Overexpression of the OsERF71 Transcription Factor Alters Rice Root Structure and Drought Resistance

ETHYLENE RESPONSE FACTOR39-MYB8 complex regulates low-temperature-induced lignification of loquat fruit

Flesh lignification is a specific chilling response that causes deterioration in the quality of stored red-fleshed loquat fruit (Eribotrya japonica) and is one aspect of wider chilling injury. APETALA2/ETHLENE RESPONSIVE FACTOR (AP2/ERF) transcription factors are important regulators of plant low-temperature responses and lignin biosynthesis. In this study, the expression and action of 27 AP2/ERF genes from the red-fleshed loquat cultivar 'Luoyangqing' were investigated in order to identify transcription factors regulating low-temperature-induced lignification. EjERF27, EjERF30, EjERF36, and EjERF39 were significantly induced by storage at 0 °C but inhibited by a low-temperature conditioning treatment (pre-storage at 5 °C for 6 days before storage at 0 °C, which reduces low-temperature-induced lignification), and their transcript levels positively correlated with flesh lignification. A dual-luciferase assay indicated that EjERF39 could transactivate the promoter of the lignin biosynthetic gene Ej4CL1, and an electrophoretic mobility shift assay confirmed that EjERF39 recognizes the DRE element in the promoter region of Ej4CL1. Furthermore, the combination of EjERF39 and the previously characterized EjMYB8 synergistically transactivated the Ej4CL1 promoter, and both transcription factors showed expression patterns correlated with lignification in postharvest treatments and red-fleshed 'Luoyangqing' and white-fleshed 'Ninghaibai' cultivars with different lignification responses. Bimolecular fluorescence complementation and luciferase complementation imaging assays confirmed direct protein-protein interaction between EjERF39 and EjMYB8. These data indicate that EjERF39 is a novel cold-responsive transcriptional activator of Ej4CL1 that forms a synergistic activator complex with EjMYB8 and contributes to loquat fruit lignification at low temperatures.

An APETALA2/ethylene responsive factor, OsEBP89 knockout enhances adaptation to direct-seeding on wet land and tolerance to drought stress in rice

Water stress is the most important adverse factor limiting rice production. Too much water leads to flood and too little leads to drought. Floods and droughts can severely damage crop at different times of the rice life cycle. So the research on submergence tolerance and drought resistance of rice is particularly urgent. In this study, we reported that OsEBP89 (Oryza sativa Ethylene-responsive element binding protein, clone 89), a member of the AP2/ERF subfamily, is involved in a novel signal transduction associated with the tolerance to drought and submergence stress. OsEBP89 was found to be strongly inhibited by drought stress and promoted by submergence. The OsEBP89 protein was located at the nucleus in the rice protoplast. Loss of OsEBP89 was found to improve the seed germination under submerged conditions and also enhanced the tolerance to drought stress throughout growth stage. Additionally, OsEBP89 knockout rice plants increased the accumulation of proline, improved the ability to scavenge ROS compared to overexpression lines and wild type after PEG treatment. Transcriptome data indicates that knockout of OsEBP89 improved the expression of specific genes in response to adverse factors, such as OsAPX1, OsHsfA3, and OsP5CS. Further results indicate that OsEBP89 can interact with and be phosphorylated by SnRK1α (sucrose non-fermenting-1-related protein kinase-1 gene). These findings provide insight into the mechanism of abiotic stress tolerance, and suggest OsEBP89 as a new genetic engineering resource to improve abiotic stress tolerance in rice.

Ethylene-responsive factor 4 is associated with the desirable rind hardness trait conferring cracking resistance in fresh fruits of watermelon

Fruit rind plays a pivotal role in alleviating water loss and disease and particularly in cracking resistance as well as the transportability, storability and shelf-life quality of the fruit. High susceptibility to cracking due to low rind hardness is largely responsible for severe annual yield losses of fresh fruits such as watermelon in the field and during the postharvest process. However, the candidate gene controlling the rind hardness phenotype remains unclear to date. Herein, we report, for the first time, an ethylene-responsive transcription factor 4 (ClERF4) associated with variation in rind hardness via a combinatory genetic map with bulk segregant analysis (BSA). Strikingly, our fine-mapping approach revealed an InDel of 11 bp and a neighbouring SNP in the ClERF4 gene on chromosome 10, conferring cracking resistance in F₂ populations with variable rind hardness. Furthermore, the concomitant kompetitive/competitive allele-specific PCR (KASP) genotyping data sets of 104 germplasm accessions strongly supported candidate ClERF4 as a causative gene associated with fruit rind hardness variability. In conclusion, our results provide new insight into the underlying mechanism controlling rind hardness, a desirable trait in fresh fruit. Moreover, the findings will further enable the molecular improvement of fruit cracking resistance in watermelon via precisely targeting the causative gene relevant to rind hardness, ClERF4.

