Abscisic acid-responsive element binding transcription factors contribute to proline synthesis and stress adaptation in Arabidopsis

Genetic engineering of drought- and salt-tolerant tomato via Δ1-pyrroline-5-carboxylate reductase S-nitrosylation

Drought and soil salinization substantially impact agriculture. While proline's role in enhancing stress tolerance is known, the exact molecular mechanism by which plants process stress signals and control proline synthesis under stress is still not fully understood. In tomato (Solanum lycopersicum L.), drought and salt stress stimulate nitric oxide (NO) production, which boosts proline synthesis by activating Δ1-pyrroline-5-carboxylate synthetase (SlP5CS) and Δ1-pyrroline-5-carboxylate reductase (SlP5CR) genes and the P5CR enzyme. The crucial factor is stress-triggered NO production, which regulates the S-nitrosylation of SlP5CR at Cys-5, thereby increasing its NAD(P)H affinity and enzymatic activity. S-nitrosylation of SlP5CR enables tomato plants to better adapt to changing NAD(P)H levels, boosting both SlP5CR activity and proline synthesis during stress. By comparing tomato lines genetically modified to express different forms of SlP5CR, including a variant mimicking S-nitrosylation (SlP5CRC5W), we found that SlP5CRC5W plants show superior growth and stress tolerance. This is attributed to better P5CR activity, proline production, water use efficiency, reactive oxygen species scavenging, and sodium excretion. Overall, this study demonstrates that tomato engineered to mimic S-nitrosylated SlP5CR exhibits enhanced growth and yield under drought and salt stress conditions, highlighting a promising approach for stress-tolerant tomato cultivation.

Identification of a sugarcane bacilliform virus promoter that is activated by drought stress in plants

Sugarcane (Saccharum spp.) is an important sugar and biofuel crop in the world. It is frequently subjected to drought stress, thus causing considerable economic losses. Transgenic technology is an effective breeding approach to improve sugarcane tolerance to drought using drought-inducible promoter(s) to activate drought-resistance gene(s). In this study, six different promoters were cloned from sugarcane bacilliform virus (SCBV) genotypes exhibiting high genetic diversity. In β-glucuronidase (GUS) assays, expression of one of these promoters (PSCBV-YZ2060) is similar to the one driven by the CaMV 35S promoter and >90% higher compared to the other cloned promoters and Ubi1. Three SCBV promoters (PSCBV-YZ2060, PSCBV-TX, and PSCBV-CHN2) function as drought-induced promoters in transgenic Arabidopsis plants. In Arabidopsis, GUS activity driven by promoter PSCBV-YZ2060 is also upregulated by abscisic acid (ABA) and is 2.2-5.5-fold higher when compared to the same activity of two plant native promoters (PScRD29A from sugarcane and PAtRD29A from Arabidopsis). Mutation analysis revealed that a putative promoter region 1 (PPR1) and two ABA response elements (ABREs) are required in promoter PSCBV-YZ2060 to confer drought stress response and ABA induction. Yeast one-hybrid and electrophoretic mobility shift assays uncovered that transcription factors ScbZIP72 from sugarcane and AREB1 from Arabidopsis bind with two ABREs of promoter PSCBV-YZ2060. After ABA treatment or drought stress, the expression levels of endogenous ScbZIP72 and heterologous GUS are significantly increased in PSCBV-YZ2060:GUS transgenic sugarcane plants. Consequently, promoter PSCBV-YZ2060 is a possible alternative promoter for genetic engineering of drought-resistant transgenic crops such as sugarcane.

Bioinformatics and expression analysis of proline metabolism-related gene families in alfalfa under saline-alkali stress

