 12 traits under osmotic-stress and control conditions, respectively) and for three shoot traits under both conditions. In total, 52 QTL (multi-trait or identified by at least two different GWAS approaches) were investigated to identify genes representing promising candidates with a role in root development and adaptation to drought stress.

Recent insights into metabolic and signalling events of directional root growth regulation and its implications for sustainable crop production systems

Roots are sensors evolved to simultaneously respond to manifold signals, which allow the plant to survive. Root growth responses, including the modulation of directional root growth, were shown to be differently regulated when the root is exposed to a combination of exogenous stimuli compared to an individual stress trigger. Several studies pointed especially to the impact of the negative phototropic response of roots, which interferes with the adaptation of directional root growth upon additional gravitropic, halotropic or mechanical triggers. This review will provide a general overview of known cellular, molecular and signalling mechanisms involved in directional root growth regulation upon exogenous stimuli. Furthermore, we summarise recent experimental approaches to dissect which root growth responses are regulated upon which individual trigger. Finally, we provide a general overview of how to implement the knowledge gained to improve plant breeding.

SUPPRESSOR of MAX2 1 (SMAX1) and SMAX1-LIKE2 (SMXL2) Negatively Regulate Drought Resistance in Arabidopsis thaliana

Recent investigations in Arabidopsis thaliana suggest that SUPPRESSOR of MORE AXILLARY GROWTH 2 1 (SMAX1) and SMAX1-LIKE2 (SMXL2) are negative regulators of karrikin (KAR) and strigolactone (SL) signaling during plant growth and development, but their functions in drought resistance and related mechanisms of action remain unclear. To understand the roles and mechanisms of SMAX1 and SMXL2 in drought resistance, we investigated the drought-resistance phenotypes and transcriptome profiles of smax1 smxl2 (s1,2) double-mutant plants in response to drought stress. The s1,2 mutant plants showed enhanced drought-resistance and lower leaf water loss when compared with wild-type (WT) plants. Transcriptome comparison of rosette leaves from the s1,2 mutant and the WT under normal and dehydration conditions suggested that the mechanism related to cuticle formation was involved in drought resistance. This possibility was supported by enhanced cuticle formation in the rosette leaves of the s1,2 mutant. We also found that the s1,2 mutant plants were more sensitive to abscisic acid in assays of stomatal closure, cotyledon opening, chlorophyll degradation and growth inhibition, and they showed a higher reactive oxygen species detoxification capacity than WT plants. In addition, the s1,2 mutant plants had longer root hairs and a higher root-to-shoot ratio than the WT plants, suggesting that the mutant had a greater capacity for water absorption than the WT. Taken together, our results indicate that SMAX1 and SMXL2 negatively regulate drought resistance, and disruption of these KAR- and SL-signaling-related genes may therefore provide a novel means for improving crop drought resistance.

KARRIKIN UPREGULATED F-BOX 1 negatively regulates drought tolerance in Arabidopsis

The karrikin (KAR) receptor and several related signaling components have been identified by forward genetic screening, but only a few studies have reported on upstream and downstream KAR signaling components and their roles in drought tolerance. Here, we characterized the functions of KAR UPREGULATED F-BOX 1 (KUF1) in drought tolerance using a reverse genetics approach in Arabidopsis (Arabidopsis thaliana). We observed that kuf1 mutant plants were more tolerant to drought stress than wild-type (WT) plants. To clarify the mechanisms by which KUF1 negatively regulates drought tolerance, we performed physiological, transcriptome, and morphological analyses. We found that kuf1 plants limited leaf water loss by reducing stomatal aperture and cuticular permeability. In addition, kuf1 plants showed increased sensitivity of stomatal closure, seed germination, primary root growth, and leaf senescence to abscisic acid (ABA). Genome-wide transcriptome comparisons of kuf1 and WT rosette leaves before and after dehydration showed that the differences in various drought tolerance-related traits were accompanied by differences in the expression of genes associated with stomatal closure (e.g. OPEN STOMATA 1), lipid and fatty acid metabolism (e.g. WAX ESTER SYNTHASE), and ABA responsiveness (e.g. ABA-RESPONSIVE ELEMENT 3). The kuf1 mutant plants had higher root/shoot ratios and root hair densities than WT plants, suggesting that they could absorb more water than WT plants. Together, these results demonstrate that KUF1 negatively regulates drought tolerance by modulating various physiological traits, morphological adjustments, and ABA responses and that the genetic manipulation of KUF1 in crops is a potential means of enhancing their drought tolerance.

