Synergistic and antagonistic pleiotropy of STOP1 in stress tolerance

TaMYB-CC5 gene specifically expressed in root improve tolerance of phosphorus deficiency and drought stress in wheat

Phosphate deficiency and drought are significant environmental constraints that impact both the productivity and quality of wheat. The interaction between phosphorus and water facilitates their mutual absorption processes in plants. Under conditions of both phosphorus deficiency and drought stress, we observed a significant upregulation in the expression of wheat MYB-CC transcription factors through the transcriptome analysis. 52 TaMYB-CC genes in wheat were identified and analyzed their evolutionary relationships, structures, and expression patterns. The TaMYB-CC5 gene exhibited specific expression in roots and demonstrated significant upregulation under phosphorus deficiency and drought stress compared to other TaMYB-CC genes. The overexpression of TaMYB-CC5A in Arabidopsis resulted in a significant increase of root length under stress conditions, thereby enhancing tolerance to phosphate starvation and drought stress. The wheat lines with silenced TaMYB-CC5 genes exhibited reduced root length under stress conditions and increased sensitivity to phosphate deficiency and drought stress. In addition, silencing the TaMYB-CC5 genes resulted in altered phosphorus content in leaves but did not lead to a reduction in phosphorus content in roots. Enrichment analysis the co-expression genes of TaMYB-CC5 transcription factors, we found the zinc-induced facilitator-like (ZIFL) genes were prominent associated with TaMYB-CC5 gene. The TaZIFL1, TaZIFL2, and TaZIFL5 genes were verified specifically expressed in roots and regulated by TaMYB-CC5 transcript factor. Our study reveals the pivotal role of the TaMYB-CC5 gene in regulating TaZIFL genes, which is crucial for maintaining normal root growth under phosphorus deficiency and drought stress, thereby enhanced resistance to these abiotic stresses in wheat.

STOP1 regulates CCX1-mediated Ca2+ homeostasis for plant adaptation to Ca2+ deprivation

Calcium (Ca) is essential for plant growth and stress adaptation, yet its availability is often limited in acidic soils, posing a major threat to crop production. Understanding the intricate mechanisms orchestrating plant adaptation to Ca deficiency remains elusive. Here, we show that the Ca deficiency-enhanced nuclear accumulation of the transcription factor SENSITIVE TO PROTON RHIZOTOXICITY 1 (STOP1) in Arabidopsis thaliana confers tolerance to Ca deprivation, with the global transcriptional responses triggered by Ca deprivation largely impaired in the stop1 mutant. Notably, STOP1 activates the Ca deprivation-induced expression of CATION/Ca2+ EXCHANGER 1 (CCX1) by directly binding to its promoter region, which facilitates Ca2+ efflux from endoplasmic reticulum to cytosol to maintain Ca homeostasis. Consequently, the constitutive expression of CCX1 in the stop1 mutant partially rescues the Ca deficiency phenotype by increasing Ca content in the shoots. These findings uncover the pivotal role of the STOP1-CCX1 axis in plant adaptation to low Ca, offering alternative manipulating strategies to improve plant Ca nutrition in acidic soils and extending our understanding of the multifaceted role of STOP1.

Pleiotropy alleviates the fitness costs associated with resource allocation trade-offs in immune signalling networks

Many genes and signalling pathways within plant and animal taxa drive the expression of multiple organismal traits. This form of genetic pleiotropy instigates trade-offs among life-history traits if a mutation in the pleiotropic gene improves the fitness contribution of one trait at the expense of another. Whether or not pleiotropy gives rise to conflict among traits, however, likely depends on the resource costs and timing of trait deployment during organismal development. To investigate factors that could influence the evolutionary maintenance of pleiotropy in gene networks, we developed an agent-based model of co-evolution between parasites and hosts. Hosts comprise signalling networks that must faithfully complete a developmental programme while also defending against parasites, and trait signalling networks could be independent or share a pleiotropic component as they evolved to improve host fitness. We found that hosts with independent developmental and immune networks were significantly more fit than hosts with pleiotropic networks when traits were deployed asynchronously during development. When host genotypes directly competed against each other, however, pleiotropic hosts were victorious regardless of trait synchrony because the pleiotropic networks were more robust to parasite manipulation, potentially explaining the abundance of pleiotropy in immune systems despite its contribution to life history trade-offs.

