Identification and Expression Analysis of Strigolactone Biosynthetic and Signaling Genes in Response to Salt and Alkaline Stresses in Soybean (Glycine max)

The Glycine soja cytochrome P450 gene GsCYP82C4 confers alkaline tolerance by promoting reactive oxygen species scavenging

Recent studies have demonstrated the crucial role of Cytochrome P450 enzymes (CYPs) in the production of secondary metabolites, phytohormones and antioxidants in plants. However, their functional characterization specifically under alkaline stress remains elusive. CYP82C4 was the key gene screened from a family of wild soybean CYPs in our previous studies. The aim of this present study was to clone the Glycine soja GsCYP82C4 gene and characterize its functions in Arabidopsis and Glycine max. The results showed that the GsCYP82C4 gene displayed a high expression in different plant tissues at mature stages compared to young stages. Further, higher temporal expression of the GsCYP82C4 gene was noted at 6, 12 and 24 h time points after alkali treatment in leaves compared to roots. In addition, overexpression of GsCYP82C4 improved alkaline stress tolerance in Arabidopsis via increased root lengths and fresh biomass and strengthened the antioxidant defense system via a reduction in superoxide radicals in transgenic lines compared to wild type (WT) and atcyp82c4 mutants. Further, the expression levels of stress-related marker genes were up-regulated in GsCYP82C4 OX lines under alkali stress. The functional analysis of GsCYP82C4 overexpression in soybean displayed better hairy root growth, increased fresh weight, higher antioxidant enzyme activities and reduced lipid peroxidation rates in OX lines compared to the soybean WT (K599) line. In total, our study displayed positive roles of GsCYP82C4 overexpression in both Arabidopsis and Glycine max to alleviate alkaline stress via altering expression abundance of stress responsive genes, stronger roots, higher antioxidant enzyme activities as well as reduced rates of lipid peroxidation and superoxide radicals.

Effect of strigolactone on growth, photosynthetic efficiency, antioxidant activity, and osmolytes accumulation in different maize (Zea mays L.) hybrids grown under drought stress

Drought alters plant physiology, morphology, and biochemical pathways, necessitating effective mitigation strategies. Strigolactones (SLs) are phytohormones known to enhance plant growth under abiotic stress. However, their specific impact on drought stress in maize remains unclear. This study aimed to determine the optimal SL concentration for mitigating drought stress in two maize hybrids (HY-1898, FH-1046). Maize plants were subjected to 60% field capacity drought stress in a pot experiment. After 40 d, different concentrations (0, 0.001, 0.01, and 0.1 mg L-1) of the synthetic SL analogue GR24 were applied to evaluate their effects on growth features, photosynthesis attributes, and osmolyte accumulation in the maize hybrids. Results showed that exogenous SL application significantly increased photosynthetic pigments in maize hybrids under drought stress. Chlorophyll content, gas exchange characteristics, net CO2 assimilation rate, stomatal conductance, water use efficiency, and antioxidant activities were enhanced by GR24. Leaf ascorbic acid and total phenolics also increased with SL application. Organic osmolytes, such as glycine betaine and free proline, were elevated in both maize hybrids under drought stress. Yield-related parameters, including cob diameter, cob weight, number of seeds per cob, and number of seeds per plant, were significantly increased by GR24 under drought stress. Our findings highlight the potential of GR24 foliar application to mitigate drought stress and promote maize growth and grain yield in a concentration-dependent manner. The minimum effective SL concentration against drought stress was determined to be 0.01 mg L-1. Overall, foliar application of GR24 could serve as a sustainable approach for drought tolerance in agriculture.

Strigolactones can be a potential tool to fight environmental stresses in arid lands

BACKGROUND

Environmental stresses pose a significant threat to plant growth and ecosystem productivity, particularly in arid lands that are more susceptible to climate change. Strigolactones (SLs), carotenoid-derived plant hormones, have emerged as a potential tool for mitigating environmental stresses.

METHODS

This review aimed to gather information on SLs' role in enhancing plant tolerance to ecological stresses and their possible use in improving the resistance mechanisms of arid land plant species to intense aridity in the face of climate change.

RESULTS

Roots exude SLs under different environmental stresses, including macronutrient deficiency, especially phosphorus (P), which facilitates a symbiotic association with arbuscular mycorrhiza fungi (AMF). SLs, in association with AMF, improve root system architecture, nutrient acquisition, water uptake, stomatal conductance, antioxidant mechanisms, morphological traits, and overall stress tolerance in plants. Transcriptomic analysis revealed that SL-mediated acclimatization to abiotic stresses involves multiple hormonal pathways, including abscisic acid (ABA), cytokinins (CK), gibberellic acid (GA), and auxin. However, most of the experiments have been conducted on crops, and little attention has been paid to the dominant vegetation in arid lands that plays a crucial role in reducing soil erosion, desertification, and land degradation. All the environmental gradients (nutrient starvation, drought, salinity, and temperature) that trigger SL biosynthesis/exudation prevail in arid regions. The above-mentioned functions of SLs can potentially be used to improve vegetation restoration and sustainable agriculture.

