Comparative functional analyses of DWARF14 and KARRIKIN INSENSITIVE 2 in drought adaptation of Arabidopsis thaliana

Strigolactone insensitivity affects differential shoot and root transcriptome in barley

Strigolactones (SLs) are plant hormones that play a crucial role in regulating various aspects of plant architecture, such as shoot and root branching. However, the knowledge of SL-responsive genes and transcription factors (TFs) that control the shaping of plant architecture remains elusive. Here, transcriptomic analysis was conducted using the SL-insensitive barley mutant hvd14.d (carried mutation in SL receptor DWARF14, HvD14) and its wild-type (WT) to unravel the differences in gene expression separately in root and shoot tissues. This approach enabled us to select more than six thousand SL-dependent genes that were exclusive to each studied organ or not tissue-specific. The data obtained, along with in silico analyses, found several TFs that exhibited changed expression between the analyzed genotypes and that recognized binding sites in promoters of other identified differentially expressed genes (DEGs). In total, 28 TFs that recognize motifs over-represented in DEG promoters were identified. Moreover, nearly half of the identified TFs were connected in a single network of known and predicted interactions, highlighting the complexity and multidimensionality of SL-related signalling in barley. Finally, the SL control on the expression of one of the identified TFs in HvD14- and dose-dependent manners was proved. Obtained results bring us closer to understanding the signalling pathways regulating SL-dependent plant development.

Karrikin signalling: impacts on plant development and abiotic stress tolerance

Plants rely upon a diverse range of metabolites to control growth and development, and to overcome stress that results from suboptimal conditions. Karrikins (KARs) are a class of butenolide compounds found in smoke that stimulate seed germination and regulate various developmental processes in plants. KARs are perceived via a plant α/β-hydrolase called KARRIKIN INSENSITIVE2 (KAI2), which also functions as a receptor for a postulated phytohormone, provisionally termed KAI2 ligand (KL). Considered natural analogues of KL, KARs have been extensively studied for their effects on plant growth and their crosstalk with plant hormones. The perception and response pathway for KAR-KL signalling is closely related to that of strigolactones, another class of butenolides with numerous functions in regulating plant growth. KAR-KL signalling influences seed germination, seedling photomorphogenesis, root system architecture, abiotic stress responses, and arbuscular mycorrhizal symbiosis. Here, we summarize current knowledge of KAR-KL signalling, focusing on its role in plant development, its effects on stress tolerance, and its interaction with other signalling mechanisms.

Plant growth regulator extracts from seaweeds promote plant growth and confer drought tolerance in canola (Brassica napus)

Brassica napus, commonly known as canola, is an important oilseed crop in Canada contributing over $29.9 billion CAD to the Canadian economy annually. A major challenge facing Canadian canola is drought, which has become increasingly prevalent in recent years due to the changing climate. Research investigating novel agronomic techniques in mitigating drought is key to securing yields and sustainability in canola and other crops. One such technique is the use of bio-stimulant sprays to help offset biotic and abiotic stresses in plants through promoting stand establishment. Previous studies have shown that the application of seaweed extracts as bio-stimulant sprays to Brassicaceae has been successful in improving plant growth and development along with stress tolerance. However, this method has yet to be tested on canola. The organic nutrients that are waste products from processed seaweed help stimulate plant growth, yielding higher quality plants as a result. In association with Le Groupe Roullier, this study demonstrates that the Roullier extracts (RE) help increase plant growth characteristics and drought tolerance in canola when sprayed 3 times over a 3-week period. A high yielding but drought sensitive mutant of canola, d14 (developed through gene editing) was used for drought assays after 8 weeks of growth and where water was withheld for 6 days. Application of the REs prior to drought resulted in plants having enhanced survival rate and improved biomass retention indicating high drought tolerance. Subsequent RNA sequencing and gene ontological term analysis performed using RE treated plants in triplicates, revealed substantial levels of differential expression of growth-related genes along with stress-related genes. These REs elicited responses in plants that had previously only been achieved through gene editing and transgenic methodologies. Using bio-stimulant sprays provides a novel platform to promote beneficial agronomic traits, independent of genetic manipulation.

Strigolactones promote the localization of the ABA exporter ABCG25 at the plasma membrane in root epidermal cells of Arabidopsis thaliana

The phytohormones strigolactones crosstalk with abscisic acid (ABA) in acclimation to osmotic stress, as ascertained in leaves. However, our knowledge about underground tissues is limited, and lacking in Arabidopsis: whether strigolactones affect ABA transport across plasma membranes has never been addressed. We evaluated the effect of strigolactones on the localization of ATP BINDING CASSETTE G25 (ABCG25), an ABA exporter in Arabidopsis thaliana. Wild-type, strigolactone-insensitive, and strigolactone-depleted seedlings expressing a green fluorescent protein:ABCG25 construct were treated with ABA or strigolactones, and green fluorescent protein was quantified by confocal microscopy in different subcellular compartments of epidermal root cells. We show that strigolactones promote the localization of an ABA transporter at the plasma membrane by enhancing its endosomal recycling. Genotypes altered in strigolactone synthesis or perception are not impaired in ABCG25 recycling promotion by ABA, which acts downstream or independent of strigolactones in this respect. Additionally, we confirm that osmotic stress decreases strigolactone synthesis in A. thaliana root cells, and that this decrease may support local ABA retention under low water availability by allowing ABCG25 internalization. Thus, we propose a new mechanism for ABA homeostasis regulation in the context of osmotic stress acclimation: the fine-tuning by strigolactones of ABCG25 localization in root cells.

Multi-omics insights into the positive role of strigolactone perception in barley drought response

BACKGROUND

Drought is a major environmental stress that affects crop productivity worldwide. Although previous research demonstrated links between strigolactones (SLs) and drought, here we used barley (Hordeum vulgare) SL-insensitive mutant hvd14 (dwarf14) to scrutinize the SL-dependent mechanisms associated with water deficit response.

RESULTS

We have employed a combination of transcriptomics, proteomics, phytohormonomics analyses, and physiological data to unravel differences between wild-type and hvd14 plants under drought. Our research revealed that drought sensitivity of hvd14 is related to weaker induction of abscisic acid-responsive genes/proteins, lower jasmonic acid content, higher reactive oxygen species content, and lower wax biosynthetic and deposition mechanisms than wild-type plants. In addition, we identified a set of transcription factors (TFs) that are exclusively drought-induced in the wild-type barley.

CONCLUSIONS

Critically, we resolved a comprehensive series of interactions between the drought-induced barley transcriptome and proteome responses, allowing us to understand the profound effects of SLs in alleviating water-limiting conditions. Several new avenues have opened for developing barley more resilient to drought through the information provided. Moreover, our study contributes to a better understanding of the complex interplay between genes, proteins, and hormones in response to drought, and underscores the importance of a multidisciplinary approach to studying plant stress response mechanisms.