Review: Nutrient-nutrient interactions governing underground plant adaptation strategies in a heterogeneous environment

Plant growth relies on the mineral nutrients present in the rhizosphere. The distribution of nutrients in soils varies depending on their mobility and capacity to bind with soil particles. Consequently, plants often encounter either low or high levels of nutrients in the rhizosphere. Plant roots are the essential organs that sense changes in soil mineral content, leading to the activation of signaling pathways associated with the adjustment of plant architecture and metabolic responses. During differential availability of minerals in the rhizosphere, plants trigger adaptation strategies such as cellular remobilization of minerals, secretion of organic molecules, and the attenuation or enhancement of root growth to balance nutrient uptake. The interdependency, availability, and uptake of minerals, such as phosphorus (P), iron (Fe), zinc (Zn), potassium (K), nitrogen (N) forms, nitrate (NO3-), and ammonium (NH4+), modulate the root architecture and metabolic functioning of plants. Here, we summarized the interactions of major nutrients (N, P, K, Fe, Zn) in shaping root architecture, physiological responses, genetic components involved, and address the current challenges associated with nutrient-nutrient interactions. Furthermore, we discuss the major gaps and opportunities in the field for developing plants with improved nutrient uptake and use efficiency for sustainable agriculture.

Strategies to identify and dissect trade-offs in plants

Trade-offs between traits arise and reflect constraints imposed by the environment and physicochemical laws. Trade-off situations are expected to be highly relevant for sessile plants, which have to respond to changes in the environment to ensure survival. Despite increasing interest in determining the genetic and molecular basis of plant trade-offs, there are still gaps and differences with respect to how trade-offs are defined, how they are measured, and how their genetic architecture is dissected. The first step to fill these gaps is to establish what is meant by trade-offs. In this review we provide a classification of the existing definitions of trade-offs according to: (1) the measures used for their quantification, (2) the dependence of trade-offs on environment, and (3) experimental designed used (i.e. a single individual across different environments or a population of individuals in single or multiple environments). We then compare the approaches for quantification of trade-offs based on phenotypic, between-individual, and genetic correlations, and stress the need for developing further quantification indices particularly for trade-offs between multiple traits. Lastly, we highlight the genetic mechanisms underpinning trade-offs and experimental designs that facilitate their discovery in plants, with focus on usage of natural variability. This review also offers a perspective for future research aimed at identification of plant trade-offs, dissection of their genetic architecture, and development of strategies to overcome trade-offs, with applications in crop breeding.

Arabidopsis transcription factor STOP1 directly activates expression of NOD26-LIKE MAJOR INTRINSIC PROTEIN5;1, and is involved in the regulation of tolerance to low-boron stress

Transcriptional regulation is a crucial component of plant adaptation to numerous different stresses; however, its role in how plants adapt to low-boron (B) stress remains unclear. In this study, we show that the C2H2-type transcription factor SENSITIVE TO PROTON RHIZOTOXICITY1 (STOP1) in Arabidopsis is essential for improving plant growth under low-B conditions. STOP1 and the boric acid-channel protein NOD26-LIKE MAJOR INTRINSIC PROTEIN5;1 (NIP5;1) were found to co-localize in root epidermal cells, and STOP1 binds to the 5´-untranslated region of NIP5;1 to activate its expression and enhance B uptake by the roots. Overexpression of STOP1 increased tolerance to low-B stress by up-regulating NIP5;1 transcript levels. Further genetic analyses revealed that STOP1 and NIP5;1 function together in the same pathway to confer low-B tolerance. These results highlight the importance of the STOP1-NIP5;1 module in improving plant growth under low-B conditions.