CONCLUSIONS

Present review concluded that knowledge on SL-mediated tolerance in plants is developed, but still in-depth research is needed on downstream signaling components in plants, SL molecular mechanisms and physiological interactions, efficient methods of synthetic SLs production, and their effective application in field conditions. This review also invites researchers to explore the possible application of SLs in improving the survival rate of indigenous vegetation in arid lands, which can potentially help combat land degradation problems.

Strigolactone signaling gene from soybean GmMAX2a enhances the drought and salt-alkaline resistance in Arabidopsis via regulating transcriptional profiles of stress-related genes

Strigolactone (SL) is a new plant hormone, which not only plays an important role in stimulating seed germination, plant branching, and regulating root development, but also plays an important role in the response of plants to abiotic stresses. In this study, the full-length cDNA of a soybean SL signal transduction gene (GmMAX2a) was isolated, cloned and revealed an important role in abiotic stress responses. Tissue-specific expression analysis by qRT-PCR indicated that GmMAX2a was expressed in all tissues of soybean, but highest expression was detected in seedling stems. Moreover, upregulation of GmMAX2a transcript expression under salt, alkali, and drought conditions were noted at different time points in soybean leaves compared to roots. Additionally, histochemical GUS staining studies revealed the deep staining in P_GmMAX2a: GUS transgenic lines compared to WT indicating active involvement of GmMAX2a promoter region to stress responses. To further investigate the function of GmMAX2a gene in transgenic Arabidopsis, Petri-plate experiments were performed and GmMAX2a OX lines appeared with longer roots and improved fresh biomass compared to WT plants to NaCl, NaHCO₃, and mannitol supplementation. Furthermore, the expression of several stress-related genes such as RD29B, SOS1, NXH1, AtRD22, KIN1, COR15A, RD29A, COR47, H+-APase, NADP-ME, NCED3, and P5CS were significantly high in GmMAX2a OX plants after stress treatment compared to WT plants. In conclusion, GmMAX2a improves soybean tolerance towards abiotic stresses (salt, alkali, and drought). Hence, GmMAX2a can be considered a candidate gene for transgenic breeding against various abiotic stresses in plants.

SLs signal transduction gene CsMAX2 of cucumber positively regulated to salt, drought and ABA stress in Arabidopsis thaliana L

Recent studies have demonstrated that strigolactones (SLs) participate in the regulation of stress adaptation, however, the mechanisms remain elusive. MAX2 (MORE AXILLARY GROWTH2) is the key gene in the signal transduction pathway of SLs. This study aimed to clone and functionally characterize the CsMAX2 gene of cucumber in Arabidopsis. The results showed that the expression levels of the CsMAX2 gene changed significantly after salt, drought, and ABA stresses in cucumber. Moreover, the overexpression of CsMAX2 promoted stress tolerance and increased the germination rate and root length of Arabidopsis thaliana. Meanwhile, the content of chlorophyll increased and malondialdehyde decreased in CsMAX2 OE lines under salt and drought stresses. Additionally, the expression levels of stress-related marker genes, especially AREB1 and COR15A, were significantly upregulated under salt stress, while the expression levels of all genes were upregulated under drought stress, except ABI4 and ABI5 genes. The level of NCED3 continued to rise under both salt and drought stresses. In addition, D10 and D27 gene expression level also showed a continuous increase under ABA stress. The result suggested the interaction between SL and ABA in the process of adapting to stress. Overall, CsMAX2 could positively regulate salt, drought, and ABA stress resistance, and this process correlated with ABA transduction.

Genome-wide identification and characterization of NHL gene family in response to alkaline stress, ABA and MEJA treatments in wild soybean (Glycine soja)

Background

NDR1/HIN1-like (NHL) family genes are known to be involved in pathogen induced plant responses to biotic stress. Even though the NHL family genes have been identified and characterized in plant defense responses in some plants, the roles of these genes associated with the plant abiotic stress tolerance in wild soybean is not fully established yet, especially in response to alkaline stress.

Methods

We identified the potential NHL family genes by using the Hidden Markov model and wild soybean genome. The maximum-likelihood phylogenetic tree and conserved motifs were generated by using the MEME online server and MEGA 7.0 software, respectively. Furthermore, the syntenic analysis was generated with Circos-0.69. Then we used the PlantCARE online software to predict and analyze the regulatory cis-acting elements in promoter regions. Hierarchical clustering trees was generated using TM4: MeV4.9 software. Additionally, the expression levels of NHL family genes under alkaline stress, ABA and MEJA treatment were identified by qRT-PCR.