SUPPRESSOR OF MAX2 LIKE 6, 7, and 8 Interact with DDB1 BINDING WD REPEAT DOMAIN HYPERSENSITIVE TO ABA DEFICIENT 1 to Regulate the Drought Tolerance and Target SUCROSE NONFERMENTING 1 RELATED PROTEIN KINASE 2.3 to Abscisic Acid Response in Arabidopsis

SUPPRESSOR OF MAX2-LIKE 6, 7, and 8 (SMXL6,7,8) function as repressors and transcription factors of the strigolactone (SL) signaling pathway, playing an important role in the development and stress tolerance in Arabidopsis thaliana. However, the molecular mechanism by which SMXL6,7,8 negatively regulate drought tolerance and ABA response remains largely unexplored. In the present study, the interacting protein and downstream target genes of SMXL6,7,8 were investigated. Our results showed that the substrate receptor for the CUL4-based E3 ligase DDB1-BINDING WD-REPEAT DOMAIN (DWD) HYPERSENSITIVE TO ABA DEFICIENT 1 (ABA1) (DWA1) physically interacted with SMXL6,7,8. The degradation of SMXL6,7,8 proteins were partially dependent on DWA1. Disruption of SMXL6,7,8 resulted in increased drought tolerance and could restore the drought-sensitive phenotype of the dwa1 mutant. In addition, SMXL6,7,8 could directly bind to the promoter of SUCROSE NONFERMENTING 1 (SNF1)-RELATED PROTEIN KINASE 2.3 (SnRK2.3) to repress its transcription. The mutations in SnRK2.2/2.3 significantly suppressed the hypersensitivity of smxl6/7/8 to ABA-mediated inhibition of seed germination. Conclusively, SMXL6,7,8 interact with DWA1 to negatively regulate drought tolerance and target ABA-response genes. These data provide insights into drought tolerance and ABA response in Arabidopsis via the SMXL6,7,8-mediated SL signaling pathway.

KAI2 Can Do: Karrikin Receptor Function in Plant Development and Response to Abiotic and Biotic Factors

The α/β hydrolase KARRIKIN INSENSITIVE 2 (KAI2) functions as a receptor for a yet undiscovered phytohormone, provisionally termed KAI2 ligand (KL). In addition, it perceives karrikin, a butenolide compound found in the smoke of burnt plant material. KAI2-mediated signaling is involved in regulating seed germination and in shaping seedling and adult plant morphology, both above and below ground. It also governs responses to various abiotic stimuli and stresses and shapes biotic interactions. KAI2-mediated signaling is being linked to an elaborate cross-talk with other phytohormone pathways such as auxin, gibberellin, abscisic acid, ethylene and salicylic acid signaling, in addition to light and nutrient starvation signaling. Further connections will likely be revealed in the future. This article summarizes recent advances in unraveling the function of KAI2-mediated signaling and its interaction with other signaling pathways.

Complexity of SMAX1 signaling during seedling establishment

Karrikins (KARs) are small butenolide compounds identified in the smoke of burning vegetation. Along with the stimulating effects on seed germination, KARs also regulate seedling vigor and adaptive behaviors, such as seedling morphogenesis, root hair development, and stress acclimation. The pivotal KAR signaling repressor, SUPPRESSOR OF MAX2 1 (SMAX1), plays central roles in these developmental and morphogenic processes through an extensive signaling network that governs seedling responses to endogenous and environmental cues. Here, we summarize the versatile roles of SMAX1 reported in recent years and discuss how SMAX1 integrates multiple growth hormone signals into optimizing seedling establishment. We also discuss the evolutionary relevance of the SMAX1-mediated signaling pathways during the colonization of aqueous plants to terrestrial environments.

The strigolactone pathway plays a crucial role in integrating metabolic and nutritional signals in plants

Strigolactones are rhizosphere signals and phytohormones that play crucial roles in plant development. They are also well known for their role in integrating nitrate and phosphate signals to regulate shoot and root development. More recently, sugars and citrate (an intermediate of the tricarboxylic acid cycle) were reported to inhibit the strigolactone response, with dramatic effects on shoot architecture. This Review summarizes the discoveries recently made concerning the mechanisms through which the strigolactone pathway integrates sugar, metabolite and nutrient signals. We highlight here that strigolactones and MAX2-dependent signalling play crucial roles in mediating the impacts of nutritional and metabolic cues on plant development and metabolism. We also discuss and speculate concerning the role of these interactions in plant evolution and adaptation to their environment.

Strigolactones and abscisic acid interactions affect plant development and response to abiotic stresses

Strigolactones (SL) are the youngest group of plant hormones responsible for shaping plant architecture, especially the branching of shoots. However, recent studies provided new insights into the functioning of SL, confirming their participation in regulating the plant response to various types of abiotic stresses, including water deficit, soil salinity and osmotic stress. On the other hand, abscisic acid (ABA), commonly referred as a stress hormone, is the molecule that crucially controls the plant response to adverse environmental conditions. Since the SL and ABA share a common precursor in their biosynthetic pathways, the interaction between both phytohormones has been largely studied in the literature. Under optimal growth conditions, the balance between ABA and SL content is maintained to ensure proper plant development. At the same time, the water deficit tends to inhibit SL accumulation in the roots, which serves as a sensing mechanism for drought, and empowers the ABA production, which is necessary for plant defense responses. The SL-ABA cross-talk at the signaling level, especially regarding the closing of the stomata under drought conditions, still remains poorly understood. Enhanced SL content in shoots is likely to stimulate the plant sensitivity to ABA, thus reducing the stomatal conductance and improving the plant survival rate. Besides, it was proposed that SL might promote the closing of stomata in an ABA-independent way. Here, we summarize the current knowledge regarding the SL and ABA interactions by providing new insights into the function, perception and regulation of both phytohormones during abiotic stress response of plants, as well as revealing the gaps in the current knowledge of SL-ABA cross-talk.

Strigolactones positively regulate Verticillium wilt resistance in cotton via crosstalk with other hormones

Verticillium wilt caused by Verticillium dahliae is a serious vascular disease in cotton (Gossypium spp.). V. dahliae induces the expression of the CAROTENOID CLEAVAGE DIOXYGENASE 7 (GauCCD7) gene involved in strigolactone (SL) biosynthesis in Gossypium australe, suggesting a role for SLs in Verticillium wilt resistance. We found that the SL analog rac-GR24 enhanced while the SL biosynthesis inhibitor TIS108 decreased cotton resistance to Verticillium wilt. Knock-down of GbCCD7 and GbCCD8b genes in island cotton (Gossypium barbadense) decreased resistance, whereas overexpression of GbCCD8b in upland cotton (Gossypium hirsutum) increased resistance to Verticillium wilt. Additionally, Arabidopsis (Arabidopsis thaliana) SL mutants defective in CCD7 and CCD8 putative orthologs were susceptible, whereas both Arabidopsis GbCCD7- and GbCCD8b-overexpressing plants were more resistant to Verticillium wilt than wild-type (WT) plants. Transcriptome analyses showed that several genes related to the jasmonic acid (JA)- and abscisic acid (ABA)-signaling pathways, such as MYELOCYTOMATOSIS 2 (GbMYC2) and ABA-INSENSITIVE 5, respectively, were upregulated in the roots of WT cotton plants in responses to rac-GR24 and V. dahliae infection but downregulated in the roots of both GbCCD7- and GbCCD8b-silenced cotton plants. Furthermore, GbMYC2 suppressed the expression of GbCCD7 and GbCCD8b by binding to their promoters, which might regulate the homeostasis of SLs in cotton through a negative feedback loop. We also found that GbCCD7- and GbCCD8b-silenced cotton plants were impaired in V. dahliae-induced reactive oxygen species (ROS) accumulation. Taken together, our results suggest that SLs positively regulate cotton resistance to Verticillium wilt through crosstalk with the JA- and ABA-signaling pathways and by inducing ROS accumulation.