H2O2 negatively regulates aluminum resistance via oxidation and degradation of the transcription factor STOP1

Aluminum (Al) stress triggers the accumulation of hydrogen peroxide (H2O2) in roots. However, whether H2O2 plays a regulatory role in aluminum resistance remains unclear. In this study, we show that H2O2 plays a crucial role in regulation of Al resistance, which is modulated by the mitochondrion-localized pentatricopeptide repeat protein REGULATION OF ALMT1 EXPRESSION 6 (RAE6). Mutation in RAE6 impairs the activity of complex I of the mitochondrial electron transport chain, resulting in the accumulation of H2O2 and increased sensitivity to Al. Our results suggest that higher H2O2 concentrations promote the oxidation of SENSITIVE TO PROTON RHIZOTOXICITY 1 (STOP1), an essential transcription factor that promotes Al resistance, thereby promoting its degradation by enhancing the interaction between STOP1 and the F-box protein RAE1. Conversely, decreasing H2O2 levels or blocking the oxidation of STOP1 leads to greater STOP1 stability and increased Al resistance. Moreover, we show that the thioredoxin TRX1 interacts with STOP1 to catalyze its chemical reduction. Thus, our results highlight the importance of H2O2 in Al resistance and regulation of STOP1 stability in Arabidopsis (Arabidopsis thaliana).

Unlocking dynamic root phenotypes for simultaneous enhancement of water and phosphorus uptake

Phosphorus (P) and water are crucial for plant growth, but their availability is challenged by climate change, leading to reduced crop production and global food security. In many agricultural soils, crop productivity is confronted by both water and P limitations. The diminished soil moisture decreases available P due to reduced P diffusion, and inadequate P availability diminishes tissue water status through modifications in stomatal conductance and a decrease in root hydraulic conductance. P and water display contrasting distributions in the soil, with P being concentrated in the topsoil and water in the subsoil. Plants adapt to water- and P-limited environments by efficiently exploring localized resource hotspots of P and water through the adaptation of their root system. Thus, developing cultivars with improved root architecture is crucial for accessing and utilizing P and water from arid and P-deficient soils. To meet this goal, breeding towards multiple advantageous root traits can lead to better cultivars for water- and P-limited environments. This review discusses the interplay of P and water availability and highlights specific root traits that enhance the exploration and exploitation of optimal resource-rich soil strata while reducing metabolic costs. We propose root ideotype models, including 'topsoil foraging', 'subsoil foraging', and 'topsoil/subsoil foraging' for maize (monocot) and common bean (dicot). These models integrate beneficial root traits and guide the development of water- and P-efficient cultivars for challenging environments.

STOP1-regulated SMALL AUXIN UP RNA55 (SAUR55) is involved in proton/malate co-secretion for Al tolerance in Arabidopsis

Proton (H+) release is linked to aluminum (Al)-enhanced organic acids (OAs) excretion from the roots under Al rhizotoxicity in plants. It is well-reported that the Al-enhanced organic acid excretion mechanism is regulated by SENSITIVE TO PROTON RHIZOTOXICITY1 (STOP1), a zinc-finger TF that regulates major Al tolerance genes. However, the mechanism of H+ release linked to OAs excretion under Al stress has not been fully elucidated. Recent physiological and molecular-genetic studies have implicated the involvement of SMALL AUXIN UP RNAs (SAURs) in the activation of plasma membrane H+-ATPases for stress responses in plants. We hypothesized that STOP1 is involved in the regulation of Al-responsive SAURs, which may contribute to the co-secretion of protons and malate under Al stress conditions. In our transcriptome analysis of the roots of the stop1 (sensitive to proton rhizotoxicity1) mutant, we found that STOP1 regulates the transcription of one of the SAURs, namely SAUR55. Furthermore, we observed that the expression of SAUR55 was induced by Al and repressed in the STOP1 T-DNA insertion knockout (KO) mutant (STOP1-KO). Through in silico analysis, we identified a functional STOP1-binding site in the promoter of SAUR55. Subsequent in vitro and in vivo studies confirmed that STOP1 directly binds to the promoter of SAUR55. This suggests that STOP1 directly regulates the expression of SAUR55 under Al stress. We next examined proton release in the rhizosphere and malate excretion in the T-DNA insertion KO mutant of SAUR55 (saur55), in conjunction with STOP1-KO. Both saur55 and STOP1-KO suppressed rhizosphere acidification and malate release under Al stress. Additionally, the root growth of saur55 was sensitive to Al-containing media. In contrast, the overexpressed line of SAUR55 enhanced rhizosphere acidification and malate release, leading to increased Al tolerance. These associations with Al tolerance were also observed in natural variations of Arabidopsis. These findings demonstrate that transcriptional regulation of SAUR55 by STOP1 positively regulates H+ excretion via PM H+-ATPase 2 which enhances Al tolerance by malate secretion from the roots of Arabidopsis. The activation of PM H+-ATPase 2 by SAUR55 was suggested to be due to PP2C.D2/D5 inhibition by interaction on the plasma membrane with its phosphatase. Furthermore, RNAi-suppression of NtSTOP1 in tobacco shows suppression of rhizosphere acidification under Al stress, which was associated with the suppression of SAUR55 orthologs, which are inducible by Al in tobacco. It suggests that transcriptional regulation of Al-inducible SAURs by STOP1 plays a critical role in OAs excretion in several plant species as an Al tolerance mechanism.

