The karrikin receptor KAI2 promotes drought resistance in Arabidopsis thaliana

ζ-Carotene Isomerase Suppresses Tillering in Rice through the Coordinated Biosynthesis of Strigolactone and Abscisic Acid

Rice tillering is an important agronomic trait affecting grain yield. Here, we identified a high-tillering mutant tillering20 (t20), which could be restored to the wild type by treatment with the strigolactone (SL) analog rac-GR24. T20 encodes a chloroplast ζ-carotene isomerase (Z-ISO), which is involved in the biosynthesis of carotenoids and their metabolites, SL and abscisic acid (ABA). The t20 mutant has reduced SL and ABA, raising the question of how SL and ABA biosynthesis is coordinated, and whether they have overlapping functions in tillering. We discovered that rac-GR24 stimulated T20 expression and enhanced all-trans-β-carotene biosynthesis. Importantly, rac-GR24 also stimulated expression of Oryza sativa 9-CIS-EPOXYCAROTENOID DIOXYGENASE 1 (OsNCED1) through induction of Oryza sativa HOMEOBOX12 (OsHOX12), promoting ABA biosynthesis in shoot base. On the other hand, ABA treatment significantly repressed SL biosynthesis and the ABA biosynthetic mutants displayed elevated SL biosynthesis. ABA treatment reduced the number of basal tillers in both t20 and wild-type plants. Furthermore, while ABA-deficient mutants aba1 and aba2 had the same number of basal tillers as wild type, they had more unproductive upper tillers at maturity. This work demonstrates complex interactions in the biosynthesis of carotenoid, SLs and ABA, and reveals a role for ABA in the regulation of rice tillering.

Butanolide alleviated cadmium stress by improving plant growth, photosynthetic parameters and antioxidant defense system of brassica oleracea

Current study was performed to explore the effect of butanolide (KAR1) in mitigation of cadmium (Cd) induced toxicity in Brussels sprout (Brassica oleracea L.). Brussels sprout seeds, treated with 10-5 M, 10-7 M and 10-10 M solution of KAR1 were allowed to grow in Cd-contaminated (5 mg L-1) regimes for 25 d. Cadmium toxicity decreased seed germination and growth in B. oleracea seedlings. Elevated intensity of electrolyte leakage (EL), malondialdehyde (MDA) and hydrogen peroxide (H₂O₂) were observed in Cd-stressed seedlings. Additionally, reduced level of stomatal conductivity, transpiration rate, photosynthesis rate, intercellular carbon dioxide concentration, and leaf relative water content (LRWC) was also observed in Cd-stressed seedlings. Nevertheless, KAR1 improved seed germination, seedling growth and biomass production in Cd stressed plants. KAR1 application showed elevated LRWC, osmotic potential, and higher membranous stability index (MSI) in seedlings under Cd regime. Furthermore, seedlings developed by KAR1 treatment exhibited higher stomatal conductivity, and intercellular carbon dioxide concentration together with improved rate of transpiration and photosynthetic rate in B. oleracea under Cd stress. These findings elucidate that the reduced level of MDA, EL and H₂O₂, as well as improvement in antioxidative machinery increased growth and alleviated Cd toxicity in KAR1 treated seedlings under Cd stress.

Lotus japonicus karrikin receptors display divergent ligand-binding specificities and organ-dependent redundancy

Karrikins (KARs), smoke-derived butenolides, are perceived by the α/β-fold hydrolase KARRIKIN INSENSITIVE2 (KAI2) and thought to mimic endogenous, yet elusive plant hormones tentatively called KAI2-ligands (KLs). The sensitivity to different karrikin types as well as the number of KAI2 paralogs varies among plant species, suggesting diversification and co-evolution of ligand-receptor relationships. We found that the genomes of legumes, comprising a number of important crops with protein-rich, nutritious seed, contain two or more KAI2 copies. We uncover sub-functionalization of the two KAI2 versions in the model legume Lotus japonicus and demonstrate differences in their ability to bind the synthetic ligand GR24ent-5DS in vitro and in genetic assays with Lotus japonicus and the heterologous Arabidopsis thaliana background. These differences can be explained by the exchange of a widely conserved phenylalanine in the binding pocket of KAI2a with a tryptophan in KAI2b, which arose independently in KAI2 proteins of several unrelated angiosperms. Furthermore, two polymorphic residues in the binding pocket are conserved across a number of legumes and may contribute to ligand binding preferences. The diversification of KAI2 binding pockets suggests the occurrence of several different KLs acting in non-fire following plants, or an escape from possible antagonistic exogenous molecules. Unexpectedly, L. japonicus responds to diverse synthetic KAI2-ligands in an organ-specific manner. Hypocotyl growth responds to KAR1, KAR2 and rac-GR24, while root system development responds only to KAR1. This differential responsiveness cannot be explained by receptor-ligand preferences alone, because LjKAI2a is sufficient for karrikin responses in the hypocotyl, while LjKAI2a and LjKAI2b operate redundantly in roots. Instead, it likely reflects differences between plant organs in their ability to transport or metabolise the synthetic KLs. Our findings provide new insights into the evolution and diversity of butenolide ligand-receptor relationships, and open novel research avenues into their ecological significance and the mechanisms controlling developmental responses to divergent KLs.

Synergistic Effect of Bacillus thuringiensis IAGS 199 and Putrescine on Alleviating Cadmium-Induced Phytotoxicity in Capsicum annum

Plant growth-promoting bacteria (PGPB) and putrescine (Put) have shown a promising role in the mitigation of abiotic stresses in plants. The present study was anticipated to elucidate the potential of Bacillus thuringiensis IAGS 199 and Put in mitigation of cadmium (Cd)-induced toxicity in Capsicum annum. Cadmium toxicity decreased growth, photosynthetic rate, gas exchange attributes and activity of antioxidant enzymes in C. annum seedlings. Moreover, higher levels of protein and non-protein bound thiols besides increased Cd contents were also observed in Cd-stressed plants. B. thuringiensis IAGS 199 and Put, alone or in combination, reduced electrolyte leakage (EL), hydrogen peroxide (H₂O₂) and malondialdehyde (MDA) level in treated plants. Synergistic effect of B. thuringiensis IAGS 199 and Put significantly enhanced the activity of stress-responsive enzymes including peroxidase (POD), ascorbate peroxidase (APX), catalase (CAT) and superoxide dismutase (SOD). Furthermore, Put and microbial interaction enhanced the amount of proline, soluble sugars, and total soluble proteins in C. annum plants grown in Cd-contaminated soil. Data obtained during the current study advocates that application of B. thuringiensis IAGS 199 and Put establish a synergistic role in the mitigation of Cd-induced stress through modulating physiochemical features of C. annum plants.

Tanned or Sunburned: How Excessive Light Triggers Plant Cell Death

Plants often encounter light intensities exceeding the capacity of photosynthesis (excessive light) mainly due to biotic and abiotic factors, which lower CO₂ fixation and reduce light energy sinks. Under excessive light, the photosynthetic electron transport chain generates damaging molecules, hence leading to photooxidative stress and eventually to cell death. In this review, we summarize the mechanisms linking the excessive absorption of light energy in chloroplasts to programmed cell death in plant leaves. We highlight the importance of reactive carbonyl species generated by lipid photooxidation, their detoxification, and the integrating role of the endoplasmic reticulum in the adoption of phototolerance or cell-death pathways. Finally, we invite the scientific community to standardize the conditions of excessive light treatments.