Karrikin increases tomato cold tolerance via strigolactone and the abscisic acid signaling network

As a class of biostimulants, karrikins (KARs) were first identified from plant-derived smoke to regulate plant growth, development, and stress tolerance. However, the roles of KARs in plant cold tolerance and their crosstalk with strigolactones (SLs) and abscisic acid (ABA) remain elusive. We studied the interaction among KAR, SLs, and ABA in cold acclimatization with KAI2-, MAX1-, SnRK2.5-silenced, or cosilenced plant materials. KAI2 is involved in smoke-water- (SW-) and KAR-mediated cold tolerance. MAX1 acts downstream of KAR in cold acclimation. ABA biosynthesis and sensitivity are regulated by KAR and SLs, which improve cold acclimation through the SnRK2.5 component. The physiological mechanisms of SW and KAR in improving growth, yield, and tolerance under a long-term sublow temperature environment were also studied. SW and KAR were shown to improve tomato growth and yield under sublow temperature conditions by regulating nutritional uptake, leaf temperature control, photosynthetic defense, ROS scavenging, and CBF transcriptional activation. Together, SW, which functions via the KAR-mediated SL and ABA signaling network, has potential application value for increasing cold tolerance in tomato production.

Karrikin perception and signalling

Karrikins (KARs) are a class of butenolide compounds found in smoke that were first identified as seed germination stimulants for fire-following species. Early studies of KARs classified the germination and postgermination responses of many plant species and investigated crosstalk with plant hormones that regulate germination. The discovery that Arabidopsis thaliana responds to KARs laid the foundation for identifying mutants with altered KAR responses. Genetic analysis of KAR signalling revealed an unexpected link to strigolactones (SLs), a class of carotenoid-derived plant hormones. Substantial progress has since been made towards understanding how KARs are perceived and regulate plant growth, in no small part due to advances in understanding SL perception. KAR and SL signalling systems are evolutionarily related and retain a high degree of similarity. There is strong evidence that KARs are natural analogues of an endogenous signal(s), KAI2 ligand (KL), which remains unknown. KAR/KL signalling regulates many developmental processes in plants including germination, seedling photomorphogenesis, and root and root hair growth. KAR/KL signalling also affects abiotic stress responses and arbuscular mycorrhizal symbiosis. Here, we summarise the current knowledge of KAR/KL signalling and discuss current controversies and unanswered questions in this field.

The MAX2-KAI2 module promotes salicylic acid-mediated immune responses in Arabidopsis

Arabidopsis MORE AXILLARY GROWTH2 (MAX2) is a key component in the strigolactone (SL) and karrikin (KAR) signaling pathways and regulates the degradation of SUPPRESSOR OF MAX2 1/SMAX1-like (SMAX1/SMXL) proteins, which are transcriptional co-repressors that regulate plant architecture, as well as abiotic and biotic stress responses. The max2 mutation reduces resistance against Pseudomonas syringae pv. tomato (Pst). To uncover the mechanism of MAX2-mediated resistance, we evaluated the resistance of various SL and KAR signaling pathway mutants. The resistance of SL-deficient mutants and of dwarf 14 (d14) was similar to that of the wild-type, whereas the resistance of the karrikin insensitive 2 (kai2) mutant was compromised, demonstrating that the KAR signaling pathway, not the SL signaling pathway, positively regulates the immune response. We measured the resistance of smax1 and smxl mutants, as well as the double, triple, and quadruple mutants with max2, which revealed that both the smax1 mutant and smxl6/7/8 triple mutant rescue the low resistance phenotype of max2 and that SMAX1 accumulation diminishes resistance. The susceptibility of smax1D, containing a degradation-insensitive form of SMAX1, further confirmed the SMAX1 function in the resistance. The relationship between the accumulation of SMAX1/SMXLs and disease resistance suggested that the inhibitory activity of SMAX1 to resistance requires SMXL6/7/8. Moreover, the exogenous application of KAR2 enhanced resistance against Pst, but KAR-induced resistance depended on salicylic acid (SA) signaling. Inhibition of karrikin signaling delayed SA-mediated defense responses and inhibited pathogen-induced protein biosynthesis. Together, we propose that the MAX2-KAI2-SMAX1 complex regulates resistance with the assistance of SMXL6/7/8 and SA signaling and that SMAX1/SMXLs possibly form a multimeric complex with their target transcription factors to fine tune immune responses.

Transcriptome Analysis Points to BES1 as a Transducer of Strigolactone Effects on Drought Memory in Arabidopsis thaliana

Strigolactones (SLs) are carotenoid-derived phytohormones governing a wide range of physiological processes, including drought-associated stomatal closure. We have previously shown in tomato that SLs regulate the so-called after-effect of drought, whereby stomatal conductance is not completely restored for some time during recovery after a drought spell, irrespective of the water potential. To ease the elucidation of its molecular underpinnings, we investigated whether this SL effect is conserved in Arabidopsis thaliana by contrasting the physiological performances of the wild-type with SL-depleted (more axillary growth 4, max4) and insensitive (dwarf 14, d14) mutants in a drought and recovery protocol. Physiological analyses showed that SLs are important to achieve a complete after-effect in A. thaliana, while transcriptome results suggested that the SL-dependent modulation of drought responses extends to a large subset (about 4/5) of genes displaying memory transcription patterns. Among these, we show that the activation of over 30 genes related to abscisic acid metabolism and signaling strongly depends on SL signaling. Furthermore, by using promoter-enrichment tools, we identified putative cis- and trans-acting factors that may be important in the SL-dependent and SL-independent regulation of genes during drought and recovery. Finally, in order to test the accuracy of our bioinformatic prediction, we confirmed one of the most promising transcription factor candidates mediating SL signaling effects on transcriptional drought memory-BRI-EMS SUPPRESSOR1 (BES1). Our findings reveal that SLs are master regulators of Arabidopsis transcriptional memory upon drought and that this role is partially mediated by the BES1 transcription factor.