Regulation of the proline metabolic pathway is essential for the accumulation of proline under abiotic stress and for the amelioration of plant stress resistance. Δ1-pyrroline-5-carboxylate synthase (P5CS), pyrroline-5-carboxylate reductase (P5CR), ornithine transaminase (δ-OAT), proline dehydrogenase (PDH), pyrroline-5-carboxylate dehydrogenase (P5CDH), and proline transporter (ProT) are the key enzymes in the proline metabolic pathway. However, the gene families responsible for proline metabolism have not yet been identified or reported in alfalfa. In this study, a total of 12 MsP5CSs, 4 MsP5CRs, 3 MsOATs, 6 MsPDHs, 2 MsP5CDHs, and 5 MsProTs were identified in the genome of alfalfa, and the members of the same subfamily had similar gene structures and conserved motifs. Analysis of cis-regulatory elements revealed the presence of light-responsive, hormone-regulated, and stress-responsive elements in the promoter regions of alfalfa proline metabolism-related genes. Following treatment with saline-alkali, the expression of MsP5CSs, MsP5CRs, MsOATs, and MsProTs was significantly upregulated, whereas the expression of MsPDH1.1, MsPDH1.3, and MsP5CDH was significantly downregulated. The proline content and enzyme activity of P5CS gradually increased, whereas the enzyme activity of PDH gradually decreased as the duration of stress increased. Root growth rates decreased upon MsP5CS1a suppression (MsP5CS1a-RNAi) in the hairy roots of alfalfa compared to the empty vector line under saline-alkali stress. These results show that proline metabolism-related genes play an important role in the saline-alkali stress tolerance of alfalfa and provide a theoretical basis for further research on the functions of proline metabolism-related genes in alfalfa in response to saline-alkali stress.

Integrated metabolome, full-length sequencing, and transcriptome analyses unveil the molecular mechanisms of color formation of the canary yellow and red bracts of Bougainvillea × buttiana 'Chitra'

Bougainvillea is a typical tropical flower of great ornamental value due to its colorful bracts. The molecular mechanism behind color formation is not well-understood. Therefore, this research conducted metabolome analysis, transcriptome analysis, and multi-flux full-length sequencing in two color bracts of Bougainvillea × buttiana 'Chitra' to investigate the significantly different metabolites (SDMs) and differentially expressed genes (DEGs). Overall, 261 SDMs, including 62 flavonoids and 26 alkaloids, were detected, and flavonols and betalains were significantly differentially accumulated among the two bracts. Furthermore, the complete-length transcriptome of Bougainvillea × buttiana was also developed, which contained 512 493 non-redundant isoforms. Among them, 341 210 (66.58%) displayed multiple annotations in the KOG, GO, NR, KEGG, Pfam, Swissprot, and NT databases. RNA-seq findings revealed that 3610 DEGs were identified between two bracts. Co-expression analysis demonstrated that the DEGs and SDMs involved in flavonol metabolism (such as CHS, CHI, F3H, FLS, CYP75B1, kaempferol, and quercetin) and betacyanin metabolism (DODA, betanidin, and betacyanins) were the main contributors for the canary yellow and red bract formation, respectively. Further investigation revealed that several putative transcription factors (TFs) might interact with the promoters of the genes mentioned above. The expression profiles of the putative TFs displayed that they may positively and negatively regulate the structural genes' expression profiles. The data revealed a potential regulatory network between important genes, putative TFs, and metabolites in the flavonol and betacyanin biosynthesis of Bougainvillea × buttiana 'Chitra' bracts. These findings will serve as a rich genetic resource for future studies that could create new color bracts.

A novel ABA-induced transcript factor from Millettia pinnata, MpAITR1, enhances salt and drought tolerance through ABA signaling in transgenic Arabidopsis

Abiotic stress, such as salt and drought stress, seriously limits plant growth and crop yield. Abscisic acid (ABA) is essential in regulating plant responses to abiotic stress via signal perception, transduction, and transcriptional regulation. Pongamia (Millettia pinnata) is a kind of semi-mangrove plant with strong stress tolerance and can grow in fresh and sea water. However, the molecular mechanism of the ABA signaling pathway mediating the environmental tolerance of Pongamia is still scarce so far. AITR (ABA-Induced Transcription Repressor) was a recently identified small conserved family of transcription factor in angiosperms, which played controversial roles in response to abiotic stresses in different species. Here, we identified an ABA-induced gene, MpAITR1, which encoded a nucleus localization transcriptional factor in Pongamia. MpAITR1 was highly induced by ABA and salt treatments in roots and leaves. Heterologous expression of MpAITR1 in Arabidopsis increased sensitivity to ABA, moreover, enhanced tolerance to salt and drought stress. The expression levels of some ABA-responsive and stress-responsive genes were altered in transgenic plants compared to wild-type plants under the ABA, salt, and drought stress, which was consistent with the stress-tolerant phenotype of transgenic plants. These results reveal that MpAITR1 positively modulates ABA signaling pathways and enhances the tolerance to salt and drought stress by regulating downstream target genes. Taken together, MpAITR1 from the semi-mangrove plant Pongamia serves as a potential candidate for stress-tolerant crop breeding.