Multi-Omics Strategies for Decoding Smoke-Assisted Germination Pathways and Seed Vigour

The success of seed germination and the successful establishment of seedlings across diverse environmental conditions depends on seed vigour, which is of both economic and ecologic importance. The smoke-derived exogenous compound karrikins (KARs) and the endogenous plant hormone strigolactone (SL) are two classes of butanolide-containing molecules that follow highly similar signalling pathways to control diverse biological activities in plants. Unravelling the precise mode-of-action of these two classes of molecules in model species has been a key research objective. However, the specific and dynamic expression of biomolecules upon stimulation by these signalling molecules remains largely unknown. Genomic and post-genomic profiling approaches have enabled mining and association studies across the vast genetic diversity and phenotypic plasticity. Here, we review the background of smoke-assisted germination and vigour and the current knowledge of how plants perceive KAR and SL signalling and initiate the crosstalk with the germination-associated hormone pathways. The recent advancement of 'multi-omics' applications are discussed in the context of KAR signalling and with relevance to their adoption for superior agronomic trait development. The remaining challenges and future opportunities for integrating multi-omics datasets associated with their application in KAR-dependent seed germination and abiotic stress tolerance are also discussed.

6-Benzylaminopurine (6-BA) ameliorates drought stress response in tall fescue via the influencing of biochemicals and strigolactone-signaling genes

Drought is a major agricultural and societal concern that causes farmers worldwide billions of dollars in annual losses. By revealing the as-of-yet unknown details of the biochemical and phytohormonal crosstalk occurring in drought-stressed plants, novel strategies can be pioneered to enhance drought tolerance in crop plants. Toward this goal, exogenous treatments containing the synthetic cytokinin 6-Benzylaminopurine (6-BA) were applied to the perennial monocot grass Festuca arundinacea (Tall Fescue). These plants were subjected to three irrigation levels: 100% ± 5%, 50% ± 5%, and 25% ± 5% of field capacity, at which a number of morpho-physiological and biochemical responses were evaluated. Furthermore, to elucidate the crosstalk between cytokinin (CK) and strigolactone (SL), we evaluated the activities of several SL-responsive genes. Drought conditions were shown to have widespread effects on morpho-physiological and biochemical indices. However, foliar application of 6-BA on tall fescue largely ameliorated drought stress symptoms. Water-soluble carbohydrates also declined significantly in response to CK over the course of drought progression, with virtually no change to starch content. Severe drought stress also upregulated a number of SL-response genes in the leaves of plants, indicating a correlation between the degree of drought severity and the quantity of SLs in tall fescue. Furthermore, the drought‒mediated induction of SL-signaling genes (including FaD14 and FaMax2) was inhibited in response to exogenous application of 6-BA, implying that 6-BA is a drought-dependent suppressor of SL-signaling genes. However, our results also hint at the existence of an as-of-yet poorly-characterized system of complex phytohormonal responses coordinated from multiple signaling pathways in response to drought.

Synthetic strigolactone (rac-GR24) alleviates the adverse effects of heat stress on seed germination and photosystem II function in lupine seedlings

There is increasing experimental evidence that strigolactones, a class of carotenoid-derived sesquiterpenoid hormones, and their downstream signal components play a role in plant resilience to abiotic stress. Strigolactones positively influence plant coping mechanisms in response to abiotic stressors like drought and high salinity. In this study, we examined the effects of rac-GR24 (a synthetic strigolactone analog) and strigolactone inhibitors on the physiological and molecular responses associated with thermotolerance during seed germination and seedling development in Lupinus angustifolius under heat stress. Photosystem I & II functions were also evaluated via Chl a fluorescence transient analysis in heat stressed lupine seedlings. Our results suggest a putative role for GR24 in mediating tolerance to heat stress during seed germination and seedling development albeit these responses appeared independent of D14-mediated signalling. Seeds primed with GR24 had the highest of all germination indices, enhanced proline content and reduced peroxidation of lipids. GR24 also enhanced the activities of enzymes of the antioxidant and glyoxalase systems in lupine seedlings. The JIP-test indicated that GR24 conferred resistance to heat stress-induced damage to the oxygen evolution complex while also preventing the inactivation of PSII reaction centres thus ensuring PSII thermotolerance.

Karrikin Signaling Acts Parallel to and Additively with Strigolactone Signaling to Regulate Rice Mesocotyl Elongation in Darkness

Seedling emergence in monocots depends mainly on mesocotyl elongation, requiring coordination between developmental signals and environmental stimuli. Strigolactones (SLs) and karrikins are butenolide compounds that regulate various developmental processes; both are able to negatively regulate rice (Oryza sativa) mesocotyl elongation in the dark. Here, we report that a karrikin signaling complex, DWARF14-LIKE (D14L)-DWARF3 (D3)-O. sativa SUPPRESSOR OF MAX2 1 (OsSMAX1) mediates the regulation of rice mesocotyl elongation in the dark. We demonstrate that D14L recognizes the karrikin signal and recruits the SCFD3 ubiquitin ligase for the ubiquitination and degradation of OsSMAX1, mirroring the SL-induced and D14- and D3-dependent ubiquitination and degradation of D53. Overexpression of OsSMAX1 promoted mesocotyl elongation in the dark, whereas knockout of OsSMAX1 suppressed the elongated-mesocotyl phenotypes of d14l and d3 OsSMAX1 localizes to the nucleus and interacts with TOPLESS-RELATED PROTEINs, regulating downstream gene expression. Moreover, we showed that the GR24 enantiomers GR245DS and GR24 ent-5DS specifically inhibit mesocotyl elongation and regulate downstream gene expression in a D14- and D14L-dependent manner, respectively. Our work revealed that karrikin and SL signaling play parallel and additive roles in modulating downstream gene expression and negatively regulating mesocotyl elongation in the dark.

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.

Phenotyping in Arabidopsis and Crops-Are We Addressing the Same Traits? A Case Study in Tomato

The convenient model Arabidopsis thaliana has allowed tremendous advances in plant genetics and physiology, in spite of only being a weed. It has also unveiled the main molecular networks governing, among others, abiotic stress responses. Through the use of the latest genomic tools, Arabidopsis research is nowadays being translated to agronomically interesting crop models such as tomato, but at a lagging pace. Knowledge transfer has been hindered by invariable differences in plant architecture and behaviour, as well as the divergent direct objectives of research in Arabidopsis versus crops compromise transferability. In this sense, phenotype translation is still a very complex matter. Here, we point out the challenges of "translational phenotyping" in the case study of drought stress phenotyping in Arabidopsis and tomato. After briefly defining and describing drought stress and survival strategies, we compare drought stress protocols and phenotyping techniques most commonly used in the two species, and discuss their potential to gain insights, which are truly transferable between species. This review is intended to be a starting point for discussion about translational phenotyping approaches among plant scientists, and provides a useful compendium of methods and techniques used in modern phenotyping for this specific plant pair as a case study.

The R2R3-MYB transcription factor AtMYB49 modulates salt tolerance in Arabidopsis by modulating the cuticle formation and antioxidant defence

Salt stress activates defence responses in plants, including changes in leaf surface structure. Here, we showed that the transcriptional activation of cutin deposition and antioxidant defence by the R2R3-type MYB transcription factor AtMYB49 contributed to salt tolerance in Arabidopsis thaliana. Characterization of loss-of-function myb49 mutants, and chimeric AtMYB49-SRDX-overexpressing SRDX49 transcriptional repressor and AtMYB49-overexpressing (OX49) overexpressor plants demonstrated a positive role of AtMYB49 in salt tolerance. Transcriptome analysis revealed that many genes belonging to the category "cutin, suberin and wax biosyntheses" were markedly up-regulated and down-regulated in OX49 and SRDX49 plants, respectively, under normal and/or salt stress conditions. Some of these differentially expressed genes, including MYB41, ASFT, FACT and CYP86B1, were also shown to be the direct targets of AtMYB49 and activated by AtMYB49. Biochemical analysis indicated that AtMYB49 modulated cutin deposition in the leaves. Importantly, cuticular transpiration, chlorophyll leaching and toluidine blue-staining assays revealed a link between increased AtMYB49-mediated cutin deposition in leaves and enhanced salt tolerance. Additionally, increased AtMYB49 expression elevated Ca²⁺ level in leaves and improved antioxidant capacity by up-regulating genes encoding peroxidases and late embryogenesis abundant proteins. These results suggest that genetic manipulation of AtMYB49 may provide a novel way to improve salt tolerance in plants.