Karrikin Receptor KAI2 Coordinates Salt Tolerance Mechanisms in Arabidopsis thaliana

Plants activate a myriad of signaling cascades to tailor adaptive responses under environmental stresses, such as salinity. While the roles of exogenous karrikins (KARs) in salt stress mitigation are well comprehended, genetic evidence of KAR signaling during salinity responses in plants remains unresolved. Here, we explore the functions of the possible KAR receptor KARRIKIN-INSENSITIVE2 (KAI2) in Arabidopsis thaliana tolerance to salt stress by investigating comparative responses of wild-type (WT) and kai2-mutant plants under a gradient of NaCl. Defects in KAI2 functions resulted in delayed and inhibited cotyledon opening in kai2 seeds compared with WT seeds, suggesting that KAI2 played an important role in enhancing seed germination under salinity. Salt-stressed kai2 plants displayed more phenotypic aberrations, biomass reduction, water loss and oxidative damage than WT plants. kai2 shoots accumulated significantly more Na+ and thus had a lower K+/Na+ ratio, than WT, indicating severe ion toxicity in salt-stressed kai2 plants. Accordingly, kai2 plants displayed a lower expression of genes associated with Na+ homeostasis, such as SALT OVERLY SENSITIVE (SOS) 1, SOS2, HIGH-AFFINITY POTASSIUM TRANSPORTER 1;1 (HKT1;1) and CATION-HYDROGEN EXCHANGER 1 (NHX1) than WT plants. WT plants maintained a better glutathione level, glutathione-related redox status and antioxidant enzyme activities relative to kai2 plants, implying KAI2's function in oxidative stress mitigation in response to salinity. kai2 shoots had lower expression levels of genes involved in the biosynthesis of strigolactones (SLs), salicylic acid and jasmonic acid and the signaling of abscisic acid and SLs than those of WT plants, indicating interactive functions of KAI2 signaling with other hormone signaling in modulating plant responses to salinity. Collectively, these results underpin the likely roles of KAI2 in the alleviation of salinity effects in plants by regulating several physiological and biochemical mechanisms involved in ionic and osmotic balance, oxidative stress tolerance and hormonal crosstalk.

The Karrikin Receptor Karrikin Insensitive2 Positively Regulates Heat Stress Tolerance in Arabidopsis thaliana

In this study, we investigated the potential role of the karrikin receptor KARRIKIN INSENSITIVE2 (KAI2) in the response of Arabidopsis seedlings to high-temperature stress. We performed phenotypic, physiological and transcriptome analyses of Arabidopsis kai2 mutants and wild-type (WT) plants under control (kai2_C and WT_C, respectively) and 6- and 24-h heat stress conditions (kai2_H6, kai2_H24, WT_H6 and WT_H24, respectively) to understand the basis for KAI2-regulated heat stress tolerance. We discovered that the kai2 mutants exhibited hypersensitivity to high-temperature stress relative to WT plants, which might be associated with a more highly increased leaf surface temperature and cell membrane damage in kai2 mutant plants. Next, we performed comparative transcriptome analysis of kai2_C, kai2_H6, kai2_H24, WT_C, WT_H6 and WT_H24 to identify transcriptome differences between WT and kai2 mutants in response to heat stress. K-mean clustering of normalized gene expression separated the investigated genotypes into three clusters based on heat-treated and non-treated control conditions. Within each cluster, the kai2 mutants were separated from WT plants, implying that kai2 mutants exhibited distinct transcriptome profiles relative to WT plants. Gene Ontology and Kyoto Encyclopedia of Genes and Genomes enrichment analyses showed a repression in 'misfolded protein binding', 'heat shock protein binding', 'unfolded protein binding' and 'protein processing in endoplasmic reticulum' pathways, which was consistent with the downregulation of several genes encoding heat shock proteins and heat shock transcription factors in the kai2 mutant versus WT plants under control and heat stress conditions. Our findings suggest that chemical or genetic manipulation of KAI2 signaling may provide a novel way to improve heat tolerance in plants.

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.

Strigolactones as a hormonal hub for the acclimation and priming to environmental stress in plants

Strigolactones are phytohormones with many attributed roles in development, and more recently in responses to environmental stress. We will review evidence of the latter in the frame of the classic distinction among the three main stress acclimation strategies (i.e., avoidance, tolerance and escape), by taking osmotic stress in its several facets as a non-exclusive case study. The picture we will sketch is that of a hormonal family playing important roles in each of the mechanisms tested so far, and influencing as well the build-up of environmental memory through priming. Thus, strigolactones appear to be backstage operators rather than frontstage players, setting the tune of acclimation responses by fitting them to the plant individual history of stress experience.

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.

TaFDL2-1A confers drought stress tolerance by promoting ABA biosynthesis, ABA responses, and ROS scavenging in transgenic wheat

Plants have developed various protective mechanisms to survive drought stress. Previously, it was shown that a wheat bZIP transcription factor gene TaFD-Like2-1A (TaFDL2-1A) can confer drought tolerance in Arabidopsis. However, the biological functions related to drought stress tolerance of TaFDL2-1A in wheat (Triticum aestivum L.) remain unclear. In the present study, overexpression of TaFDL2-1A in the wheat cultivar Fielder improved drought resistance and conferred abscisic acid (ABA) hypersensitivity. Further analysis showed that overexpression of TaFDL2-1A increased the hypersensitivity of stomata to drought stress and endogenous ABA content under drought conditions. Genetic analysis and transcriptional regulation analysis indicated that TaFDL2-1A binds directly to the promoter fragments of TaRAB21s and TaNCED2s via ACGT core cis-elements, thereby activating their expression, leading to enhanced ABA responses and endogenous ABA accumulation. In addition, our results demonstrate that overexpression of TaFDL2-1A results in higher SOD and GPX activities in wheat under drought conditions by promoting the expression of TaSOD1 and TaGPx1-D, indicating enhanced reactive oxygen species (ROS) scavenging. These results imply that TaFDL2-1A positively regulates ABA biosynthesis, ABA responses, and ROS scavenging to improve drought stress tolerance in transgenic wheat. Our findings improve our understanding of the mechanisms that allow the wheat bZIP transcription factor to improve drought resistance and provide a useful reference gene for breeding programs to enhance drought resistance.

The strigolactone receptor SlDWARF14 plays a role in photosynthetic pigment accumulation and photosynthesis in tomato

Tomato DWARF14 regulates the development of roots, shoot branches and leaves, and also plays a role in photosynthetic pigment accumulation and photosynthetic capacity. Strigolactones (SLs) are a novel class of plant hormones. DWARF14 (D14) is the only SL receptor identified to date, but it is not functionally analyzed in tomato (Solanum lycopersicum). In the present study, we identified the potential SL receptor in tomato by bioinformatic analysis, which was designated as SlD14. SlD14 was expressed in roots, stems, flowers and developing fruits, with the highest expression level in leaves. sld14 mutant plants produced by the CRISPR/Cas9 system displayed reduced plant height and root biomass, increased shoot branching and altered leaf shape comparing with WT plants. The cytokinin biosynthetic gene ISOPENTENYLTRANSFERASE 3 (SlIPT3), auxin biosynthetic genes FLOOZY (SlFZY) and TRYPTOPHAN AMINOTRANSFERASE RELATED 1 (SlTAR1) and several auxin transport genes SlPINs, which are involved in branch formation, showed higher expression levels in the sld14 plant stem. In addition, sld14 plants exhibited light-green leaves, reduced chlorophyll and carotenoid contents, abnormal chloroplast structure and reduced photosynthetic capacity. Transcriptomic analysis showed that the transcript levels of six chlorophyll biosynthetic genes, three carotenoid biosynthetic genes and numerous chlorophyll a/b-binding protein genes were decreased in sld14 plants. These results suggest that tomato SL receptor gene SlD14 not only regulates the development of roots, shoot branches and leaves, but also plays a role in regulating photosynthetic pigment accumulation and photosynthetic capacity.