Host- and virus-induced gene silencing of HOG1-MAPK cascade genes in Rhizophagus irregularis inhibit arbuscule development and reduce resistance of plants to drought stress

Arbuscular mycorrhizal (AM) fungi can form beneficial associations with the most terrestrial vascular plant species. AM fungi not only facilitate plant nutrient acquisition but also enhance plant tolerance to various environmental stresses such as drought stress. However, the molecular mechanisms by which AM fungal mitogen-activated protein kinase (MAPK) cascades mediate the host adaptation to drought stimulus remains to be investigated. Recently, many studies have shown that virus-induced gene silencing (VIGS) and host-induced gene silencing (HIGS) strategies are used for functional studies of AM fungi. Here, we identify the three HOG1 (High Osmolarity Glycerol 1)-MAPK cascade genes RiSte11, RiPbs2 and RiHog1 from Rhizophagus irregularis. The expression levels of the three HOG1-MAPK genes are significantly increased in mycorrhizal roots of the plant Astragalus sinicus under severe drought stress. RiHog1 protein was predominantly localized in the nucleus of yeast in response to 1 M sorbitol treatment, and RiPbs2 interacts with RiSte11 or RiHog1 directly by pull-down assay. Importantly, VIGS or HIGS of RiSte11, RiPbs2 or RiHog1 hampers arbuscule development and decreases relative water content in plants during AM symbiosis. Moreover, silencing of HOG1-MAPK cascade genes led to the decreased expression of drought-resistant genes (RiAQPs, RiTPSs, RiNTH1 and Ri14-3-3) in the AM fungal symbiont in response to drought stress. Taken together, this study demonstrates that VIGS or HIGS of AM fungal HOG1-MAPK cascade inhibits arbuscule development and expression of AM fungal drought-resistant genes under drought stress.

Natural diversity uncovers P5CS1 regulation and its role in drought stress tolerance and yield sustainability in barley

Proline accumulation is one of the major responses of plants to many abiotic stresses. However, the significance of proline accumulation for drought stress tolerance remains enigmatic in crop plants. First, we examined the natural variation of the pyrolline-5-carboxylate synthase (P5CS1) among 49 barley genotypes. Allele mining identified a previously unknown allelic series that showed polymorphisms at 42 cis-elements across the P5CS1 promoter. Selected haplotypes had quantitative variation in P5CS1 gene expression and proline accumulation, putatively influenced by both  abscisic acid-dependent and independent pathways under drought stress. Next, we introgressed the P5CS1 allele from a high proline accumulating wild barley accession ISR42-8 into cultivar Scarlett developing a near-isogenic line (NIL-143). NIL-143 accumulated higher proline concentrations than Scarlett under drought stress at seedling and reproductive stages. Under drought stress, NIL-143 showed less pigment damage, sustained photosynthetic health, and higher drought stress recovery compared to Scarlett. Further, the drought-induced damage to yield-related traits, mainly thousand-grain weight, was lower in NIL-143 compared with Scarlett in field conditions. Our data uncovered new variants of the P5CS1 promoter and the significance of the increased proline accumulation regulated by the P5CS1 allele of ISR42-8 in drought stress tolerance and yield stability in barley.

FtbZIP12 Positively Regulates Responses to Osmotic Stress in Tartary Buckwheat

ABFs play a key role in regulating plant osmotic stress. However, in Tartary buckwheat, data on the role of ABF genes in osmotic stress remain limited and its associated mechanism in osmoregulation remain nebulous. Herein, a novel ABF family in Tartary buckwheat, FtbZIP12, was cloned and characterized. FtbZIP12 is a transcriptional activator located in the nucleus; its expression is induced by NaCl, mannitol, and abscisic acid (ABA). Atopic expression of FtbZIP12 in Arabidopsis promoted seed germination, reduced damage to primary roots, and improved the tolerance of seedlings to osmotic stress. The quantitative realtime polymerase chain reaction (RT-qPCR) results showed that the expressions of the typical genes related to stress, the SOS pathway, and the proline synthesis pathway in Arabidopsis were significantly (p < 0.05) upregulated under osmotic stress. FtbZIP12 improved the osmotic pressure resistance by reducing the damage caused by reactive oxygen species to plants and maintained plant homeostasis by upregulating the expression of genes related to stress, osmotic regulation, and ion homeostasis. This study identified a key candidate gene for understanding the mechanism underlying osmotic-stress-regulated function in Tartary buckwheat, thereby providing a theoretical basis for improving its yield and quality.