Structure-Function Analysis of SMAX1 Reveals Domains That Mediate Its Karrikin-Induced Proteolysis and Interaction with the Receptor KAI2

Karrikins (KARs) are butenolides found in smoke that can influence germination and seedling development of many plants. The KAR signaling mechanism is hypothesized to be very similar to that of the plant hormone strigolactone (SL). Both pathways require the F-box protein MORE AXILLARY GROWTH2 (MAX2), and other core signaling components have shared ancestry. Putatively, KAR activates the receptor KARRIKIN INSENSITIVE2 (KAI2), triggering its association with the E3 ubiquitin ligase complex SCFMAX2 and downstream targets SUPPRESSOR OF MAX2 1 (SMAX1) and SMAX1-LIKE2 (SMXL2). Polyubiquitination and proteolysis of SMAX1 and SMXL2 then enable growth responses to KAR. However, many of the assumptions of this model have not been demonstrated. Therefore, we investigated the posttranslational regulation of SMAX1 from the model plant Arabidopsis (Arabidopsis thaliana). We find evidence that SMAX1 is degraded by KAI2-SCFMAX2 but is also subject to MAX2-independent turnover. We identify SMAX1 domains that are responsible for its nuclear localization, KAR-induced degradation, association with KAI2, and ability to interact with other SMXL proteins. KAI2 undergoes MAX2-independent degradation after KAR treatment, which we propose results from its association with SMAX1 and SMXL2. Finally, we discover an SMXL domain that mediates receptor-target interaction preferences in KAR and SL signaling, laying the foundation for understanding how these highly similar pathways evolved to fulfill different roles.

The SUPPRESSOR of MAX2 1 (SMAX1)-Like SMXL6, SMXL7 and SMXL8 Act as Negative Regulators in Response to Drought Stress in Arabidopsis

Drought represents a major threat to crop growth and yields. Strigolactones (SLs) contribute to regulating shoot branching by targeting the SUPPRESSOR OF MORE AXILLARY GROWTH2 (MAX2)-LIKE6 (SMXL6), SMXL7 and SMXL8 for degradation in a MAX2-dependent manner in Arabidopsis. Although SLs are implicated in plant drought response, the functions of the SMXL6, 7 and 8 in the SL-regulated plant response to drought stress have remained unclear. Here, we performed transcriptomic, physiological and biochemical analyses of smxl6, 7, 8 and max2 plants to understand the basis for SMXL6/7/8-regulated drought response. We found that three D53 (DWARF53)-Like SMXL members, SMXL6, 7 and 8, are involved in drought response as the smxl6smxl7smxl8 triple mutants showed markedly enhanced drought tolerance compared to wild type (WT). The smxl6smxl7smxl8 plants exhibited decreased leaf stomatal index, cuticular permeability and water loss, and increased anthocyanin biosynthesis during dehydration. Moreover, smxl6smxl7smxl8 were hypersensitive to ABA-induced stomatal closure and ABA responsiveness during and after germination. In addition, RNA-sequencing analysis of the leaves of the D53-like smxl mutants, SL-response max2 mutant and WT plants under normal and dehydration conditions revealed an SMXL6/7/8-mediated network controlling plant adaptation to drought stress via many stress- and/or ABA-responsive and SL-related genes. These data further provide evidence for crosstalk between ABA- and SL-dependent signaling pathways in regulating plant responses to drought. Our results demonstrate that SMXL6, 7 and 8 are vital components of SL signaling and are negatively involved in drought responses, suggesting that genetic manipulation of SMXL6/7/8-dependent SL signaling may provide novel ways to improve drought resistance.

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

Functional analyses of various strigolactone-deficient mutants have demonstrated that strigolactones enhance drought resistance; however, the mechanistic involvement of the strigolactone receptor DWARF14 (D14) in this trait remains elusive. In this study, loss-of-function analysis of the D14 gene in Arabidopsis thaliana revealed that d14 mutant plants were more drought-susceptible than wild-type plants, which was associated with their larger stomatal aperture, slower abscisic acid (ABA)-mediated stomatal closure, lower anthocyanin content and delayed senescence under drought stress. Transcriptome analysis revealed a consistent alteration in the expression levels of many genes related to the observed physiological and biochemical changes in d14 plants when compared with the wild type under normal and dehydration conditions. A comparative drought resistance assay confirmed that D14 plays a less critical role in Arabidopsis drought resistance than its paralog karrikin receptor KARRIKIN INSENSITIVE 2 (KAI2). In-depth comparative analyses of the single mutants d14 and kai2 and the double mutant d14 kai2, in relation to various drought resistance-associated mechanisms, revealed that D14 and KAI2 exhibited a similar effect on stomatal closure. On the other hand, D14 had a lesser role in the maintenance of cell membrane integrity, leaf cuticle structure and ABA-induced leaf senescence, but a greater role in drought-induced anthocyanin biosynthesis, than KAI2. Interestingly, a possible additive relationship between D14 and KAI2 could be observed in regulating cell membrane integrity and leaf cuticle development. In addition, our findings also suggest the existence of a complex interaction between the D14 and ABA signaling pathways in the adaptation of Arabidopsis to drought.

A novel strigolactone-miR156 module controls stomatal behaviour during drought recovery

miR156 is a conserved microRNA whose role and induction mechanisms under stress are poorly known. Strigolactones are phytohormones needed in shoots for drought acclimation. They promote stomatal closure ABA-dependently and independently; however, downstream effectors for the former have not been identified. Linkage between miR156 and strigolactones under stress has not been reported. We compared ABA accumulation and sensitivity as well as performances of wt and miR156-overexpressing (miR156-oe) tomato plants during drought. We also quantified miR156 levels in wt, strigolactone-depleted and strigolactone-treated plants, exposed to drought stress. Under irrigated conditions, miR156 overexpression and strigolactone treatment led to lower stomatal conductance and higher ABA sensitivity. Exogenous strigolactones were sufficient for miR156 accumulation in leaves, while endogenous strigolactones were required for miR156 induction by drought. The "after-effect" of drought, by which stomata do not completely re-open after rewatering, was enhanced by both strigolactones and miR156. The transcript profiles of several miR156 targets were altered in strigolactone-depleted plants. Our results show that strigolactones act as a molecular link between drought and miR156 in tomato, and identify miR156 as a mediator of ABA-dependent effect of strigolactones on the after-effect of drought on stomata. Thus, we provide insights into both strigolactone and miR156 action on stomata.

Strigolactone and Karrikin Signaling Pathways Elicit Ubiquitination and Proteolysis of SMXL2 to Regulate Hypocotyl Elongation in Arabidopsis

Strigolactones (SLs) and karrikins (KARs) are related butenolide signaling molecules that control plant development. In Arabidopsis (Arabidopsis thaliana), they are recognized separately by two closely related receptors but use the same F-box protein MORE AXILLARY GROWTH2 (MAX2) for signal transduction, targeting different members of the SMAX1-LIKE (SMXL) family of transcriptional repressors for degradation. Both signals inhibit hypocotyl elongation in seedlings, raising the question of whether signaling is convergent or parallel. Here, we show that synthetic SL analog GR24^4DO enhanced the interaction between the SL receptor DWARF14 (D14) and SMXL2, while the KAR surrogate GR24 ^ent-5DS induced association of the KAR receptor KARRIKIN INSENSITIVE2 (KAI2) with SMAX1 and SMXL2. Both signals trigger polyubiquitination and degradation of SMXL2, with GR24^4DO dependent on D14 and GR24 ^ent-5DS dependent mainly on KAI2. SMXL2 is critical for hypocotyl responses to GR24^4DO and functions redundantly with SMAX1 in hypocotyl response to GR24 ^ent-5DS Furthermore, GR24^4DO induced response of D14-LIKE2 and KAR-UP F-BOX1 through SMXL2, whereas GR24 ^ent-5DS induced expression of these genes via both SMAX1 and SMXL2. These findings demonstrate that both SLs and KARs could trigger polyubiquitination and degradation of SMXL2, thus uncovering an unexpected but important convergent pathway in SL- and KAR-regulated gene expression and hypocotyl elongation.