A KARRIKIN INSENSITIVE2 paralog in lettuce mediates highly sensitive germination responses to karrikinolide

Karrikins (KARs) are chemicals in smoke that can enhance germination of many plants. Lettuce (Lactuca sativa) cv. Grand Rapids germinates in response to nanomolar karrikinolide (KAR1). Lettuce is much less responsive to KAR2 or a mixture of synthetic strigolactone analogs, rac-GR24. We investigated the molecular basis of selective and sensitive KAR1 perception in lettuce. The lettuce genome contains two copies of KARRIKIN INSENSITIVE2 (KAI2), which in Arabidopsis (Arabidopsis thaliana) encodes a receptor that is required for KAR responses. LsKAI2b is more highly expressed than LsKAI2a in dry achenes and during early stages of imbibition. Through cross-species complementation assays in Arabidopsis, we found that an LsKAI2b transgene confers robust responses to KAR1, but LsKAI2a does not. Therefore, LsKAI2b likely mediates KAR1 responses in lettuce. We compared homology models of KAI2 proteins from lettuce and a fire-follower, whispering bells (Emmenanthe penduliflora). This identified pocket residues 96, 124, 139, and 161 as candidates that influence the ligand specificity of KAI2. Further support for the importance of these residues was found through a broader comparison of pocket residues among 281 KAI2 proteins from 184 asterid species. Almost all KAI2 proteins had either Tyr or Phe identity at position 124. Genes encoding Y124-type KAI2 are more broadly distributed in asterids than in F124-type KAI2. Substitutions at residues 96, 124, 139, and 161 in Arabidopsis KAI2 produced a broad array of responses to KAR1, KAR2, and rac-GR24. This suggests that the diverse ligand preferences observed among KAI2 proteins in plants could have evolved through relatively few mutations.

The histidine phosphotransfer AHP4 plays a negative role in Arabidopsis plant response to drought

Cytokinin plays an important role in plant stress responses via a multistep signaling pathway, involving the histidine phosphotransfer proteins (HPs). In Arabidopsis thaliana, the AHP2, AHP3 and AHP5 proteins are known to affect drought responses; however, the role of AHP4 in drought adaptation remains undetermined. In the present study, using a loss-of-function approach we showed that AHP4 possesses an important role in the response of Arabidopsis to drought. This is evidenced by the higher survival rates of ahp4 than wild-type (WT) plants under drought conditions, which is accompanied by the downregulated AHP4 expression in WT during periods of dehydration. Comparative transcriptome analysis of ahp4 and WT plants revealed AHP4-mediated expression of several dehydration- and/or abscisic acid-responsive genes involved in modulation of various physiological and biochemical processes important for plant drought acclimation. In comparison with WT, ahp4 plants showed increased wax crystal accumulation in stems, thicker cuticles in leaves, greater sensitivity to exogenous abscisic acid at germination, narrow stomatal apertures, heightened leaf temperatures during dehydration, and longer root length under osmotic stress. In addition, ahp4 plants showed greater photosynthetic efficiency, lower levels of reactive oxygen species, reduced electrolyte leakage and lipid peroxidation, and increased anthocyanin contents under drought, when compared with WT. These differences displayed in ahp4 plants are likely due to upregulation of genes that encode enzymes involved in reactive oxygen species scavenging and non-enzymatic antioxidant metabolism. Overall, our findings suggest that AHP4 plays a crucial role in plant drought adaptation.

Exogenous strigolactones enhance tolerance in soybean seedlings in response to alkaline stress

The plant hormone strigolactones (SLs) play crucial roles in regulating plant development and adaptations to abiotic stresses. Even though the functional roles of SLs have been identified in response to abiotic stresses, the function, and mechanism of SLs are not fully established under alkaline stress. In this study, we identified that exogenous SL could improve alkaline tolerance of soybean seedlings, especially when treated with 0.5 μM SL. The application of SL remarkably reduced the malondialdehyde content, hydrogen peroxide content, and increased the activity of antioxidant enzymes under alkaline stress, suggesting that SL improved the alkaline tolerance by regulating the antioxidant defense capacity. The RNA sequencing data showed 530 special differentially expressed genes under SL treatment and alkaline stress, mainly were associated with antioxidant processes and phenylpropanoid biosynthetic pathway. Some transcription factors were also induced by SL under alkaline stress as confirmed by quantitative real-time PCR (qRT-PCR). Furthermore, SL largely increased the Na content in leaves and decreased Na content in roots under alkaline stress, which suggested that SL might promote the transport of Na from the roots to the leaves of the soybean seedlings. Meanwhile, exogenous SL decreased the content of other elements such as K, Mg, Fe, and Cu in leaves or roots under alkaline stress. Collectively, our results suggested a role of SL in regulating antioxidant defense capacity, specific gene expression, and alterations in ionic contents to alleviate harmful effects of alkaline stress in soybean seedlings.

Strigolactones are chemoattractants for host tropism in Orobanchaceae parasitic plants

Parasitic plants are worldwide threats that damage major agricultural crops. To initiate infection, parasitic plants have developed the ability to locate hosts and grow towards them. This ability, called host tropism, is critical for parasite survival, but its underlying mechanism remains mostly unresolved. To characterise host tropism, we used the model facultative root parasite Phtheirospermum japonicum, a member of the Orobanchaceae. Here, we show that strigolactones (SLs) function as host-derived chemoattractants. Chemotropism to SLs is also found in Striga hermonthica, a parasitic member of the Orobanchaceae, but not in non-parasites. Intriguingly, chemotropism to SLs in P. japonicum is attenuated in ammonium ion-rich conditions, where SLs are perceived, but the resulting asymmetrical accumulation of the auxin transporter PIN2 is diminished. P. japonicum encodes putative receptors that sense exogenous SLs, whereas expression of a dominant-negative form reduces its chemotropic ability. We propose a function for SLs as navigators for parasite roots.

Effects of Strigolactone on Torreya grandis Gene Expression and Soil Microbial Community Structure Under Simulated Nitrogen Deposition

Nitrogen enters the terrestrial ecosystem through deposition. High nitrogen levels can affect physical and chemical properties of soil and inhibit normal growth and reproduction of forest plants. Nitrogen modulates the composition of soil microorganisms. Strigolactones inhibits plant branching, promotes root growth, nutrient absorption, and promotes arbuscular fungal mycelia branching. Plants are subjected to increasing atmospheric nitrogen deposition. Therefore, it is imperative to explore the relationship between strigolactone and nitrogen deposition of plants and abundance of soil microorganisms. In the present study, the effects of strigolactone on genetic responses and soil microorganisms of Torreya grandis, under simulated nitrogen deposition were explored using high-throughput sequencing techniques. T. grandis is a subtropical economic tree species in China. A total of 4,008 differentially expressed genes were identified in additional N deposition and GR24 treatment. These genes were associated with multiple GO terms and metabolic pathways. GO enrichment analysis showed that several DEGs were associated with enrichment of the transporter activity term. Both additional nitrogen deposition and GR24 treatment modulated the content of nutrient elements. The content of K reduced in leaves after additional N deposition treatment. The content of P increased in leaves after GR24 treatment. A total of 20 families and 29 DEGs associated with transporters were identified. These transporters may be regulated by transcription factors. A total of 1,402,819 clean reads and 1,778 amplicon sequence variants (ASVs) were generated through Bacterial 16S rRNA sequencing. Random forest classification revealed that Legionella, Lacunisphaera, Klebsiella, Bryobacter, and Janthinobacterium were significantly enriched in the soil in the additional N deposition group and the GR24 treatment group. Co-occurrence network analysis showed significant differences in composition of soil microbial community under different treatments. These results indicate a relationship between N deposition and strigolactones effect. The results provide new insights on the role of strigolactones in plants and composition of soil microorganisms under nitrogen deposition.