Pleiotropy alleviates the fitness costs associated with resource allocation trade-offs in immune signaling networks

Many genes and signaling pathways within plant and animal taxa drive the expression of multiple organismal traits. This form of genetic pleiotropy instigates trade-offs among life-history traits if a mutation in the pleiotropic gene improves the fitness contribution of one trait at the expense of another. Whether or not pleiotropy gives rise to conflict among traits, however, likely depends on the resource costs and timing of trait deployment during organismal development. To investigate factors that could influence the evolutionary maintenance of pleiotropy in gene networks, we developed an agent-based model of co-evolution between parasites and hosts. Hosts comprise signaling networks that must faithfully complete a developmental program while also defending against parasites, and trait signaling networks could be independent or share a pleiotropic component as they evolved to improve host fitness. We found that hosts with independent developmental and immune networks were significantly more fit than hosts with pleiotropic networks when traits were deployed asynchronously during development. When host genotypes directly competed against each other, however, pleiotropic hosts were victorious regardless of trait synchrony because the pleiotropic networks were more robust to parasite manipulation, potentially explaining the abundance of pleiotropy in immune systems despite its contribution to life history trade-offs.

Finding Balance in Adversity: Nitrate Signaling as the Key to Plant Growth, Resilience, and Stress Response

To effectively adapt to changing environments, plants must maintain a delicate balance between growth and resistance or tolerance to various stresses. Nitrate, a significant inorganic nitrogen source in soils, not only acts as an essential nutrient but also functions as a critical signaling molecule that regulates multiple aspects of plant growth and development. In recent years, substantial advancements have been made in understanding nitrate sensing, calcium-dependent nitrate signal transmission, and nitrate-induced transcriptional cascades. Mounting evidence suggests that the primary response to nitrate is influenced by environmental conditions, while nitrate availability plays a pivotal role in stress tolerance responses. Therefore, this review aims to provide an overview of the transcriptional and post-transcriptional regulation of key components in the nitrate signaling pathway, namely, NRT1.1, NLP7, and CIPK23, under abiotic stresses. Additionally, we discuss the specificity of nitrate sensing and signaling as well as the involvement of epigenetic regulators. A comprehensive understanding of the integration between nitrate signaling transduction and abiotic stress responses is crucial for developing future crops with enhanced nitrogen-use efficiency and heightened resilience.

Genome-Wide Analysis of Strictosidine Synthase-like Gene Family Revealed Their Response to Biotic/Abiotic Stress in Poplar

The strictosidine synthase-like (SSL) gene family is a small plant immune-regulated gene family that plays a critical role in plant resistance to biotic/abiotic stresses. To date, very little has been reported on the SSL gene in plants. In this study, a total of thirteen SSLs genes were identified from poplar, and these were classified into four subgroups based on multiple sequence alignment and phylogenetic tree analysis, and members of the same subgroup were found to have similar gene structures and motifs. The results of the collinearity analysis showed that poplar SSLs had more collinear genes in the woody plants Salix purpurea and Eucalyptus grandis. The promoter analysis revealed that the promoter region of PtrSSLs contains a large number of biotic/abiotic stress response elements. Subsequently, we examined the expression patterns of PtrSSLs following drought, salt, and leaf blight stress, using RT-qPCR to validate the response of PtrSSLs to biotic/abiotic stresses. In addition, the prediction of transcription factor (TF) regulatory networks identified several TFs, such as ATMYB46, ATMYB15, AGL20, STOP1, ATWRKY65, and so on, that may be induced in the expression of PtrSSLs in response to adversity stress. In conclusion, this study provides a solid basis for a functional analysis of the SSL gene family in response to biotic/abiotic stresses in poplar.