A series of dual-reporter vectors for ratiometric analysis of protein abundance in plants

Ratiometric reporter systems enable comparisons of the abundance of a protein of interest, or "target," relative to a reference protein. Both proteins are encoded on a single transcript but are separated during translation. This arrangement bypasses the potential for discordant expression that can arise when the target and reference proteins are encoded by separate genes. We generated a set of 18 Gateway-compatible vectors termed pRATIO that combine a variety of promoters, fluorescent, and bioluminescent reporters, and 2A "self-cleaving" peptides. These constructs are easily modified to produce additional combinations or introduce new reporter proteins. We found that mScarlet-I provides the best signal-to-noise ratio among several fluorescent reporter proteins during transient expression experiments in Nicotiana benthamiana. Firefly and Gaussia luciferase also produce high signal-to-noise in N. benthamiana. As proof of concept, we used this system to investigate whether degradation of the receptor KAI2 after karrikin treatment is influenced by its subcellular localization. KAI2 is normally found in the cytoplasm and the nucleus of plant cells. In N. benthamiana, karrikin-induced degradation of KAI2 was only observed when it was retained in the nucleus. These vectors are tools to easily monitor in vivo the abundance of a protein that is transiently expressed in plants, and will be particularly useful for investigating protein turnover in response to different stimuli.

Recent progress in the chemistry and biochemistry of strigolactones

Strigolactones (SLs) are plant secondary metabolites derived from carotenoids. SLs play important roles in the regulation of plant growth and development in planta and coordinate interactions between plants and other organisms including root parasitic plants, and symbiotic and pathogenic microbes in the rhizosphere. In the 50 years since the discovery of the first SL, strigol, our knowledge about the chemistry and biochemistry of SLs has advanced explosively, especially over the last two decades. In this review, recent advances in the chemistry and biology of SLs are summarized and possible future outcomes are discussed.

The negative regulator SMAX1 controls mycorrhizal symbiosis and strigolactone biosynthesis in rice

Most plants associate with beneficial arbuscular mycorrhizal (AM) fungi that facilitate soil nutrient acquisition. Prior to contact, partner recognition triggers reciprocal genetic remodelling to enable colonisation. The plant Dwarf14-Like (D14L) receptor conditions pre-symbiotic perception of AM fungi, and also detects the smoke constituent karrikin. D14L-dependent signalling mechanisms, underpinning AM symbiosis are unknown. Here, we present the identification of a negative regulator from rice, which operates downstream of the D14L receptor, corresponding to the homologue of the Arabidopsis thaliana Suppressor of MAX2-1 (AtSMAX1) that functions in karrikin signalling. We demonstrate that rice SMAX1 is a suppressor of AM symbiosis, negatively regulating fungal colonisation and transcription of crucial signalling components and conserved symbiosis genes. Similarly, rice SMAX1 negatively controls strigolactone biosynthesis, demonstrating an unexpected crosstalk between the strigolactone and karrikin signalling pathways. We conclude that removal of SMAX1, resulting from D14L signalling activation, de-represses essential symbiotic programmes and increases strigolactone hormone production.

Signaling via the D14L karrikin receptor conditions rice roots for association with arbuscular mycorrhizal fungi. Here, Choi et al. show that SMAX1, a rice homolog of an Arabidopsis repressor of karrikin signaling, acts downstream of D14L to suppress mycorrhizal symbiosis and strigolactone biosynthesis.

Evolution of Plant Hormone Response Pathways

This review focuses on the evolution of plant hormone signaling pathways. Like the chemical nature of the hormones themselves, the signaling pathways are diverse. Therefore, we focus on a group of hormones whose primary perception mechanism involves an Skp1/Cullin/F-box-type ubiquitin ligase: auxin, jasmonic acid, gibberellic acid, and strigolactone. We begin with a comparison of the core signaling pathways of these four hormones, which have been established through studies conducted in model organisms in the Angiosperms. With the advent of next-generation sequencing and advanced tools for genetic manipulation, the door to understanding the origins of hormone signaling mechanisms in plants beyond these few model systems has opened. For example, in-depth phylogenetic analyses of hormone signaling components are now being complemented by genetic studies in early diverging land plants. Here we discuss recent investigations of how basal land plants make and sense hormones. Finally, we propose connections between the emergence of hormone signaling complexity and major developmental transitions in plant evolution.

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.

Perception of karrikins by plants: a continuing enigma

Karrikins are small butenolide molecules with the capacity to promote germination and enhance seedling establishment. Generated abiotically from partial combustion of vegetation, karrikins are comparatively rare in the environment, but studying their mode of action has been most informative in revealing a new regulatory pathway for plant development that uses the karrikin perception machinery. Recent studies suggest that the karrikin receptor protein KAI2 and downstream transcriptional co-repressors in the SMXL family influence seed germination, seedling photomorphogenesis, root morphology, and responses to abiotic stress such as drought. Based on taxonomic distribution, this pathway is ubiquitous and likely to be evolutionarily ancient, originating prior to land plants. However, we still do not have a good grasp on how karrikins actually activate the receptor protein, and we have yet to discover the assumed endogenous ligand for KAI2 that karrikins are thought to mimic. This review covers recent progress in this field, as well as current gaps in our knowledge.

Karrikin Improves Osmotic and Salt Stress Tolerance via the Regulation of the Redox Homeostasis in the Oil Plant Sapium sebiferum

Karrikins are reported to stimulate seed germination, regulate seedling growth, and increase the seedling vigor in abiotic stress conditions in plants. Nevertheless, how karrikins alleviate abiotic stress remains largely elusive. In this study, we found that karrikin (KAR1) could significantly alleviate both drought and salt stress in the important oil plant Sapium sebiferum. KAR1 supplementation in growth medium at a nanomolar (nM) concentration was enough to recover seed germination under salt and osmotic stress conditions. One nanomolar of KAR1 improved seedling biomass, increased the taproot length, and increased the number of lateral roots under abiotic stresses, suggesting that KAR1 is a potent alleviator of abiotic stresses in plants. Under abiotic stresses, KAR1-treated seedlings had a higher activity of the key antioxidative enzymes, such as superoxide dismutase, peroxidase, catalase, and ascorbate peroxidase, in comparison with the control, which leads to a lower level of hydrogen peroxide, malondialdehyde, and electrolyte leakage. Moreover, the metabolome analysis showed that KAR1 treatment significantly increased the level of organic acids and amino acids, which played important roles in redox homeostasis under stresses, suggesting that karrikins might alleviate abiotic stresses via the regulation of redox homeostasis. Under abiotic stresses, applications of karrikins did not increase the endogenous abscisic acid level but altered the expression of several ABA signaling genes, such as SNF1-RELATED PROTEIN KINASE2.3, SNF1-RELATED PROTEIN KINASE2.6, ABI3, and ABI5, suggesting potential interactions between karrikins and ABA signaling in the stress responses. Conclusively, we not only provided the physiological and molecular evidence to clarify the mechanism of karrikins in the regulation of stress adaptation in S. sebiferum but also showed the potential value of karrikins in agricultural practices, which will lay a foundation for further studies about the role of karrikins in abiotic stress alleviation in plants.

Divergent receptor proteins confer responses to different karrikins in two ephemeral weeds

Wildfires can encourage the establishment of invasive plants by releasing potent germination stimulants, such as karrikins. Seed germination of Brassica tournefortii, a noxious weed of Mediterranean climates, is strongly stimulated by KAR₁, the archetypal karrikin produced from burning vegetation. In contrast, the closely-related yet non-fire-associated ephemeral Arabidopsisthaliana is unusual because it responds preferentially to KAR₂. The α/β-hydrolase KARRIKIN INSENSITIVE 2 (KAI2) is the putative karrikin receptor identified in Arabidopsis. Here we show that B. tournefortii expresses three KAI2 homologues, and the most highly-expressed homologue is sufficient to confer enhanced responses to KAR₁ relative to KAR₂ when expressed in Arabidopsis. We identify two amino acid residues near the KAI2 active site that explain the ligand selectivity, and show that this combination has arisen independently multiple times within dicots. Our results suggest that duplication and diversification of KAI2 proteins could confer differential responses to chemical cues produced by environmental disturbance, including fire.