Diversity of root hydrotropism among natural variants of Arabidopsis thaliana

Root gravitropism affects root hydrotropism. The interference intensity of root gravitropism with root hydrotropism differs among plant species. However, these differences have not been well compared within a single plant species. In this study, we compared root hydrotropism in various natural variants of Arabidopsis under stationary conditions. As a result, we detected a range of root hydrotropism under stationary conditions among natural Arabidopsis variants. Comparison of root gravitropism and root hydrotropism among several Arabidopsis natural variants classified natural variants that decreased root hydrotropism into two types; namely one type that expresses root gravitropism and root hydrotropism weaker than Col-0, and the other type that expresses weaker root hydrotropism than Col-0 but expresses similar root gravitropism with Col-0. However, root hydrotropism of all examined Arabidopsis natural variants was facilitated by clinorotation. These results suggested that the interference of root gravitropism with root hydrotropism is conserved among Arabidopsis natural variants, although the intensity of root gravitropism interference with root hydrotropism differs.

Physiological and Multi-Omics Approaches for Explaining Drought Stress Tolerance and Supporting Sustainable Production of Rice

Drought differs from other natural disasters in several respects, largely because of the complexity of a crop's response to it and also because we have the least understanding of a crop's inductive mechanism for addressing drought tolerance among all abiotic stressors. Overall, the growth and productivity of crops at a global level is now thought to be an issue that is more severe and arises more frequently due to climatic change-induced drought stress. Among the major crops, rice is a frontline staple cereal crop of the developing world and is critical to sustaining populations on a daily basis. Worldwide, studies have reported a reduction in rice productivity over the years as a consequence of drought. Plants are evolutionarily primed to withstand a substantial number of environmental cues by undergoing a wide range of changes at the molecular level, involving gene, protein and metabolite interactions to protect the growing plant. Currently, an in-depth, precise and systemic understanding of fundamental biological and cellular mechanisms activated by crop plants during stress is accomplished by an umbrella of -omics technologies, such as transcriptomics, metabolomics and proteomics. This combination of multi-omics approaches provides a comprehensive understanding of cellular dynamics during drought or other stress conditions in comparison to a single -omics approach. Thus a greater need to utilize information (big-omics data) from various molecular pathways to develop drought-resilient crop varieties for cultivation in ever-changing climatic conditions. This review article is focused on assembling current peer-reviewed published knowledge on the use of multi-omics approaches toward expediting the development of drought-tolerant rice plants for sustainable rice production and realizing global food security.

Comparative analysis reveals gravity is involved in the MIZ1-regulated root hydrotropism

Hydrotropism is the directed growth of roots toward the water found in the soil. However, mechanisms governing interactions between hydrotropism and gravitropism remain largely unclear. In this study, we found that an air system and an agar-sorbitol system induced only oblique water-potential gradients; an agar-glycerol system induced only vertical water-potential gradients; and a sand system established both oblique and vertical water-potential gradients. We employed obliquely oriented and vertically oriented experimental systems to study hydrotropism in Arabidopsis and tomato plants. Comparative analyses using different hydrotropic systems showed that gravity hindered the ability of roots to search for obliquely oriented water, whilst facilitating roots' search for vertically oriented water. We found that the gravitropism-deficient mutant aux1 showed enhanced hydrotropism in the oblique orientation but impaired root elongation towards water in the vertical orientation. The miz1 mutant exhibited deficient hydrotropism in the oblique orientation but normal root elongation towards water in the vertical orientation. Importantly, in contrast to miz1, the miz1/aux1 double mutant exhibited hydrotropic bending in the oblique orientation and attenuated root elongation towards water in the vertical orientation. Our results suggest that gravitropism is required for MIZ1-regulated root hydrotropism in both the oblique orientation and the vertical orientation, providing further insight into the role of gravity in root hydrotropism.