Genome-Wide Characterization and Gene Expression Analyses of Malate Dehydrogenase (MDH) Genes in Low-Phosphorus Stress Tolerance of Chinese Fir (Cunninghamia lanceolata)

Malate dehydrogenase (MDH) genes play vital roles in developmental control and environmental stress tolerance in sessile plants by modulating the organic acid-malic acid level. However, MDH genes have not yet been characterized in gymnosperm, and their roles in nutrient deficiency are largely unexplored. In this study, 12 MDH genes were identified in Chinese fir (Cunninghamia lanceolata), namely, ClMDH-1, -2, -3, …, and -12. Chinese fir is one of the most abundant commercial timber trees in China, and low phosphorus has limited its growth and production due to the acidic soil of southern China. According to the phylogenetic analysis, MDH genes were classified into five groups, and Group 2 genes (ClMDH-7, -8, -9, and 10) were only found to be present in Chinese fir but not in Arabidopsis thaliana and Populus trichocarpa. In particular, the Group 2 MDHs also had specific functional domains-Ldh_1_N (malidase NAD-binding functional domain) and Ldh_1_C (malate enzyme C-terminal functional domain)-indicating a specific function of ClMDHs in the accumulation of malate. All ClMDH genes contained the conserved MDH gene characteristic functional domains Ldh_1_N and Ldh_1_C, and all ClMDH proteins exhibited similar structures. Twelve ClMDH genes were identified from eight chromosomes, involving fifteen ClMDH homologous gene pairs, each with a Ka/Ks ratio of <1. The analysis of cis-elements, protein interactions, and transcription factor interactions of MDHs showed that the ClMDH gene might play a role in plant growth and development, and in response to stress mechanisms. The results of transcriptome data and qRT-PCR validation based on low-phosphorus stress showed that ClMDH1, ClMDH6, ClMDH7, ClMDH2, ClMDH4, ClMDH5, ClMDH10 and ClMDH11 were upregulated under low-phosphorus stress and played a role in the response of fir to low-phosphorus stress. In conclusion, these findings lay a foundation for further improving the genetic mechanism of the ClMDH gene family in response to low-phosphorus stress, exploring the potential function of this gene, promoting the improvement of fir genetics and breeding, and improving production efficiency.

The MEKK1-MKK1/2-MPK4 cascade phosphorylates and stabilizes STOP1 to confer aluminum resistance in Arabidopsis

Aluminum (Al) toxicity can seriously restrict crop production on acidic soils, which comprise 40% of the world's potentially arable land. The zinc finger transcription factor STOP1 has a conserved and essential function in mediating plant Al resistance. Al stress induces STOP1 accumulation via post-transcriptional regulatory mechanisms. However, the upstream signaling pathway involved in Al-triggered STOP1 accumulation remains unclear. Here, we report that the MEKK1-MKK1/2-MPK4 cascade positively regulates STOP1 phosphorylation and stability. Mutations of MEKK1, MKK1/2, or MPK4 lead to decreased STOP1 stability and Al resistance. Al stress induces the kinase activity of MPK4, which interacts with and phosphorylates STOP1. The phosphorylation of STOP1 reduces its interaction with the F-box protein RAE1 that mediates STOP1 degradation, thereby leading to enhanced STOP1 stability and Al resistance. Taken together, our results suggest that the MEKK1-MKK1/2-MPK4 cascade is important for Al signaling and confers Al resistance through phosphorylation-mediated enhancement of STOP1 accumulation in Arabidopsis.