Karrikins are germination stimulants perceived by KAI2 in Arabidopsis. Here the authors show that Brassica tournefortii, a close relative to Arabidopsis, has multiple copies of KAI2 with amino acid substitutions that confer responsiveness to the specific karrikin compounds found in wildfire smoke.

Differential role of MAX2 and strigolactones in pathogen, ozone, and stomatal responses

Strigolactones are a group of phytohormones that control developmental processes including shoot branching and various plant-environment interactions in plants. We previously showed that the strigolactone perception mutant more axillary branches 2 (max2) has increased susceptibility to plant pathogenic bacteria. Here we show that both strigolactone biosynthesis (max3 and max4) and perception mutants (max2 and dwarf14) are significantly more sensitive to Pseudomonas syringae DC3000. Moreover, in response to P. syringae infection, high levels of SA accumulated in max2 and this mutant was ozone sensitive. Further analysis of gene expression revealed no major role for strigolactone in regulation of defense gene expression. In contrast, guard cell function was clearly impaired in max2 and depending on the assay used, also in max3, max4, and d14 mutants. We analyzed stomatal responses to stimuli that cause stomatal closure. While the response to abscisic acid (ABA) was not impaired in any of the mutants, the response to darkness and high CO2 was impaired in max2 and d14-1 mutants, and to CO2 also in strigolactone synthesis (max3, max4) mutants. To position the role of MAX2 in the guard cell signaling network, max2 was crossed with mutants defective in ABA biosynthesis or signaling. This revealed that MAX2 acts in a signaling pathway that functions in parallel to the guard cell ABA signaling pathway. We propose that the impaired defense responses of max2 are related to higher stomatal conductance that allows increased entry of bacteria or air pollutants like ozone. Furthermore, as MAX2 appears to act in a specific branch of guard cell signaling (related to CO2 signaling), this protein could be one of the components that allow guard cells to distinguish between different environmental conditions.

Fellowship of the rings: a saga of strigolactones and other small signals

Strigolactones are an important class of plant signalling molecule with both external rhizospheric and internal hormonal functions in flowering plants. The past decade has seen staggering progress in strigolactone biology, permitting highly detailed understanding of their signalling, synthesis and biological roles - or so it seems. However, phylogenetic analyses show that strigolactone signalling mediated by the D14-SCF^MAX2 -SMXL7 complex is only one of a number of closely related signalling pathways, and is much less ubiquitous in land plants than might be expected. The existence of closely related pathways, such as the KAI2-SMAX1 module, challenges many of our assumptions about strigolactones, and in particular emphasises how little we understand about the specificity of strigolactone signalling with respect to related signalling pathways. In this review, we examine recent advances in strigolactone signalling, taking a holistic evolutionary view to identify the ambiguities and uncertainties in our understanding. We highlight that while we now have highly detailed molecular models for the core mechanism of D14-SMXL7 signalling, we still do not understand the ligand specificity of D14, the specificity of its interaction with SMXL7, nor the specificity of SMXL7 function. Our analysis therefore identifies key areas requiring further study.

GmMAX2-D14 and -KAI interaction-mediated SL and KAR signaling play essential roles in soybean root nodulation

Despite of important functions of strigolactones (SLs) and karrikins (KARs) in plant development, plant-parasite and plant-fungi interactions, their roles in soybean-rhizobia interaction remain elusive. SL/KAR signaling genes GmMAX2a, GmD14s, and GmKAIs are activated by rhizobia infection. GmMAX2a restored atmax2 root hair defects and soybean root hairs were changed in GmMAX2a overexpression (GmMAX2a-OE) or knockdown (GmMAX2a-KD) mutants. GmMAX2a-KD gave fewer, whereas GmMAX2a-OE produced more nodules than GUS hairy roots. Mutation of GmMAX2a in its KD or OE transgenic hairy roots affected the rhizobia infection-induced increases in early nodulation gene expression. Both mutant hairy roots also displayed the altered auxin, jasmonate and abscisic acid levels, as further verified by transcriptomic analyses of their synthetic genes. Overexpression of an auxin synthetic gene GmYUC2a also affected SL and KAR signaling genes. GmMAX2a physically interacted with SL/KAR receptors GmD14s, GmKAIs, and GmD14Ls with different binding affinities, depending on variations in the critical amino acids, forming active D14/KAI-SCFMAX2 complexes. The knockdown mutant roots of the nodule-specifically expressing GmKAIs and GmD14Ls gave fewer nodules, with altered expression of several early nodulation genes. The expression levels of GmKAIs, and GmD14Ls were markedly changed in GmMAX2a mutant roots, so did their target repressor genes GmD53s and GmSMAX1s. Thus, SL and KAR signaling were involved in soybean-rhizobia interaction and nodulation partly through interactions with hormones, and this may explain the different effects of MXA2 orthologs on legume determinate and indeterminate nodulation. The study provides fresh insights into the roles of GmMAX2-mediated SL/KAR signaling in soybean root hair and nodule formation.

Potential of Karrikins as Novel Plant Growth Regulators in Agriculture

Karrikins (KARs) have been identified as molecules derived from plant material smoke, which have the capacity to enhance seed germination for a wide range of plant species. However, KARs were observed to not only impact seed germination but also observed to influence several biological processes. The plants defected in the KARs signaling pathway were observed to grow differently with several morphological changes. The observation of KARs as a growth regulator in plants leads to the search for an endogenous KAR-like molecule. Due to its simple genomic structure, Arabidopsis (Arabidopsis thaliana L.) helps to understand the signaling mechanism of KARs and phenotypic responses caused by them. However, different species have a different phenotypic response to KARs treatment. Therefore, in the current work, updated information about the KARs effect is presented. Results of research on agricultural and horticultural crops are summarized and compared with the findings of Arabidopsis studies. In this article, we suggested that KARs may be more important in coping with modern problems than one could imagine.

Biofilms Positively Contribute to Bacillus amyloliquefaciens 54-induced Drought Tolerance in Tomato Plants

Drought stress is a major obstacle to agriculture. Although many studies have reported on plant drought tolerance achieved via genetic modification, application of plant growth-promoting rhizobacteria (PGPR) to achieve tolerance has rarely been studied. In this study, the ability of three isolates, including Bacillus amyloliquefaciens 54, from 30 potential PGPR to induce drought tolerance in tomato plants was examined via greenhouse screening. The results indicated that B. amyloliquefaciens 54 significantly enhanced drought tolerance by increasing survival rate, relative water content and root vigor. Coordinated changes were also observed in cellular defense responses, including decreased concentration of malondialdehyde and elevated concentration of antioxidant enzyme activities. Moreover, expression levels of stress-responsive genes, such as lea, tdi65, and ltpg2, increased in B. amyloliquefaciens 54-treated plants. In addition, B. amyloliquefaciens 54 induced stomatal closure through an abscisic acid-regulated pathway. Furthermore, we constructed biofilm formation mutants and determined the role of biofilm formation in B. amyloliquefaciens 54-induced drought tolerance. The results showed that biofilm-forming ability was positively correlated with plant root colonization. Moreover, plants inoculated with hyper-robust biofilm (ΔabrB and ΔywcC) mutants were better able to resist drought stress, while defective biofilm (ΔepsA-O and ΔtasA) mutants were more vulnerable to drought stress. Taken altogether, these results suggest that biofilm formation is crucial to B. amyloliquefaciens 54 root colonization and drought tolerance in tomato plants.