Plant hormone regulation of abiotic stress responses

Plant hormones are signalling compounds that regulate crucial aspects of growth, development and environmental stress responses. Abiotic stresses, such as drought, salinity, heat, cold and flooding, have profound effects on plant growth and survival. Adaptation and tolerance to such stresses require sophisticated sensing, signalling and stress response mechanisms. In this Review, we discuss recent advances in understanding how diverse plant hormones control abiotic stress responses in plants and highlight points of hormonal crosstalk during abiotic stress signalling. Control mechanisms and stress responses mediated by plant hormones including abscisic acid, auxin, brassinosteroids, cytokinins, ethylene and gibberellins are discussed. We discuss new insights into osmotic stress sensing and signalling mechanisms, hormonal control of gene regulation and plant development during stress, hormone-regulated submergence tolerance and stomatal movements. We further explore how innovative imaging approaches are providing insights into single-cell and tissue hormone dynamics. Understanding stress tolerance mechanisms opens new opportunities for agricultural applications.

The strigolactone receptor D14 targets SMAX1 for degradation in response to GR24 treatment and osmotic stress

The effects of the phytohormone strigolactone (SL) and smoke-derived karrikins (KARs) on plants are generally distinct, despite the fact that they are perceived through very similar mechanisms. The homologous receptors DWARF14 (D14) and KARRIKIN-INSENSITIVE2 (KAI2), together with the F-box protein MORE AXILLARY GROWTH2 (MAX2), mediate SL and KAR responses, respectively, by targeting different SMAX1-LIKE (SMXL) family proteins for degradation. These mechanisms are putatively well-insulated, with D14-MAX2 targeting SMXL6, SMXL7, and SMXL8 and KAI2-MAX2 targeting SMAX1 and SMXL2 in Arabidopsis thaliana. Recent evidence challenges this model. We investigated whether D14 can target SMAX1 and whether this occurs naturally. Genetic analysis indicates that the SL analog GR24 promotes D14-SMAX1 crosstalk. Although D14 shows weaker interactions with SMAX1 than with SMXL2 or SMXL7, D14 mediates GR24-induced degradation of SMAX1 in plants. Osmotic stress triggers SMAX1 degradation, which is protective, through SL biosynthesis and signaling genes. Thus, D14-SMAX1 crosstalk may be beneficial and not simply a vestige of the evolution of the SL pathway.

Rapid analysis of strigolactone receptor activity in a Nicotiana benthamiana dwarf14 mutant

DWARF14 (D14) is an ɑ/β-hydrolase and receptor for the plant hormone strigolactone (SL) in angiosperms. Upon SL perception, D14 works with MORE AXILLARY GROWTH2 (MAX2) to trigger polyubiquitination and degradation of DWARF53(D53)-type proteins in the SUPPRESSOR OF MAX2 1-LIKE (SMXL) family. We used CRISPR-Cas9 to generate knockout alleles of the two homoeologous D14 genes in the Nicotiana benthamiana genome. The Nbd14a,b double mutant had several phenotypes that are consistent with the loss of SL perception in other plants, including increased axillary bud outgrowth, reduced height, shortened petioles, and smaller leaves. A ratiometric fluorescent reporter system was used to monitor degradation of SMXL7 from Arabidopsis thaliana (AtSMXL7) after transient expression in N. benthamiana and treatment with the strigolactone analog GR24. AtSMXL7 was degraded after treatment with GR24^5DS, which has the stereochemical configuration of natural SLs, as well as its enantiomer GR24 ^ent-5DS. In Nbd14a,b leaves, AtSMXL7 abundance was unaffected by rac-GR24 or either GR24 stereoisomer. Transient coexpression of AtD14 with the AtSMXL7 reporter in Nbd14a,b restored the degradation response to rac-GR24, but required an active catalytic triad. We used this platform to evaluate the ability of several AtD14 mutants that had not been characterized in plants to target AtSMXL7 for degradation.

Strigo-D2-a bio-sensor for monitoring spatio-temporal strigolactone signaling patterns in intact plants

Strigolactones (SLs) are a class of plant hormones that mediate biotic interactions and modulate developmental programs in response to endogenous and exogenous stimuli. However, a comprehensive view on the spatio-temporal pattern of SL signaling has not been established, and tools for a systematic in planta analysis do not exist. Here, we present Strigo-D2, a genetically encoded ratiometric SL signaling sensor that enables the examination of SL signaling distribution at cellular resolution and is capable of rapid response to altered SL levels in intact Arabidopsis (Arabidopsis thaliana) plants. By monitoring the abundance of a truncated and fluorescently labeled SUPPRESSOR OF MAX2 1-LIKE 6 (SMXL6) protein, a proteolytic target of the SL signaling machinery, we show that all cell types investigated have the capacity to respond to changes in SL levels but with very different dynamics. In particular, SL signaling is pronounced in vascular cells but low in guard cells and the meristematic region of the root. We also show that other hormones leave Strigo-D2 activity unchanged, indicating that initial SL signaling steps work in isolation from other hormonal signaling pathways. The specificity and spatio-temporal resolution of Strigo-D2 underline the value of the sensor for monitoring SL signaling in a broad range of biological contexts with highly instructive analytical depth.

Physiological mechanism of strigolactone enhancing tolerance to low light stress in cucumber seedlings

Strigolactone is a newly discovered type of plant hormone that has multiple roles in modulating plant responses to abiotic stress. Herein, we aimed to investigate the effects of exogenous GR24 (a synthetic analogue of strigolactone) on plant growth, photosynthetic characteristics, carbohydrate levels, endogenous strigolactone content and antioxidant metabolism in cucumber seedlings under low light stress. The results showed that the application of 10 μM GR24 can increase the photosynthetic efficiency and plant biomass of low light-stressed cucumber seedlings. GR24 increased the accumulation of carbohydrates and the synthesis of sucrose-related enzyme activities, enhanced antioxidant enzyme activities and antioxidant substance contents, and reduced the levels of H₂O₂ and MDA in cucumber seedlings under low light stress. These results indicate that exogenous GR24 might alleviate low light stress-induced growth inhibition by regulating the assimilation of carbon and antioxidants and endogenous strigolactone contents, thereby enhancing the tolerance of cucumber seedlings to low light stress.