Genome-Wide Identification, Primary Functional Characterization of the NHX Gene Family in Canavalia rosea, and Their Possible Roles for Adaptation to Tropical Coral Reefs

Canavalia rosea, distributed in the coastal areas of tropical and subtropical regions, is an extremophile halophyte with good adaptability to high salinity/alkaline and drought tolerance. Plant sodium/hydrogen (Na⁺/H⁺) exchanger (NHX) genes encode membrane transporters involved in sodium ion (Na⁺), potassium ion (K⁺), and lithium ion (Li⁺) transport and pH homeostasis, thereby playing key roles in salinity tolerance. However, the NHX family has not been reported in this leguminous halophyte. In the present study, a genome-wide comprehensive analysis was conducted and finally eight CrNHXs were identified in C. rosea genome. Based on the bioinformatics analysis about the chromosomal location, protein domain, motif organization, and phylogenetic relationships of CrNHXs and their coding proteins, as well as the comparison with plant NHXs from other species, the CrNHXs were grouped into three major subfamilies (Vac-, Endo-, and PM-NHX). Promoter analyses of cis-regulatory elements indicated that the expression of different CrNHXs was affected by a series of stress challenges. Six CrNHXs showed high expression levels in five tested tissues of C. rosea in different levels, while CrNHX1 and CrNHX3 were expressed at extremely low levels, indicating that CrNHXs might be involved in regulating the development of C. rosea plant. The expression analysis based on RNA-seq showed that the transcripts of most CrNHXs were obviously decreased in mature leaves of C. rosea plant growing on tropical coral reefs, which suggested their involvement in this species' adaptation to reefs and specialized islands habitats. Furthermore, in the single-factor stress treatments mimicking the extreme environments of tropical coral reefs, the RNA-seq data also implied CrNHXs holding possible gene-specific regulatory roles in the environmental adaptation. The qRT-PCR based expression profiling exhibited that CrNHXs responded to different stresses to varying degrees, which further confirmed the specificity of CrNHXs' in responding to abiotic stresses. Moreover, the yeast functional complementation test proved that some CrNHXs could partially restore the salt tolerance of the salt-sensitive yeast mutant AXT3. This study provides comprehensive bio-information and primary functional identification of NHXs in C. rosea, which could help improve the salt/alkaline tolerance of genetically modified plants for further studies. This research also contributes to our understanding of the possible molecular mechanism whereby NHXs maintain the ion balance in the natural ecological adaptability of C. rosea to tropical coral islands and reefs.

Glutamate Dehydrogenase Functions in Glutamic Acid Metabolism and Stress Resistance in Pyropia haitanensis

Pyropia haitanensis is an important laver species in China. Its quality traits are closely related to the content of glutamic acid. Glutamate dehydrogenase (GDH) is a crucial enzyme in the glutamic acid metabolism. In this study, two GDH genes from P. haitanensis, PhGDH1 and PhGDH2, were cloned and successfully expressed in Escherichia coli. The in vitro enzyme activity assay demonstrated that the catalytic activity of PhGDHs is mainly in the direction of ammonium assimilation. The measured Km values of PhGDH1 for NADH, (NH4)2SO4, and α-oxoglutarate were 0.12, 4.99, and 0.16 mM, respectively, while the corresponding Km values of PhGDH2 were 0.02, 3.98, and 0.104 mM, respectively. Site-directed mutagenesis results showed that Gly193 and Thr361 were important catalytic residues for PhGDH2. Moreover, expression levels of both PhGDHs were significantly increased under abiotic stresses. These results suggest that PhGDHs can convert α-oxoglutarate to glutamic acid, and enhance the flavor and stress resistance of P. haitanensis.