Comparing and Contrasting the Multiple Roles of Butenolide Plant Growth Regulators: Strigolactones and Karrikins in Plant Development and Adaptation to Abiotic Stresses

Strigolactones (SLs) and karrikins (KARs) are both butenolide molecules that play essential roles in plant growth and development. SLs are phytohormones, with SLs having known functions within the plant they are produced in, while KARs are found in smoke emitted from burning plant matter and affect seeds and seedlings in areas of wildfire. It has been suggested that SL and KAR signaling may share similar mechanisms. The α/β hydrolases DWARF14 (D14) and KARRIKIN INSENSITIVE 2 (KAI2), which act as receptors of SL and KAR, respectively, both interact with the F-box protein MORE AXILLARY GROWTH 2 (MAX2) in order to target SUPPRESSOR OF MAX2 1 (SMAX1)-LIKE/D53 family members for degradation via the 26S proteasome. Recent reports suggest that SLs and/or KARs are also involved in regulating plant responses and adaptation to various abiotic stresses, particularly nutrient deficiency, drought, salinity, and chilling. There is also crosstalk with other hormone signaling pathways, including auxin, gibberellic acid (GA), abscisic acid (ABA), cytokinin (CK), and ethylene (ET), under normal and abiotic stress conditions. This review briefly covers the biosynthetic and signaling pathways of SLs and KARs, compares their functions in plant growth and development, and reviews the effects of any crosstalk between SLs or KARs and other plant hormones at various stages of plant development. We also focus on the distinct responses, adaptations, and regulatory mechanisms related to SLs and/or KARs in response to various abiotic stresses. The review closes with discussion on ways to gain additional insights into the SL and KAR pathways and the crosstalk between these related phytohormones.

Strigolactones and their crosstalk with other phytohormones

BACKGROUND

Strigolactones (SLs) are a diverse class of butenolide-bearing phytohormones derived from the catabolism of carotenoids. They are associated with an increasing number of emerging regulatory roles in plant growth and development, including seed germination, root and shoot architecture patterning, nutrient acquisition, symbiotic and parasitic interactions, as well as mediation of plant responses to abiotic and biotic cues.

SCOPE

Here, we provide a concise overview of SL biosynthesis, signal transduction pathways and SL-mediated plant responses with a detailed discourse on the crosstalk(s) that exist between SLs/components of SL signalling and other phytohormones such as auxins, cytokinins, gibberellins, abscisic acid, ethylene, jasmonates and salicylic acid.

CONCLUSION

SLs elicit their control on physiological and morphological processes via a direct or indirect influence on the activities of other hormones and/or integrants of signalling cascades of other growth regulators. These, among many others, include modulation of hormone content, transport and distribution within plant tissues, interference with or complete dependence on downstream signal components of other phytohormones, as well as acting synergistically or antagonistically with other hormones to elicit plant responses. Although much has been done to evince the effects of SL interactions with other hormones at the cell and whole plant levels, research attention must be channelled towards elucidating the precise molecular events that underlie these processes. More especially in the case of abscisic acid, cytokinins, gibberellin, jasmonates and salicylic acid for which very little has been reported about their hormonal crosstalk with SLs.

CYP707As are effectors of karrikin and strigolactone signalling pathways in Arabidopsis thaliana and parasitic plants

Karrikins stimulate Arabidopsis thaliana germination, whereas parasitic weeds of the Orobanchaceae family have evolved to respond to host-exuded compounds such as strigolactones, dehydrocostus lactone, and 2-phenylethyl isothiocyanate. In Phelipanche ramosa, strigolactone-induced germination was shown to require one of the CYP707A proteins involved in abscisic acid catabolism. Here, germination and gene expression were analysed to investigate the role of CYP707As in germination of both parasitic plants and Arabidopsis upon perception of germination stimulants, after using pharmacological inhibitors and Arabidopsis mutants disrupting germination signals. CYP707A genes were up-regulated upon treatment with effective germination stimulants in both parasitic plants and Arabidopsis. Obligate parasitic plants exhibited both intensified up-regulation of CYP707A genes and increased sensitivity to the CYP707A inhibitor abscinazole-E2B, whereas Arabidopsis cyp707a mutants still positively responded to germination stimulation. In Arabidopsis, CYP707A regulation required the canonical karrikin signalling pathway KAI2/MAX2/SMAX1 and the transcription factor WRKY33. Finally, CYP707As and WRKY33 also modulated Arabidopsis root architecture in response to the synthetic strigolactone rac-GR24, and wrky33-1 exhibited a shoot hyperbranched phenotype. This study suggests that the lack of host-independent germination in obligate parasites is associated with an exacerbated CYP707A induction and that CYP707As and WRKY33 are new players involved in a variety of strigolactone/karrikin responses.

Impairment in karrikin but not strigolactone sensing enhances root skewing in Arabidopsis thaliana

Roots form highly complex systems varying in growth direction and branching pattern to forage for nutrients efficiently. Here mutations in the KAI2 (KARRIKIN INSENSITIVE) α/β-fold hydrolase and the MAX2 (MORE AXILLARY GROWTH 2) F-box leucine-rich protein, which together perceive karrikins (smoke-derived butenolides), caused alteration in root skewing in Arabidopsis thaliana. This phenotype was independent of endogenous strigolactones perception by the D14 α/β-fold hydrolase and MAX2. Thus, KAI2/MAX2 effect on root growth may be through the perception of endogenous KAI2-ligands (KLs), which have yet to be identified. Upon perception of a ligand, a KAI2/MAX2 complex is formed together with additional target proteins before ubiquitination and degradation through the 26S proteasome. Using a genetic approach, we show that SMAX1 (SUPPRESSOR OF MAX2-1)/SMXL2 and SMXL6,7,8 (SUPPRESSOR OF MAX2-1-LIKE) are also likely degradation targets for the KAI2/MAX2 complex in the context of root skewing. In A. thaliana therefore, KAI2 and MAX2 act to limit root skewing, while kai2's gravitropic and mechano-sensing responses remained largely unaffected. Many proteins are involved in root skewing, and we investigated the link between MAX2 and two members of the SKS/SKU family. Though KLs are yet to be identified in plants, our data support the hypothesis that they are present and can affect root skewing.

The SNAC-A Transcription Factor ANAC032 Reprograms Metabolism in Arabidopsis

Studies have indicated that the carbon starvation response leads to the reprogramming of the transcriptome and metabolome, and many genes, including several important regulators, such as the group S1 basic leucine zipper transcription factors (TFs) bZIP1, bZIP11 and bZIP53, the SNAC-A TF ATAF1, etc., are involved in these physiological processes. Here, we show that the SNAC-A TF ANAC032 also plays important roles in this process. The overexpression of ANAC032 inhibits photosynthesis and induces reactive oxygen species accumulation in chloroplasts, thereby reducing sugar accumulation and resulting in carbon starvation. ANAC032 reprograms carbon and nitrogen metabolism by increasing sugar and amino acid catabolism in plants. The ChIP-qPCR and transient dual-luciferase reporter assays indicated that ANAC032 regulates trehalose metabolism via the direct regulation of TRE1 expression. Taken together, these results show that ANAC032 is an important regulator of the carbon/energy status that represses photosynthesis to induce carbon starvation.

Strigolactones positively regulate defense against root-knot nematodes in tomato

Strigolactones (SLs) are carotenoid-derived phytohormones that are known to influence various aspects of plant growth and development. As root-derived signals, SLs can enhance symbiosis between plants and arbuscular mycorrhizal fungi (AMF). However, little is known about the roles of SLs in plant defense against soil-borne pathogens. Here, we determined that infection with root-knot nematodes (RKNs; Meloidogyne incognita) induced SL biosynthesis in roots of tomato (Solanum lycopersicum). Silencing of SL biosynthesis genes compromised plant defense against RKNs, whilst application of the SL analog racGR24 enhanced it. Accumulation of endogenous jasmonic acid (JA) and abscisic acid (ABA) in the roots in response to RKN infection was enhanced by silencing of SL biosynthetic genes and was suppressed by application of racGR24. Genetic evidence showed that JA was a positive regulator of defense against RKNs while ABA was a negative regulator. In addition, racGR24 enhanced the defense against nematode in a JA-deficient mutant but not in an ABA-deficient mutant. Silencing of SL biosynthetic genes resulted in up-regulation of MYC2, which negatively regulated defense against RKNs. Our results demonstrate that SLs play a positive role in nematode defense in tomato and that MYC2 negatively regulates this defense, potentially by mediating hormone crosstalk among SLs, ABA and JA.