Molecular Mechanisms Underlying Mimosa acutistipula Success in Amazonian Rehabilitating Minelands

Mimosa acutistipula is endemic to Brazil and grows in ferruginous outcrops (canga) in Serra dos Carajás, eastern Amazon, where one of the largest iron ore deposits in the world is located. Plants that develop in these ecosystems are subject to severe environmental conditions and must have adaptive mechanisms to grow and thrive in cangas. Mimosa acutistipula is a native species used to restore biodiversity in post-mining areas in canga. Understanding the molecular mechanisms involved in the adaptation of M. acutistipula in canga is essential to deduce the ability of native species to adapt to possible stressors in rehabilitating minelands over time. In this study, the root proteomic profiles of M. acutistipula grown in a native canga ecosystem and rehabilitating minelands were compared to identify essential proteins involved in the adaptation of this species in its native environment and that should enable its establishment in rehabilitating minelands. The results showed differentially abundant proteins, where 436 proteins with significant values (p < 0.05) and fold change ≥ 2 were more abundant in canga and 145 in roots from the rehabilitating minelands. Among them, a representative amount and diversity of proteins were related to responses to water deficit, heat, and responses to metal ions. Other identified proteins are involved in biocontrol activity against phytopathogens and symbiosis. This research provides insights into proteins involved in M. acutistipula responses to environmental stimuli, suggesting critical mechanisms to support the establishment of native canga plants in rehabilitating minelands over time.

A zinc finger protein SlSZP1 protects SlSTOP1 from SlRAE1-mediated degradation to modulate aluminum resistance

In acidic soils, aluminum (Al) toxicity is the main factor inhibiting plant root development and reducing crops yield. STOP1 (SENSITIVE TO PROTON RHIZOTOXICITY 1) was a critical factor in detoxifying Al stress. Under Al stress, STOP1 expression was not induced, although STOP1 protein accumulated, even in the presence of RAE1 (STOP1 DEGRADATION E3-LIGASE). How the Al stress triggers and stabilises the accumulation of STOP1 is still unknown. Here, we characterised SlSTOP1-interacting zinc finger protein (SlSZP1) using a yeast-two-hybrid screening, and generated slstop1, slszp1 and slstop1/slszp1 knockout mutants using clustered regularly interspaced short palindromic repeats (CRISPR) in tomato. SlSZP1 is induced by Al stress but it is not regulated by SlSTOP1. The slstop1, slszp1 and slstop1/slszp1 knockout mutants exhibited hypersensitivity to Al stress. The expression of SlSTOP1-targeted genes, such as SlRAE1 and SlASR2 (ALUMINUM SENSITIVE), was inhibited in both slstop1 and slszp1 mutants, but not directly regulated by SlSZP1. Furthermore, the degradation of SlSTOP1 by SlRAE1 was prevented by SlSZP1. Al stress increased the accumulation of SlSTOP1 in wild-type (WT) but not in slszp1 mutants. The overexpression of either SlSTOP1 or SlSZP1 did not enhance plant Al resistance. Altogether, our results show that SlSZP1 is an important factor for protecting SlSTOP1 from SlRAE1-mediated degradation.

GmTDN1 improves wheat yields by inducing dual tolerance to both drought and low-N stress

Genetically enhancing drought tolerance and nutrient use efficacy enables sustainable and stable wheat production in drought-prone areas exposed to water shortages and low soil fertility, due to global warming and declining natural resources. In this study, wheat plants, exhibiting improved drought tolerance and N-use efficacy, were developed by introducing GmTDN1, a gene encoding a DREB-like transcription factor, into two modern winter wheat varieties, cv Shi4185 and Jimai22. Overexpressing GmTDN1 in wheat resulted in significantly improved drought and low-N tolerance under drought and N-deficient conditions in the greenhouse. Field trials conducted at three different locations over a period of 2-3 consecutive years showed that both Shi4185 and Jimai22 GmTDN1 transgenic lines were agronomically superior to wild-type plants, and produced significantly higher yields under both drought and N-deficient conditions. No yield penalties were observed in these transgenic lines under normal well irrigation conditions. Overexpressing GmTDN1 enhanced photosynthetic and osmotic adjustment capacity, antioxidant metabolism, and root mass of wheat plants, compared to those of wild-type plants, by orchestrating the expression of a set of drought stress-related genes as well as the nitrate transporter, NRT2.5. Furthermore, transgenic wheat with overexpressed NRT2.5 can improve drought tolerance and nitrogen (N) absorption, suggesting that improving N absorption in GmTDN1 transgenic wheat may contribute to drought tolerance. These findings may lead to the development of new methodologies with the capacity to simultaneously improve drought tolerance and N-use efficacy in cereal crops to ensure sustainable agriculture and global food security.