Light-Dark Modulates Root Hydrotropism Associated with Gravitropism by Involving Amyloplast Response in Arabidopsis

The role of amyloplasts in the interactions between hydrotropism and gravitropism has been previously described. However, the effect of light-dark on the interactions between the two tropisms remains unclear. Here, by developing a method that makes it possible to mimic natural conditions more closely than the conventional lab conditions, we show that hydrotropism is higher in wild-type Arabidopsis seedlings whose shoots are illuminated but whose roots are grown in the dark compared with seedlings that are fully exposed to light. Root gravitropism is substantially decreased because of the reduction of amyloplast content in the root tip with decreased gene expression in PGM1 (a key starch biosynthesis gene), which may contribute to enhanced root hydrotropism under darkness. Furthermore, the starch-deficient mutant pgm1-1 exhibits greater hydrotropism compared with wild-type. Our results suggest that amyloplast response and starch reduction occur under light-dark modulation, followed by decreased gravitropism and enhanced hydrotropism in Arabidopsis root.

Early Drought-Responsive Genes Are Variable and Relevant to Drought Tolerance

Drought stress is an important crop yield limiting factor worldwide. Plant physiological responses to drought stress are driven by changes in gene expression. While drought-responsive genes (DRGs) have been identified in maize, regulation patterns of gene expression during progressive water deficits remain to be elucidated. In this study, we generated time-series transcriptomic data from the maize inbred line B73 under well-watered and drought conditions. Comparisons between the two conditions identified 8,626 DRGs and the stages (early, middle, and late drought) at which DRGs occurred. Different functional groups of genes were regulated at the three stages. Specifically, early and middle DRGs display higher copy number variation among diverse Zea mays lines, and they exhibited stronger associations with drought tolerance as compared to late DRGs. In addition, correlation of expression between small RNAs (sRNAs) and DRGs from the same samples identified 201 negatively sRNA/DRG correlated pairs, including genes showing high levels of association with drought tolerance, such as two glutamine synthetase genes, gln2 and gln6 The characterization of dynamic gene responses to progressive drought stresses indicates important adaptive roles of early and middle DRGs, as well as roles played by sRNAs in gene expression regulation upon drought stress.

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.

Overexpression of the vascular brassinosteroid receptor BRL3 confers drought resistance without penalizing plant growth

Drought represents a major threat to food security. Mechanistic data describing plant responses to drought have been studied extensively and genes conferring drought resistance have been introduced into crop plants. However, plants with enhanced drought resistance usually display lower growth, highlighting the need for strategies to uncouple drought resistance from growth. Here, we show that overexpression of BRL3, a vascular-enriched member of the brassinosteroid receptor family, can confer drought stress tolerance in Arabidopsis. Whereas loss-of-function mutations in the ubiquitously expressed BRI1 receptor leads to drought resistance at the expense of growth, overexpression of BRL3 receptor confers drought tolerance without penalizing overall growth. Systematic analyses reveal that upon drought stress, increased BRL3 triggers the accumulation of osmoprotectant metabolites including proline and sugars. Transcriptomic analysis suggests that this results from differential expression of genes in the vascular tissues. Altogether, this data suggests that manipulating BRL3 expression could be used to engineer drought tolerant crops.

Drought resistant plants typically have reduced growth. Here the authors show that overexpression of the BRL3 brassinosteroid receptor confers drought tolerance and accumulation of osmoprotectant metabolites without penalizing growth, demonstrating that drought response and growth can be uncoupled.

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.

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.

Analysis of plant hormone profiles in response to moderate dehydration stress

Plant responses to dehydration stress are mediated by highly complex molecular systems involving hormone signaling and metabolism, particularly the major stress hormone abscisic acid (ABA) and ABA-dependent gene expression. To understand the roles of plant hormones and their interactions during dehydration, we analyzed the plant hormone profiles with respect to dehydration responses in Arabidopsis thaliana wild-type (WT) plants and ABA biosynthesis mutants (nced3-2). We developed a procedure for moderate dehydration stress, and then investigated temporal changes in the profiles of ABA, jasmonic acid isoleucine (JA-Ile), salicylic acid (SA), cytokinin (trans-zeatin, tZ), auxin (indole-acetic acid, IAA), and gibberellin (GA₄ ), along with temporal changes in the expression of key genes involved in hormone biosynthesis. ABA levels increased in a bi-phasic pattern (at the early and late phases) in response to moderate dehydration stress. JA-Ile levels increased slightly in WT plants and strongly increased in nced3-2 mutant plants at 72 h after the onset of dehydration. The expression profiles of dehydration-inducible genes displayed temporal responses in an ABA-dependent manner. The early phase of ABA accumulation correlated with the expression of touch-inducible genes and was independent of factors involved in the major ABA regulatory pathway, including the ABA-responsive element-binding (AREB/ABF) transcription factor. JA-Ile, SA, and tZ were negatively regulated during the late dehydration response phase. Transcriptome analysis revealed important roles for hormone-related genes in metabolism and signaling during dehydration-induced plant responses.