The karrikin 'calisthenics': Can compounds derived from smoke help in stress tolerance?

Karrikins (KARs) are unique butenolides derived as a by-product of incomplete combustion during wildfire. Some receptive plant species respond to KARs in the form of accelerated germination. These molecules originate from stress to mediate tolerance against different sub-optimal conditions like oxidative stress, drought and low-light intensity (shade stress). KARs promote seed germination, seedling establishment and ecological diversity by accelerating the abundance of selective communities of plants. The signaling pathway is closely related, yet unique from strigolactones (SLs). Due to the structural relatedness with SLs, KARs have potential roles in mediating abiotic stress tolerance in plants. In addition, the KAR directly/indirectly interact with crucial phytohormones like abscisic acid, gibberellic acid, auxins and ethylene. This article is a summarized update on KAR research in recent times. The exhaustive discussions would be beneficial for understanding the extraordinary strategy devised by nature to protect plants from stress using a molecule which is itself generated out of stress.

Identification and functional prediction of circRNAs in Populus Euphratica Oliv. heteromorphic leaves

Populus euphratica Oliv. has typical heterophylly. Linear, lanceolate, ovate and broad-ovate leaves appeared in turn from sprouting to development, to maturity. The environmental adaptabilities of P. euphraticas with different leaves were also different. To explore the role of circRNAs on the morphogenesis of P. euphratica heteromorphic leaves (P.hl) and their stress response, the expression profile of circRNAs was analyzed by strand-specific RNA sequencing for the above four kinds of heteromorphic leaves. According to ceRNA hypothesis, 18 differentially expressed cirRNAs (DECs) could influence the expression of 84 mRNAs by antagonizing 23 miRNAs in five sample-pairs. Based on the function of 84 mRNAs, these DECs participate in development process, response to stimulus, response to hormonal et al. Therefore, these circRNAs were involved in the P.hl morphogenesis and stress response by interacting with miRNAs and mRNAs. Our study complemented the genebank of P. euphratica and provided a new strategy for studying leaf development.

The Calcium-Dependent Protein Kinase CPK33 Mediates Strigolactone-Induced Stomatal Closure in Arabidopsis thaliana

Strigolactones (SLs) are known to mediate plant acclimation to environmental stress. We recently reported that SLs acted as prominent regulators in promotion of stomatal closure. However, the detailed mechanism by which SLs induce stomatal closure requires further investigation. Here we studied the essential role of the calcium (Ca²⁺) signal mediating by the calcium-dependent protein kinase (CPK) in SL-induced stomatal closure. SL-induced stomatal closure was strongly inhibited by a Ca²⁺ chelator and Ca²⁺ channel blockers, indicating that Ca²⁺ functions in SL promotion of stomatal closure. Through examining a collection of cpk mutants, we identified CPK33, potentially acting as a Ca²⁺ transducer, which is implicated in guard cell SL signaling. SL- and Ca²⁺-induced stomatal closure were impaired in cpk33 mutants. CPK33 kinase activity is essential for SL induction of stomatal closure as SL-induced stomatal closure is blocked in the dead kinase mutant of CPK33. The cpk33 mutant is impaired in H₂O₂-induced stomatal closure, but not in SL-mediated H₂O₂ production. Our study thus uncovers an important player CPK33 which functions as an essential Ca²⁺ signals mediator in Arabidopsis guard cell SL signaling.

Methylation at the C-3' in D-Ring of Strigolactone Analogs Reduces Biological Activity in Root Parasitic Plants and Rice

Strigolactones (SLs) regulate plant development and induce seed germination in obligate root parasitic weeds, e.g. Striga spp. Because organic synthesis of natural SLs is laborious, there is a large need for easy-to-synthesize and efficient analogs. Here, we investigated the effect of a structural modification of the D-ring, a conserved structural element in SLs. We synthesized and investigated the activity of two analogs, MP13 and MP26, which differ from previously published AR8 and AR36 only in the absence of methylation at C-3'. The de-methylated MP13 and MP26 were much more efficient in regulating plant development and inducing Striga seed germination, compared with AR8. Hydrolysis assays performed with purified Striga SL receptor and docking of AR8 and MP13 to the corresponding active site confirmed and explained the higher activity. Field trials performed in a naturally Striga-infested African farmer's field unraveled MP13 as a promising candidate for combating Striga by inducing germination in host's absence. Our findings demonstrate that methylation of the C-3' in D-ring in SL analogs has a negative impact on their activity and identify MP13 and, particularly, MP26 as potent SL analogs with simple structures, which can be employed to control Striga, a major threat to global food security.

Epsc Involved in the Encoding of Exopolysaccharides Produced by Bacillus amyloliquefaciens FZB42 Act to Boost the Drought Tolerance of Arabidopsis thaliana

Bacillus amyloliquefaciens FZB42 is a plant growth-promoting rhizobacteria that stimulates plant growth, and enhances resistance to pathogens and tolerance of salt stress. Instead, the mechanistic basis of drought tolerance in Arabidopsis thaliana induced by FZB42 remains unexplored. Here, we constructed an exopolysaccharide-deficient mutant epsC and determined the role of epsC in FZB42-induced drought tolerance in A. thaliana. Results showed that FZB42 significantly enhanced growth and drought tolerance of Arabidopsis by increasing the survival rate, fresh and dry shoot weights, primary root length, root dry weight, lateral root number, and total lateral root length. Coordinated changes were also observed in cellular defense responses, including elevated concentrations of proline and activities of superoxide dismutase and peroxidase, decreased concentrations of malondialdehyde, and accumulation of hydrogen peroxide in plants treated with FZB42. The relative expression levels of drought defense-related marker genes, such as RD29A, RD17, ERD1, and LEA14, were also increased in the leaves of FZB42-treated plants. In addition, FZB42 induced the drought tolerance in Arabidopsis by the action of both ethylene and jasmonate, but not abscisic acid. However, plants inoculated with mutant strain epsC were less able to resist drought stress with respect to each of these parameters, indicating that epsC are required for the full benefit of FZB42 inoculation to be gained. Moreover, the mutant strain was less capable of supporting the formation of a biofilm and of colonizing the A. thaliana root. Therefore, epsC is an important factor that allows FZB42 to colonize the roots and induce systemic drought tolerance in Arabidopsis.

Strigolactones in plant adaptation to abiotic stresses: An emerging avenue of plant research

Phytohormones play central roles in boosting plant tolerance to environmental stresses, which negatively affect plant productivity and threaten future food security. Strigolactones (SLs), a class of carotenoid-derived phytohormones, were initially discovered as an "ecological signal" for parasitic seed germination and establishment of symbiotic relationship between plants and beneficial microbes. Subsequent characterizations have described their functional roles in various developmental processes, including root development, shoot branching, reproductive development, and leaf senescence. SLs have recently drawn much attention due to their essential roles in the regulation of various physiological and molecular processes during the adaptation of plants to abiotic stresses. Reports suggest that the production of SLs in plants is strictly regulated and dependent on the type of stresses that plants confront at various stages of development. Recently, evidence for crosstalk between SLs and other phytohormones, such as abscisic acid, in responses to abiotic stresses suggests that SLs actively participate within regulatory networks of plant stress adaptation that are governed by phytohormones. Moreover, the prospective roles of SLs in the management of plant growth and development under adverse environmental conditions have been suggested. In this review, we provide a comprehensive discussion pertaining to SL-mediated plant responses and adaptation to abiotic stresses.

Mechanisms Underlying Establishment of Arbuscular Mycorrhizal Symbioses

Most land plants engage in mutually beneficial interactions with arbuscular mycorrhizal (AM) fungi, the fungus providing phosphate and nitrogen in exchange for fixed carbon. During presymbiosis, both organisms communicate via oligosaccharides and butenolides. The requirement for a rice chitin receptor in symbiosis-induced lateral root development suggests that cell division programs operate in inner root tissues during both AM and nodule symbioses. Furthermore, the identification of transcription factors underpinning arbuscule development and degeneration reemphasized the plant's regulatory dominance in AM symbiosis. Finally, the finding that AM fungi, as lipid auxotrophs, depend on plant fatty acids (FAs) to complete their asexual life cycle revealed the basis for fungal biotrophy. Intriguingly, lipid metabolism is also central for asexual reproduction and interaction of the fungal sister clade, the Mucoromycotina, with endobacteria, indicative of an evolutionarily ancient role for lipids in fungal mutualism.