Root Development and Stress Tolerance in rice: The Key to Improving Stress Tolerance without Yield Penalties

Roots anchor plants and take up water and nutrients from the soil; therefore, root development strongly affects plant growth and productivity. Moreover, increasing evidence indicates that root development is deeply involved in plant tolerance to abiotic stresses such as drought and salinity. These findings suggest that modulating root growth and development provides a potentially useful approach to improve plant abiotic stress tolerance. Such targeted approaches may avoid the yield penalties that result from growth-defense trade-offs produced by global induction of defenses against abiotic stresses. This review summarizes the developmental mechanisms underlying root development and discusses recent studies about modulation of root growth and stress tolerance in rice.

The MicroRNA397b -LACCASE2 Module Regulates Root Lignification under Water and Phosphate Deficiency

Deficiency of water and phosphate induce lignin deposition in roots. LACCASEs, a family of cell wall-localized multicopper oxidases, are involved in lignin biosynthesis. We demonstrate here that LACCASE2 (LAC2) acts as a negative regulator of lignin deposition in root vascular tissues during water deficit. An Arabidopsis (Arabidopsis thaliana) transfer DNA insertion mutant of LAC2 displayed a short primary root and high lignin deposition in root vascular tissues. However, restoration of LAC2 expression rescued these phenotypes. LAC2 expression was significantly down-regulated under water deficit and posttranscriptionally regulated by microRNA397b (miR397b) in roots under normal and water-deficit conditions. Down-regulation of miR397b activity increased LAC2 expression and root length, and decreased lignin content in root vasculature. Similarly, phosphate (Pi) deficiency inversely affected miR397b and LAC2 expression. Lignin deposition in the root elongation zone under Pi-limited conditions was dependent on LAC2 expression. Localized iron accumulation and callose deposition in the root elongation zone under Pi deficiency increased with LAC2-dependent lignification, suggesting a direct relationship between these processes. Our study reveals a regulatory role for the miR397b-LAC2 module in root lignification during water and phosphate deficiency.

Genome-wide analysis of ethylene-response factor family in adzuki bean and functional determination of VaERF3 under saline-alkaline stress

Ethylene-response factor (ERF) proteins are members of a transcription factor family involved in plant growth and environmental stress responses, but the biological functions of ERF members in adzuki bean (Vigna angularis var. angularis) remain unknown. In addition, it is unclear whether these proteins have a role in regulating responses to abiotic stressors. Here, we identified 47 ERF genes by analyzing the adzuki bean genome. Whole-transcriptome analyses of plants under saline-alkaline stress suggested that the expression of 13 ERF genes was induced in response to saline-alkaline stress. Analysis of the cis-acting elements showed that the promoters of these saline-alkaline stress-inducible ERF genes contained LTRs, DREs, MYBs, ABREs, MYCs, CGTCA-, and TGACG-motifs, which are involved in abiotic stress responses. The expression of VaERF3 was induced by NaHCO₃, polyethylene glycol 6000, NaCl, and ABA (abscisic acid), as determined by qRT-PCR. Overexpression of VaERF3 in transgenic Arabidopsis resulted in higher levels of proline accumulation and lower malondialdehyde and reactive oxygen species contents in plants grown under saline-alkaline stress conditions. Moreover, VaERF3 encoded a nuclear-localized transcriptional activator that promoted the expression of stress-responsive genes. Collectively, these results are of great significance in elucidating the mechanisms of saline-alkaline stress responses in adzuki bean.