Poplar PtabZIP1-like enhances lateral root formation and biomass growth under drought stress

Developing drought-resistance varieties is a major goal for bioenergy crops, such as poplar (Populus), which will be grown on marginal lands with little or no water input. Root architecture can affect drought resistance, but few genes that affect root architecture in relation to water availability have been identified. Here, using activation tagging in the prime bioenergy crop poplar, we have identified a mutant that overcomes the block of lateral root (LR) formation under osmotic stress. Positioning of the tag, validation of the activation and recapitulation showed that the phenotype is caused by the poplar PtabZIP1-like (PtabZIP1L) gene with highest homology to bZIP1 from Arabidopsis. PtabZIP1L is predominantly expressed in roots, particularly in zones where lateral root primordia (LRP) initiate and LR differentiate and emerge. Transgenics overexpressing PtabZIP1L showed precocious LRP and LR development, while PtabZIP1L suppression significantly delayed both LRP and LR formation. Transgenic overexpression and suppression of PtabZIP1L also resulted in modulation of key metabolites like proline, asparagine, valine and several flavonoids. Consistently, expression of both of the poplar Proline Dehydrogenase orthologs and two of the Flavonol Synthases genes was also increased and decreased in overexpressed and suppressed transgenics, respectively. These findings suggest that PtabZIP1L mediates LR development and drought resistance through modulation of multiple metabolic pathways.

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.

The Cholodny-Went theory does not explain hydrotropism

Optimization of water foraging by plants is partially achieved by the ability of roots to direct growth towards high water potential, a phenomenon termed hydrotropism. In contrast to gravitropism and phototropism, which require auxin redistribution, as suggested by the Cholodny-Went theory, hydrotropism is not mediated by the phytohormone auxin, which raises questions about the mechanism underlying this tropic response. Here we specify the open questions in this field of research and discuss the possible interactions of abscisic acid, calcium and reactive oxygen species as part of a dynamic system of sensing water potential in the root tip, transmission of the signal to the root elongation zone and promoting root curvature towards water. We conclude that root hydrotropism is mediated by inter-cellular signals that are not explained by the Cholodny-Went theory.

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.

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.

Regulation of plant root system architecture: implications for crop advancement

Root system architecture (RSA) plays a major role in plant fitness, crop performance, and grain yield yet only recently has this role been appreciated. RSA describes the spatial arrangement of root tissue within the soil and is therefore crucial to nutrient and water uptake. Recent studies have identified many of the genetic and environmental factors influencing root growth that contribute to RSA. Some of the identified genes have the potential to limit crop loss caused by environmental extremes and are currently being used to confer drought tolerance. It is hypothesized that manipulating these and other genes that influence RSA will be pivotal for future crop advancements worldwide.

The art of being flexible: how to escape from shade, salt, and drought

Environmental stresses, such as shading of the shoot, drought, and soil salinity, threaten plant growth, yield, and survival. Plants can alleviate the impact of these stresses through various modes of phenotypic plasticity, such as shade avoidance and halotropism. Here, we review the current state of knowledge regarding the mechanisms that control plant developmental responses to shade, salt, and drought stress. We discuss plant hormones and cellular signaling pathways that control shoot branching and elongation responses to shade and root architecture modulation in response to drought and salinity. Because belowground stresses also result in aboveground changes and vice versa, we then outline how a wider palette of plant phenotypic traits is affected by the individual stresses. Consequently, we argue for a research agenda that integrates multiple plant organs, responses, and stresses. This will generate the scientific understanding needed for future crop improvement programs aiming at crops that can maintain yields under variable and suboptimal conditions.

Plant water uptake in drying soils

Over the last decade, investigations on root water uptake have evolved toward a deeper integration of the soil and roots compartment properties, with the goal of improving our understanding of water acquisition from drying soils. This evolution parallels the increasing attention of agronomists to suboptimal crop production environments. Recent results have led to the description of root system architectures that might contribute to deep-water extraction or to water-saving strategies. In addition, the manipulation of root hydraulic properties would provide further opportunities to improve water uptake. However, modeling studies highlight the role of soil hydraulics in the control of water uptake in drying soil and call for integrative soil-plant system approaches.