Transcriptome Analysis Reveals Roles of Anthocyanin- and Jasmonic Acid-Biosynthetic Pathways in Rapeseed in Response to High Light Stress

Rapeseed (Brassica napus) is one of the major important oil crops worldwide and is largely cultivated in the Qinghai-Tibetan plateau (QTP), where long and strong solar-radiation is well-known. However, the molecular mechanisms underlying rapeseed's response to light stress are largely unknown. In the present study, the color of rapeseed seedlings changed from green to purple under high light (HL) stress conditions. Therefore, changes in anthocyanin metabolism and the transcriptome of rapeseed seedlings cultured under normal light (NL) and HL conditions were analyzed to dissect how rapeseed responds to HL at the molecular level. Results indicated that the contents of anthocyanins, especially glucosides of cyanidin, delphinidin, and petunidin, which were determined by liquid chromatography-mass spectrometry (LC-MS), increased by 9.6-, 4.2-, and 59.7-fold in rapeseed seedlings exposed to HL conditions, respectively. Next, RNA-sequencing analysis identified 7390 differentially expressed genes (DEGs), which included 4393 up-regulated and 2997 down-regulated genes. Among the up-regulated genes, many genes related to the anthocyanin-biosynthetic pathway were enriched. For example, genes encoding dihydroflavonol reductase (BnDFR) and anthocyanin synthase (BnANS) were especially induced by HL conditions, which was also confirmed by RT-qPCR analysis. In addition, two PRODUCTION OF ANTHOCYANIN PIGMENTATION 2 (BnPAP2) and GLABRA3 (BnGL3) genes encoding MYB-type and bHLH-type transcription factors, respectively, whose expression was also up-regulated by HL stress, were found to be associated with the changes in anthocyanin biosynthesis. Many genes involved in the jasmonic acid (JA)-biosynthetic pathway were also up-regulated under HL conditions. This finding, which is in agreement with the well-known positive regulatory role of JA in anthocyanin biosynthesis, suggests that the JA may also play a key role in the responses of rapeseed seedlings to HL. Collectively, these data indicate that anthocyanin biosynthesis-related and JA biosynthesis-related pathways mediate HL responses in rapeseed. These findings collectively provide mechanistic insights into the mechanisms involved in the response of rapeseed to HL stress, and the identified key genes may potentially be used to improve HL tolerance of rapeseed cultivars through genetic engineering or breeding strategies.

Strigolactones positively regulate abscisic acid-dependent heat and cold tolerance in tomato

Strigolactones are carotenoid-derived phytohormones that impact plant growth and development in diverse ways. However, the roles of strigolactones in the responses to temperature stresses are largely unknown. Here, we demonstrated that strigolactone biosynthesis is induced in tomato (Solanum lycopersicum) by heat and cold stresses. Compromised strigolactone biosynthesis or signaling negatively affected heat and cold tolerance, while application of the synthetic strigolactone analog GR245DS enhanced heat and cold tolerance. Strigolactone-mediated heat and cold tolerance was associated with the induction of abscisic acid (ABA), heat shock protein 70 (HSP70) accumulation, C-REPEAT BINDING FACTOR 1 (CBF1) transcription, and antioxidant enzyme activity. Importantly, a deficiency in ABA biosynthesis compromised the GR245DS effects on heat and cold stresses and abolished the GR245DS-induced transcription of HSP70, CBF1, and antioxidant-related genes. These results support that strigolactones positively regulate tomato heat and cold tolerance and that they do so at least partially by the induction of CBFs and HSPs and the antioxidant response in an ABA-dependent manner.

Silicon-mediated heat tolerance in higher plants: A mechanistic outlook

Heat stress, resulting from global warming, is considered one of the major challenges to be addressed for increasing plant survival and productivity worldwide. Although plants have a built-in defense mechanism against heat stress, such strategy seems to be insufficient to counteract heat adversities under extreme temperature regimes. Hence, increasing heat tolerance in plants for sustainable yields is one of the biggest challenges for researchers in the coming decades. Conventional plant breeding approach to enhance heat tolerance has gained some successes; however, more efforts are needed to make plants resilient to heat stress for increasing crop production during ongoing climate change. Thus, exploring 'heat stress mitigation strategies' using cost-effective and eco-friendly approaches may be quick and sustainable alternatives. The use of silicon (Si) and Si-nanoparticles (Si-NPs) in enhancing heat tolerance in plants has recently gained much attention. Application of Si and Si-NPs can assist plants to overcome heat-induced oxidative stress through the acceleration of reactive oxygen species detoxification by modulating the antioxidant systems and regulating transcription of key genes associated with heat stress responses. In fact, molecular rationale behind Si-mediated heat tolerance in plants is largely unknown. In this minireview, we made efforts to understand the mechanistic aspects of heat-induced responses and damages in plants, and possible molecular dynamics of Si-induced heat tolerance in plants. We also highlighted recent advances on how Si induces heat tolerance potential in plants and future perspectives on how Si can contribute to sustainable crop production under the increasing threat of global climate change.

Strigolactones regulate arsenate uptake, vacuolar-sequestration and antioxidant defense responses to resist arsenic toxicity in rice roots

We explored genetic evidence for strigolactones' role in rice tolerance to arsenate-stress. Comparative analyses of roots of wild-type (WT) and strigolactone-deficient mutants d10 and d17 in response to sodium arsenate (Na₂AsO₄) revealed differential growth inhibition [WT (11.28%) vs. d10 (19.76%) and d17 (18.03%)], biomass reduction [(WT (33.65%) vs. d10 (74.86%) and d17 (60.65%)] and membrane damage (WT < d10 and d17) at 250 μM Na₂AsO₄. Microscopic and biochemical analyses showed that roots of WT accumulated lower levels of arsenic and oxidative stress indicators like reactive oxygen species and malondialdehyde than those of strigolactone-deficient mutants. qRT-PCR data indicated lower expression levels of genes (OsPT1, OsPT2, OsPT4 and OsPT8) encoding phosphate-transporters in WT roots than mutant roots, explaining the decreased arsenate and phosphate uptake by WT roots. Increased levels of glutathione and OsPCS1 and OsABCC1 transcripts indicated an efficient vacuolar-sequestration of arsenic in WT roots. Furthermore, higher activities (transcript levels) of SOD (OsCuZnSOD1 and OsCuZnSOD2), APX (OsAPX1 and OsAPX2) and CAT (OsCATA) corresponded to lower oxidative damage in WT roots compared with strigolactone-mutant roots. Collectively, these results highlight that strigolactones are involved in arsenic-stress mitigation by regulating arsenate-uptake, glutathione-biosynthesis, vacuolar-sequestration of arsenic and antioxidant defense responses in rice roots.

Crosstalk between phytohormones and secondary metabolites in the drought stress tolerance of crop plants: A review

Drought stress negatively affects crop performance and weakens global food security. It triggers the activation of downstream pathways, mainly through phytohormones homeostasis and their signaling networks, which further initiate the biosynthesis of secondary metabolites (SMs). Roots sense drought stress, the signal travels to the above-ground tissues to induce systemic phytohormones signaling. The systemic signals further trigger the biosynthesis of SMs and stomatal closure to prevent water loss. SMs primarily scavenge reactive oxygen species (ROS) to protect plants from lipid peroxidation and also perform additional defense-related functions. Moreover, drought-induced volatile SMs can alert the plant tissues to perform drought stress mitigating functions in plants. Other phytohormone-induced stress responses include cell wall and cuticle thickening, root and leaf morphology alteration, and anatomical changes of roots, stems, and leaves, which in turn minimize the oxidative stress, water loss, and other adverse effects of drought. Exogenous applications of phytohormones and genetic engineering of phytohormones signaling and biosynthesis pathways mitigate the drought stress effects. Direct modulation of the SMs biosynthetic pathway genes or indirect via phytohormones' regulation provides drought tolerance. Thus, phytohormones and SMs play key roles in plant development under the drought stress environment in crop plants.