Effects of overproduced ethylene on the contents of other phytohormones and expression of their key biosynthetic genes

Ethylene is involved in regulation of various aspects of plant growth and development. Physiological and genetic analyses have indicated the existence of crosstalk between ethylene and other phytohormones, including auxin, cytokinin (CK), abscisic acid (ABA), gibberellin (GA), salicylic acid (SA), jasmonic acid (JA), brassinosteroid (BR) and strigolactone (SL) in regulation of different developmental processes. However, the effects of ethylene on the biosynthesis and contents of these hormones are not fully understood. Here, we investigated how overproduction of ethylene may affect the contents of other plant hormones using the ethylene-overproducing mutant ethylene-overproducer 1 (eto1-1). The contents of various hormones and transcript levels of the associated biosynthetic genes in the 10-day-old Arabidopsis eto1-1 mutant and wild-type (WT) plants were determined and compared. Higher levels of CK and ABA, while lower levels of auxin, SA and GA were observed in eto1-1 plants in comparison with WT, which was supported by the up- or down-regulation of their biosynthetic genes. Although we could not quantify the BR and SL contents in Arabidopsis, we observed that the transcript levels of the potential rate-limiting BR and SL biosynthetic genes were increased in the eto1-1 versus WT plants, suggesting that BR and SL levels might be enhanced by ethylene overproduction. JA level was not affected by overproduction of ethylene, which might be explained by unaltered expression level of the proposed rate-limiting JA biosynthetic gene allene oxide synthase. Taken together, our results suggest that ET affects the levels of auxin, CK, ABA, SA and GA, and potentially BR and SL, by influencing the expression of genes involved in the rate-limiting steps of their biosynthesis.

Karrikin-KAI2 signalling provides Arabidopsis seeds with tolerance to abiotic stress and inhibits germination under conditions unfavourable to seedling establishment

The control of seed germination in response to environmental conditions is important for plant success. We investigated the role of the karrikin receptor KARRIKIN INSENSITIVE2 (KAI2) in the response of Arabidopsis seeds to osmotic stress, salinity and high temperature. Germination of the kai2 mutant was examined in response to NaCl, mannitol and elevated temperature. The effect of karrikin on germination of wild-type seeds, hypocotyl elongation and the expression of karrikin-responsive genes was also examined in response to such stresses. The kai2 seeds germinated less readily than wild-type seeds and germination was more sensitive to inhibition by abiotic stress. Karrikin-induced KAI2 signalling stimulated germination of wild-type seeds under favourable conditions, but, surprisingly, inhibited germination in the presence of osmolytes or at elevated temperature. By contrast, GA stimulated germination of wild-type seeds and mutants under all conditions. Karrikin induced expression of DLK2 and KUF1 genes and inhibited hypocotyl elongation independently of osmotic stress. Under mild osmotic stress, karrikin enhanced expression of DREB2A, WRKY33 and ERF5 genes, but not ABA signalling genes. Thus, the karrikin-KAI2 signalling system can protect against abiotic stress, first by providing stress tolerance, and second by inhibiting germination under conditions unfavourable to seedling establishment.

Regulation of Root Development and Architecture by Strigolactones under Optimal and Nutrient Deficiency Conditions

Strigolactones (SLs) constitute a group of plant hormones which are involved in multiple aspects of plant growth and development. Beside their role in shoot and root development and plant architecture in general, SLs are also involved in plant responses to nutrient deficiency by promoting interactions with symbiotic organisms and via promotion of root elongation. Recent observations on the cross talk between SLs and other hormones demonstrate that the inhibition of adventitious root formation by ethylene is independent of SLs. Additionally, it was shown that root exposure to SLs leads to the accumulation of secondary metabolites, such as flavonols or antioxidants. These data suggest pleiotropic effects of SLs, that influence root development. The discovery that the commonly used synthetic SL analogue racGR24 might also mimic the function of other plant growth regulators, such as karrikins, has led us to consider the previously published publications under the new aspects. This review summarizes present knowledge about the function of SLs in shaping root systems under optimal and nutrient deficiency conditions. Results which appear inconsistent with the various aspects of root development are singled out.

A missense allele of KARRIKIN-INSENSITIVE2 impairs ligand-binding and downstream signaling in Arabidopsis thaliana

A smoke-derived compound, karrikin (KAR), and an endogenous but as yet unidentified KARRIKIN INSENSITIVE2 (KAI2) ligand (KL) have been identified as chemical cues in higher plants that impact on multiple aspects of growth and development. Genetic screening of light-signaling mutants in Arabidopsis thaliana has identified a mutant designated as ply2 (pleiotropic long hypocotyl2) that has pleiotropic light-response defects. In this study, we used positional cloning to identify the molecular lesion of ply2 as a missense mutation of KAI2/HYPOSENSITIVE TO LIGHT, which causes a single amino acid substitution, Ala219Val. Physiological analysis and genetic epistasis analysis with the KL-signaling components MORE AXILLARY GROWTH2 (MAX2) and SUPPRESSOR OF MAX2 1 suggested that the pleiotropic phenotypes of the ply2 mutant can be ascribed to a defect in KL-signaling. Molecular and biochemical analyses revealed that the mutant KAI2ply2 protein is impaired in its ligand-binding activity. In support of this conclusion, X-ray crystallography studies suggested that the KAI2ply2 mutation not only results in a narrowed entrance gate for the ligand but also alters the structural flexibility of the helical lid domains. We discuss the structural implications of the Ala219 residue with regard to ligand-specific binding and signaling of KAI2, together with potential functions of KL-signaling in the context of the light-regulatory network in Arabidopsis thaliana.

Proteomic analysis of the effect of plant-derived smoke on soybean during recovery from flooding stress

Flooding negatively affects the growth of soybean, whereas the plant-derived smoke enhances seedling growth of crops. To clarify the mechanism underlying the recovery from flooding stress, proteomic analysis was performed based on morphological results. Growth of soybean seedlings was inhibited under flooding stress, but it recovered after water removal following treatment with plant-derived smoke. Sucrose/starch metabolism and glycolysis were suppressed in smoke-treated flooded soybean compared to flooded soybean. The protein abundance and gene expression of O-fucosyltransferase family proteins related to the cell wall were higher in smoke-treated flooded soybean than in flooded soybean. Protein abundance and gene expression of peptidyl-prolyl cis-trans isomerase and Bowman-Birk proteinase isoinhibitor D-II were lower in smoke-treated flooded soybean than in flooded soybean. Taken together, these results suggest that plant-derived smoke enhances soybean growth during recovery from flooding stress through the balance of sucrose/starch metabolism and glycolysis. Furthermore, the accumulation of cell-wall related protein might be an important factor contributing to recovery of soybean from flooding stress.

BIOLOGICAL SIGNIFICANCE

Flooding negatively affects the growth of soybean, whereas the plant-derived smoke enhances the seedling growth of crops. To clarify the mechanism underlying the recovery from flooding stress, proteomic analysis of soybean with different treatments including normal conditions, flooding stress, and flooding stress in the presence of plant-derived smoke was performed in this study. Growth of soybean seedlings was inhibited under flooding stress, however, it recovered with plant-derived smoke treatment during recovery from flooding stress. Sucrose/starch metabolism and glycolysis were suppressed in smoke-treated flooded soybean compared to flooded soybean, which suggests altered sucrose/starch metabolism and glycolysis contribute to soybean growth recovery from flood stress. Furthermore, the protein abundance and gene expression of O-fucosyltransferase family proteins related to the cell wall was higher in smoke-treated flooded soybean than in flooded soybean, which might be an important factor contributing to the recovery of soybean from flooding stress.