Overexpression of cowpea NAC transcription factors promoted growth and stress tolerance by boosting photosynthetic activity in Arabidopsis

ATAF-like NAC transcription factors are bonafide regulators of stress-signaling. However, their overexpression often exerts growth-retardation by activating ABA-hypersensitivity, chloroplast-degradation, or carbon-starvation. To improve tolerance to multiple stress complying with growth sustainability, we examined two ATAF orthologs, VuNAC1 and VuNAC2, isolated from a drought-hardy cowpea genotype, for a harmonized regulation of stress and growth signaling. The genes were induced by dehydration, NaCl, polyethylene glycol, heat, cold, ABA, and light. Analysis of the promoter-elements and regulatory network corroborated the integration of circadian, hormonal, stress, developmental, and nutrition signals, being VuNAC1/2 the central transcriptional-switch interfacing growth and stress responses. The constitutive gene overexpression in Arabidopsis resulted in an improved embryonic, rosette, and inflorescence growth, under optimum as well as limiting nutrition, in association with increased photosynthetic activity and stomatal-density. The transgenic seedlings manifested tolerance to dehydration, salinity, aluminum, cadmium, and H₂O₂ toxicity, in addition to ABA-mediated seed dormancy and hypersensitivity. The soil-grown plants survived severe drought and hypersalinity by maintaining the water-status and membrane integrity through the accumulation of stress protectants, such as proline, glutathione, and ascorbate. Unlike their orthologs from other species, VuNAC1/2 conferred tolerance to multiple abiotic stresses in line with improved growth attributes via regulation of photosynthetic controls and nutritional balance, suggesting growth being a crucial component of stress-tolerance and recovery. Such unique stress-responsive transcription factors, which also confer photosynthetic gain, could be sustainable biotechnological tools for developing stress-tolerant crops and translating the improved growth into yield without unintended trade-offs.

PpybZIP43 contributes to sucrose synthesis in pear fruits by activating PpySPS3 expression and interacts with PpySTOP1

Sucrose is an important factor affecting sweetness and flavor in pear fruits, but the molecular mechanism of sucrose synthesis regulation is relatively unknown. Here, we characterized a transcription factor gene from pear (Pyrus pyrifolia Nakai cv. "Hosui") fruits, PpybZIP43, and found that the transient overexpression of PpybZIP43 in pear fruits significantly increased the sucrose content and the relative expression level of sucrose phosphate synthase genes (PpySPS3 and PpySPS8). Subcellular localization analysis in tobacco leaves showed that PpybZIP43 was localized in the nucleus. Yeast one-hybrid, electrophoretic mobility shift assay (EMSA), and dual-luciferase reporter assays indicated that PpybZIP43 was able to activate the expression of PpySPS3 by binding specifically to the G-box (CACGTG) element in the promoter. The protein-protein interaction assays using yeast two-hybrid, bimolecular fluorescence complementation (BiFC), firefly luciferase complementation imaging (LCI), and glutathione S-transferase (GST) pull-down demonstrated that PpybZIP43 could directly interact with PpySTOP1 to form a transcription complex. This study is helpful for understanding the molecular basis of sucrose synthesis and accumulation in pear fruits and provides candidate genes for breeding.