The mechanism of host-induced germination in root parasitic plants

Chemical signals known as strigolactones (SLs) were discovered more than 50 years ago as host-derived germination stimulants of parasitic plants in the Orobanchaceae. Strigolactone-responsive germination is an essential adaptation of obligate parasites in this family, which depend upon a host for survival. Several species of obligate parasites, including witchweeds (Striga, Alectra spp.) and broomrapes (Orobanche, Phelipanche spp.), are highly destructive agricultural weeds that pose a significant threat to global food security. Understanding how parasites sense SLs and other host-derived stimulants will catalyze the development of innovative chemical and biological control methods. This review synthesizes the recent discoveries of strigolactone receptors in parasitic Orobanchaceae, their signaling mechanism, and key steps in their evolution.

Contribution of phenylpropanoid metabolism to plant development and plant-environment interactions

Phenylpropanoid metabolism is one of the most important metabolisms in plants, yielding more than 8,000 metabolites contributing to plant development and plant-environment interplay. Phenylpropanoid metabolism materialized during the evolution of early freshwater algae that were initiating terrestrialization and land plants have evolved multiple branches of this pathway, which give rise to metabolites including lignin, flavonoids, lignans, phenylpropanoid esters, hydroxycinnamic acid amides, and sporopollenin. Recent studies have revealed that many factors participate in the regulation of phenylpropanoid metabolism, and modulate phenylpropanoid homeostasis when plants undergo successive developmental processes and are subjected to stressful environments. In this review, we summarize recent progress on elucidating the contribution of phenylpropanoid metabolism to the coordination of plant development and plant-environment interaction, and metabolic flux redirection among diverse metabolic routes. In addition, our review focuses on the regulation of phenylpropanoid metabolism at the transcriptional, post-transcriptional, post-translational, and epigenetic levels, and in response to phytohormones and biotic and abiotic stresses.

Does Karrikin Signaling Shape the Rhizomicrobiome via the Strigolactone Biosynthetic Pathway?

A recent study by Choi et al. provides evidence of the interaction between the karrikin (KAR) signaling and strigolactone (SL) biosynthetic pathways. Since SLs shape rhizomicrobiome composition, it is of interest to determine whether KAR signaling could affect rhizomicrobiome composition by improving the synthesis of root-derived SLs to support climate-smart agriculture.

Different strategies of strigolactone and karrikin signals in regulating the resistance of Arabidopsis thaliana to water-deficit stress

Strigolactone and karrikin receptors, DWARF14 (D14) and KARRIKIN INSENSITIVE 2 (KAI2), respectively, have been shown to positively regulate drought resistance in Arabidopsis thaliana by modulating abscisic acid responsiveness, anthocyanin accumulation, stomatal closure, cell membrane integrity and cuticle formation. Here, we aim to identify genes specifically or commonly regulated by D14 and KAI2 under water scarcity, using comparative analysis of the transcriptome data of the A. thaliana d14-1 and kai2-2 mutants under dehydration conditions. In comparison with wild-type, under dehydration conditions, the expression levels of genes related to photosynthesis and the metabolism of glucosinolates and trehalose were significantly changed in both d14-1 and kai2-2 mutant plants, whereas the transcript levels of genes related to the metabolism of cytokinins and brassinosteroids were significantly altered in the d14-1 mutant plants only. These results suggest that cytokinin and brassinosteroid metabolism might be specifically regulated by the D14 pathway, whereas photosynthesis and metabolism of glucosinolates and trehalose are potentially regulated by both D14 and KAI2 pathways in plant response to water scarcity.

Transcriptome Analysis Reveals Potential Roles of Abscisic Acid and Polyphenols in Adaptation of Onobrychis viciifolia to Extreme Environmental Conditions in the Qinghai-Tibetan Plateau

A detailed understanding of the molecular mechanisms of plant stress resistance in the face of ever-changing environmental stimuli will be helpful for promoting the growth and production of crop and forage plants. Investigations of plant responses to various single abiotic or biotic factors, or combined stresses, have been extensively reported. However, the molecular mechanisms of plants in responses to environmental stresses under natural conditions are not clearly understood. In this study, we carried out a transcriptome analysis using RNA-sequencing to decipher the underlying molecular mechanisms of Onobrychis viciifolia responding and adapting to the extreme natural environment in the Qinghai-Tibetan Plateau (QTP). The transcriptome data of plant samples collected from two different altitudes revealed a total of 8212 differentially expressed genes (DEGs), including 5387 up-regulated and 2825 down-regulated genes. Detailed analysis of the identified DEGs uncovered that up-regulation of genes potentially leading to changes in hormone homeostasis and signaling, particularly abscisic acid-related ones, and enhanced biosynthesis of polyphenols play vital roles in the adaptive processes of O. viciifolia. Interestingly, several DEGs encoding uridine diphosphate glycosyltransferases, which putatively regulate phytohormone homeostasis to resist environmental stresses, were also discovered. Furthermore, numerous DEGs encoding transcriptional factors, such as members of the myeloblastosis (MYB), homeodomain-leucine zipper (HD-ZIP), WRKY, and nam-ataf1,2-cuc2 (NAC) families, might be involved in the adaptive responses of O. viciifolia to the extreme natural environmental conditions. The DEGs identified in this study represent candidate targets for improving environmental stress resistance of O. viciifolia grown in higher altitudes of the QTP, and can provide deep insights into the molecular mechanisms underlying the responses of this plant species to the extreme natural environmental conditions of the QTP.

Negative Roles of Strigolactone-Related SMXL6, 7 and 8 Proteins in Drought Resistance in Arabidopsis

Previous investigations have shown that the SUPPRESSORS OF MAX2 1-LIKE6, 7 and 8 (SMXL6, 7 and 8) proteins redundantly repress strigolactone (SL) signaling in plant growth and development. Recently, a growing body of evidence indicated that SLs positively regulate plant drought resistance through functional analyses of genes involved in SL biosynthesis and positive regulation of SL signaling. However, the functions of the SL-signaling negative regulators SMXL6, 7 and 8 in drought resistance and the associated mechanisms remain elusive. To reveal the functions of these SMXL proteins, we analyzed the drought-resistant phenotype of the triple smxl6,7,8 mutant plants and studied several drought resistance-related traits. Our results showed that the smxl6,7,8 mutant plants were more resistant to drought than wild-type plants. Physiological investigations indicated that the smxl6,7,8 mutant plants exhibited higher leaf surface temperature, reduced cuticle permeability, as well as decreases in drought-induced water loss and cell membrane damage in comparison with wild-type plants. Additionally, smxl6,7,8 mutant plants displayed an increase in anthocyanin biosynthesis during drought, enhanced detoxification capacity and increased sensitivity to abscisic acid in cotyledon opening and growth inhibition assays. A good correlation between the expression levels of some relevant genes and the examined physiological and biochemical traits was observed. Our findings together indicate that the SMXL6, 7 and 8 act as negative regulators of drought resistance, and that disruption of these SMXL genes in crops may provide a novel way to improve their drought resistance.