Results

In this study, we identified 59 potential NHL family genes in wild soybean. We identified that wild soybean NHL family genes could be mainly classified into five groups as well as exist with conserved motifs. Syntenic analysis of NHL family genes revealed genes location on 18 chromosomes and presence of 65 pairs of duplication genes. Moreover, NHL family genes consisted of a variety of putative hormone-related and abiotic stress responsive elements, where numbers of methyl jasmonate (MeJA) and abscisic acid (ABA) responsive elements were significantly larger than other elements. We confirmed the regulatory roles of NHL family genes in response to alkaline stress, ABA and MEJA treatment. In conclusion, we identified and provided valuable information on the wild soybean NHL family genes, and established a foundation to further explore the potential roles of NHL family genes in crosstalk with MeJA or ABA signal transduction mechanisms under alkaline stress.

Liriodendron chinense LcMAX1 regulates primary root growth and shoot branching in Arabidopsis thaliana

Strigolactones (SLs) play prominent roles in regulating shoot branching and root architecture in model plants. However, their roles in non-model (particularly woody) plants remain unclear. Liriodendron chinense is a timber tree species widely planted in southern China. The outturn percentage and wood quality of L. chinense are greatly affected by the branching characteristics of its shoot, and the rooting ability of the cuttings is key for its vegetative propagation. Here, we isolated and analyzed the function of the MORE AXILLARY GROWTH 1 (LcMAX1) gene, which is involved in L. chinense SL biosynthesis. RT-qPCR showed that LcMAX1 was highly expressed in the roots and axillary buds. LcMAX1 was located in the endoplasmic reticulum (ER) and nucleus. LcMAX1 ectopic expression promoted primary root growth, whereas there were no phenotypic differences in shoot branching between transgenic and wild-type (WT) A. thaliana plants. LcMAX1 overexpression in the max1 mutant restored them to the WT A. thaliana phenotypes. Additionally, AtPIN1, AtPIN2, and AtBRC1 expressions were significantly upregulated in transgenic A. thaliana and the max1 mutant. It was therefore speculated that LcMAX1 promotes primary root growth by regulating expression of auxin transport-related genes in A. thaliana, and LcMAX1 inhibits shoot branching by upregulating expression of AtBRC1 in the max1 mutant. Altogether, these results demonstrated that the root development and shoot branching functions of LcMAX1 were similar to those of AtMAX1. Our findings provide a foundation for obtaining further insights into root and branch development in L. chinense.

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.

Genome-wide analysis of the strigolactone biosynthetic and signaling genes in grapevine and their response to salt and drought stresses

Strigolactones (SLs) are a novel class of plant hormones that play critical roles in regulating various developmental processes and stress tolerance. Although the SL biosynthetic and signaling genes were already determined in some plants such as Arabidopsis and rice, the information of SL-related genes in grapevine (Vitis vinifera L.) remains largely unknown. In this study, the SL-related genes were identified from the whole grapevine genome, and their expression patterns under salt and drought stresses were determined. The results indicated that the five genes that involved in the SL biosynthesis included one each of the D27, CCD7, CCD8, MAX1 and LBO genes, as well as the three genes that involved in the SL signaling included one each of the D14, MAX2, D53 genes. Phylogenetic analysis suggested that these SL-related proteins are highly conserved among different plant species. Promoter analysis showed that the prevalence of a variety of cis-acting elements associated with hormones and abiotic stress existed in the promoter regions of these SL-related genes. Furthermore, the transcription expression analysis demonstrated that most SL-related genes are involved in the salt and drought stresses response in grapevine. These findings provided valuable information for further investigation and functional analysis of SL biosynthetic and signaling genes in response to salt and drought stresses in grapevine.

Biological Functions of Strigolactones and Their Crosstalk With Other Phytohormones

Phytohormones are small chemicals critical for plant development and adaptation to a changing environment. Strigolactones (SLs), carotenoid-derived small signalling molecules and a class of phytohormones, regulate multiple developmental processes and respond to diverse environmental signals. SLs also coordinate adjustments in the balance of resource distribution by strategic modification of the plant development, allowing plants to adapt to nutrient deficiency. Instead of operating independently, SL interplays with abscisic acid, cytokinin, auxin, ethylene, and some other plant phytohormones, forming elaborate signalling networks. Hormone signalling crosstalk in plant development and environmental response may occur in a fully concerted manner or as a cascade of sequential events. In many cases, the exact underlying mechanism is unclear because of the different effects of phytohormones and the varying backgrounds of their actions. In this review, we systematically summarise the synthesis, signal transduction, and biological functions of SLs and further highlight the significance of crosstalk between SLs and other phytohormones during plant development and resistance to ever-changing environments.