Jasmonic Acid, Not Salicyclic Acid Restricts Endophytic Root Colonization of Rice

Research on the interaction between the non-nodule-forming bacterial endophytes and their host plants is still in its infancy. Especially the understanding of plant control mechanisms which govern endophytic colonization is very limited. The current study sets out to determine which hormonal signaling pathway controls endophytic colonization in rice, and whether the mechanisms deviate for a pathogen. The endophyte Azoarcus olearius BH72-rice model was used to investigate root responses to endophytes in comparison to the recently established pathosystem of rice blight Xanthomonas oryzae pv. oryzae PXO99 (Xoo) in flooded roots. In the rice root transcriptome, 523 or 664 genes were found to be differentially expressed in response to Azoarcus or Xoo colonization, respectively; however, the response was drastically different, with only 6% of the differentially expressed genes (DEGs) overlapping. Overall, Xoo infection induced a much stronger defense reaction than Azoarcus colonization, with the latter leading to down-regulation of many defense related DEGs. Endophyte-induced DEGs encoded several enzymes involved in phytoalexin biosynthesis, ROS (reactive oxygen species) production, or pathogenesis-related (PR) proteins. Among putative plant markers related to signal transduction pathways modulated exclusively during Azoarcus colonization, none overlapped with previously published DEGs identified for another rice endophyte, Azospirillum sp. B510. This suggests a large variation in responses of individual genotypic combinations. Interestingly, the DEGs related to jasmonate (JA) signaling pathway were found to be consistently activated by both beneficial endophytes. In contrast, the salicylate (SA) pathway was activated only in roots infected by the pathogen. To determine the impact of SA and JA production on root colonization by the endophyte and the pathogen, rice mutants with altered hormonal responses were employed: mutant cpm2 deficient in jasmonate synthesis, and RNA interference (RNAi) knockdown lines of NPR1 decreased in salicylic acid-mediated defense responses (NPR1-kd). Only in cpm2, endophytic colonization of Azoarcus was significantly increased, while Xoo colonization was not affected. Surprisingly, NPR1-kd lines showed slightly decreased colonization by Xoo, contrary to published results for leaves. These outcomes suggest that JA but not SA signaling is involved in controlling the Azoarcus endophyte density in roots and can restrict internal root colonization, thereby shaping the beneficial root microbiome.

Drought Resistance by Engineering Plant Tissue-Specific Responses

Drought is the primary cause of agricultural loss globally, and represents a major threat to food security. Currently, plant biotechnology stands as one of the most promising fields when it comes to developing crops that are able to produce high yields in water-limited conditions. From studies of Arabidopsis thaliana whole plants, the main response mechanisms to drought stress have been uncovered, and multiple drought resistance genes have already been engineered into crops. So far, most plants with enhanced drought resistance have displayed reduced crop yield, meaning that there is still a need to search for novel approaches that can uncouple drought resistance from plant growth. Our laboratory has recently shown that the receptors of brassinosteroid (BR) hormones use tissue-specific pathways to mediate different developmental responses during root growth. In Arabidopsis, we found that increasing BR receptors in the vascular plant tissues confers resistance to drought without penalizing growth, opening up an exceptional opportunity to investigate the mechanisms that confer drought resistance with cellular specificity in plants. In this review, we provide an overview of the most promising phenotypical drought traits that could be improved biotechnologically to obtain drought-tolerant cereals. In addition, we discuss how current genome editing technologies could help to identify and manipulate novel genes that might grant resistance to drought stress. In the upcoming years, we expect that sustainable solutions for enhancing crop production in water-limited environments will be identified through joint efforts.

SiMYB56 Confers Drought Stress Tolerance in Transgenic Rice by Regulating Lignin Biosynthesis and ABA Signaling Pathway

Foxtail millet (Setaria italica) originated in China and is generally cultivated in arid and barren soil. Through long-term harsh environmental selection, foxtail millet has acquired significant drought resistance. However, the molecular mechanism of foxtail millet drought resistance is still unknown. Here, we identified a drought-induced R2R3-MYB transcription factor SiMYB56 in foxtail millet. Overexpression of SiMYB56 significantly enhances tolerance to drought stress in transgenic rice plants at both the vegetative and the reproductive stage and has no adverse effect on its normal growth. Compared with wild-type controls, SiMYB56-overexpressing rice plants had lower MDA content and higher lignin content under drought conditions. Quantitative real-time PCR and Transcriptional activity assays demonstrated that SiMYB56 could activate expression of lignin biosynthesis genes under drought conditions. Also, we found that overexpression of SiMYB56 can led to ABA accumulation in the seeds transgenic rice plants. Further experiments showed that Overexpression of SiMYB56 can upregulate the expression of ABA synthesis and response related genes under drought conditions. In conclusion, SiMYB56 may enhance the drought resistance of transgenic rice plants by regulating lignin biosynthesis and ABA signaling pathway, making SiMYB56 a candidate gene for drought resistance improvement in gramineous crops.