Involvement of cytokinins in STOP1-mediated resistance to proton toxicity

STOP1 (sensitive to proton rhizotoxicity1) is a master transcription factor that governs the expression of a set of regulatory and structural genes involved in resistance to aluminum and low pH (i.e., proton) stresses in Arabidopsis. However, the mechanisms and regulatory networks underlying STOP1-mediated resistance to proton stresses are largely unclear. Here, we report that low-pH stresses severely inhibited root growth of the stop1 plants by suppressing root meristem activities. Interestingly, the stop1 plants were less sensitive to exogenous cytokinins at normal and low pHs than the wild type. Significantly, low concentrations of cytokinins promoted root growth of the stop1 mutant under low-pH stresses. Moreover, lateral and adventitious root formation was stimulated in stop1 and by low-pH stresses but suppressed by cytokinins. Further studies of the expression patterns of a cytokinin signaling reporter suggest that both the loss-of-function mutation of STOP1 and low-pH stresses suppressed cytokinin signaling outputs in the root. Furthermore, the expression of critical genes involved in cytokinin biosynthesis, biodegradation, and signaling is altered in the stop1 mutant in response to low-pH stresses. In conclusion, our results reveal a complex network of resistance to low-pH stresses, which involves coordinated actions of STOP1, cytokinins, and an additional low-pH-resistant mechanism for controlling root meristem activities and root growth upon proton stresses.

Research Advances in the Mutual Mechanisms Regulating Response of Plant Roots to Phosphate Deficiency and Aluminum Toxicity

Low phosphate (Pi) availability and high aluminum (Al) toxicity constitute two major plant mineral nutritional stressors that limit plant productivity on acidic soils. Advances toward the identification of genes and signaling networks that are involved in both stresses in model plants such as Arabidopsis thaliana and rice (Oryza sativa), and in other plants as well have revealed that some factors such as organic acids (OAs), cell wall properties, phytohormones, and iron (Fe) homeostasis are interconnected with each other. Moreover, OAs are involved in recruiting of many plant-growth-promoting bacteria that are able to secrete both OAs and phosphatases to increase Pi availability and decrease Al toxicity. In this review paper, we summarize these mutual mechanisms by which plants deal with both Al toxicity and P starvation, with emphasis on OA secretion regulation, plant-growth-promoting bacteria, transcription factors, transporters, hormones, and cell wall-related kinases in the context of root development and root system architecture remodeling that plays a determinant role in improving P use efficiency and Al resistance on acidic soils.

Uncoupling Aluminum Toxicity From Aluminum Signals in the STOP1 Pathway

Aluminum (Al) is a major limiting factor for crop production on acidic soils, inhibiting root growth and plant development. At acidic pH (pH < 5.5), Al³⁺ ions are the main form of Al present in the media. Al³⁺ ions have an increased solubility at pH < 5.5 and result in plant toxicity. At higher pH, the free Al³⁺ fraction decreases in the media, but whether plants can detect Al at these pHs remain unknown. To cope with Al stress, the SENSITIVE TO PROTON RHIZOTOXICITY1 (STOP1) transcription factor induces AL-ACTIVATED MALATE TRANSPORTER1 (ALMT1), a malate-exuding transporter as a strategy to chelate the toxic ions in the rhizosphere. Here, we uncoupled the Al signalling pathway that controls STOP1 from Al toxicity using wild type (WT) and two stop1 mutants carrying the pALMT1:GUS construct with an agar powder naturally containing low amounts of phosphate, iron (Fe), and Al. We combined gene expression [real-time PCR (RT-PCR) and the pALMT1:GUS reporter], confocal microscopy (pSTOP1:GFP-STOP1 reporter), and root growth measurement to assess the effects of Al and Fe on the STOP1-ALMT1 pathway in roots. Our results show that Al triggers STOP1 signaling at a concentration as little as 2 μM and can be detected at a pH above 6.0. We observed that at pH 5.7, 20 μM AlCl₃ induces ALMT1 in WT but does not inhibit root growth in stop1 Al-hypersensitive mutants. Increasing AlCl₃ concentration (>50 μM) at pH 5.7 results in the inhibition of the stop1 mutants primary root. Using the green fluorescent protein (GFP)-STOP1 and ALMT1 reporters, we show that the Al signal pathway can be uncoupled from the Al toxicity on the root. Furthermore, we observe that Al strengthens the Fe-mediated inhibition of primary root growth in WT, suggesting an interaction between Fe and Al on the STOP1-ALMT1 pathway.