Integrated Analysis of the Transcriptome and Metabolome Revealed the Molecular Mechanisms Underlying the Enhanced Salt Tolerance of Rice Due to the Application of Exogenous Melatonin

High salinity is one of the major abiotic stresses limiting rice production. Melatonin has been implicated in the salt tolerance of rice. However, the molecular basis of melatonin-mediated salt tolerance in rice remains unclear. In the present study, we performed an integrated transcriptome and metabolome profiling of rice seedlings treated with salt, melatonin, or salt + melatonin. The application of exogenous melatonin increased the salt tolerance of rice plants by decreasing the sodium content to maintain Na⁺/K⁺ homeostasis, alleviating membrane lipid oxidation, and enhancing chlorophyll contention. A comparative transcriptome analysis revealed that complex molecular pathways contribute to melatonin-mediated salt tolerance. More specifically, the AP2/EREBP-HB-WRKY transcriptional cascade and phytohormone (e.g., auxin and abscisic acid) signaling pathways were activated by an exogenous melatonin treatment. On the basis of metabolome profiles, 64 metabolites, such as amino acids, organic acids, nucleotides, and secondary metabolites, were identified with increased abundances only in plants treated with salt + melatonin. Several of these metabolites including endogenous melatonin and its intermediates (5-hydroxy-L-tryptophan, N ¹-acetyl-N ²-formyl-5-methoxykynuramine), gallic acid, diosmetin, and cyanidin 3-O-galactoside had antioxidant functions, suggesting melatonin activates multiple antioxidant pathways to alleviate the detrimental effects of salt stress. Combined transcriptome and metabolome analyses revealed a few gene-metabolite networks related to various pathways, including linoleic acid metabolism and amino acid metabolism that are important for melatonin-mediated salt tolerance. The data presented herein may be useful for further elucidating the multiple regulatory roles of melatonin in plant responses to abiotic stresses.

Analysis of Dynamic Global Transcriptional Atlas Reveals Common Regulatory Networks of Hormones and Photosynthesis Across Nicotiana Varieties in Response to Long-Term Drought

Land plants evolve drought acclimation. Existing knowledge of gene regulation mainly comes from short-term drought treatment. However, common regulatory mechanism shared by multiple varieties under long-term drought is little explored. Here we investigated changes in physiology, hormones and transcriptomes in leaves of Nicotiana varieties K326 and Basma Xanthi with/without drought treatment at time courses spanning 1 month. Analyses of deep RNA-Seq data and further full-length Iso-Seq data revealed an atlas of dynamic changes of transcripts, spliced isoforms, gene expression, associated Gene Ontology, and metabolism pathways. Fewer differentially expressed genes (DEGs) were induced by drought in high tolerance variety than susceptible variety. Comparison among seven hormone signal pathways identified that genes in both abscisic acid and auxin signaling pathways were highly induced although specific genes were depended on the variety. Common hormone regulatory network analysis revealed that genes encoding clade A protein phosphatase 2C gene (PP2C) in abscisic acid pathway was the pivotal hub. Expressional regulation in photosynthesis was also common and variety specific. We conclude that long-term drought inducing gene regulatory networks of hormones and photosynthesis are variety dependent, and PP2C is the center of the common hormone regulatory network. Thus, this study improves our understanding of gene regulatory network in drought response.

Insights into Barley Root Transcriptome under Mild Drought Stress with an Emphasis on Gene Expression Regulatory Mechanisms

Root systems play a pivotal role in coupling with drought stress, which is accompanied with a substantial transcriptome rebuilding in the root tissues. Here, we present the results of global gene expression profiling of roots of two barley genotypes with contrasting abilities to cope with drought that were subjected to a mild level of the stress. We concentrate our analysis on gene expression regulation processes, which allowed the identification of 88 genes from 39 families involved in transcriptional regulation in roots upon mild drought. They include 13 genes encoding transcription factors (TFs) from AP2 family represented by ERFs, DREB, or B3 domain-containing TFs, eight WRKYs, six NACs, five of the HD-domain, MYB or MYB-related, bHLH and bZIP TFs. Also, the representatives of C3H, CPP, GRAS, LOB-domain, TCP, Tiffy, Tubby, and NF-Ys TFs, among others were found to be regulated by the mild drought in barley roots. We found that drought tolerance is accompanied with a lower number of gene expression changes than the amount observed in a susceptible genotype. The better drought acclimation may be related to the activation of transcription factors involved in the maintenance of primary root growth and in the epigenetic control of chromatin and DNA methylation. In addition, our analysis pointed to fives TFs from ERF, LOB, NAC, WRKY and bHLH families that may be important in the mild but not the severe drought response of barley roots.

An AP2/ERF transcription factor ERF139 coordinates xylem cell expansion and secondary cell wall deposition

Differentiation of xylem elements involves cell expansion, secondary cell wall (SCW) deposition and programmed cell death. Transitions between these phases require strict spatiotemporal control. The function of Populus ERF139 (Potri.013G101100) in xylem differentiation was characterized in transgenic overexpression and dominant repressor lines of ERF139 in hybrid aspen (Populus tremula × tremuloides). Xylem properties, SCW chemistry and downstream targets were analyzed in both types of transgenic trees using microscopy techniques, Fourier transform-infrared spectroscopy, pyrolysis-GC/MS, wet chemistry methods and RNA sequencing. Opposite phenotypes were observed in the secondary xylem vessel sizes and SCW chemistry in the two different types of transgenic trees, supporting the function of ERF139 in suppressing the radial expansion of vessel elements and stimulating accumulation of guaiacyl-type lignin and possibly also xylan. Comparative transcriptomics identified genes related to SCW biosynthesis (LAC5, LBD15, MYB86) and salt and drought stress-responsive genes (ANAC002, ABA1) as potential direct targets of ERF139. The phenotypes of the transgenic trees and the stem expression profiles of ERF139 potential target genes support the role of ERF139 as a transcriptional regulator of xylem cell expansion and SCW formation, possibly in response to osmotic changes of the cells.

The expanding roles of APETALA2/Ethylene Responsive Factors and their potential applications in crop improvement

Understanding the molecular basis of the gene-regulatory networks underlying agronomic traits or plant responses to abiotic/biotic stresses is very important for crop improvement. In this context, transcription factors, which either singularly or in conjugation directly control the expression of many target genes, are suitable candidates for improving agronomic traits via genetic engineering. In this regard, members of one of the largest class of plant-specific APETALA2/Ethylene Response Factor (AP2/ERF) superfamily, which is implicated in various aspects of development and plant stress adaptation responses, are considered high-value targets for crop improvement. Besides their long-known regulatory roles in mediating plant responses to abiotic stresses such as drought and submergence, the novel roles of AP2/ERFs during fruit ripening or secondary metabolites production have also recently emerged. The astounding functional plasticity of AP2/ERF members is considered to be achieved by their interplay with other regulatory networks and signalling pathways. In this review, we have integrated the recently accumulated evidence from functional genomics studies and described their newly emerged functions in plants. The key structural features of AP2/ERF proteins and the modes of their action are briefly summarized. The importance of AP2/ERFs in plant development and stress responses and a summary of the event of their successful applications in crop improvement programs are also provided. Altogether, we envisage that the synthesized information presented in this review will be useful to design effective strategies for improving agronomic traits in crop plants.

Transcriptomic analysis of early fruit development in Chinese white pear (Pyrus bretschneideri Rehd.) and functional identification of PbCCR1 in lignin biosynthesis

BACKGROUND

The content of stone cells and lignin is one of the key factors affecting the quality of pear fruit. In a previous study, we determined the developmental regularity of stone cells and lignin in 'Dangshan Su' pear fruit 15-145 days after pollination (DAP). However, the development of fruit stone cells and lignin before 15 DAP has not been heavily researched.

RESULTS

In this study, we found that primordial stone cells began to appear at 7 DAP and that the fruit had formed a large number of stone cells at 15 DAP. Subsequently, transcriptome sequencing was performed on fruits at 0, 7, and 15 DAP and identified 3834 (0 vs. 7 DAP), 4049 (7 vs. 15 DAP) and 5763 (0 vs. 15 DAP) DEGs. During the 7-15 DAP period, a large number of key enzyme genes essential for lignin biosynthesis are gradually up-regulated, and their expression pattern is consistent with the accumulation of lignin in this period. Further analysis found that the biosynthesis of S-type lignin in 'Dangshan Su' pear does not depend on the catalytic activity of PbSAD but is primarily generated by the catalytic activity of caffeoyl-CoA through CCoAOMT, CCR, F5H, and CAD. We cloned PbCCR1, 2 and analysed their functions in Chinese white pear lignin biosynthesis. PbCCR1 and 2 have a degree of functional redundancy; both demonstrate the ability to participate in lignin biosynthesis. However, PbCCR1 may be the major gene for lignin biosynthesis, while PbCCR2 has little effect on lignin biosynthesis.

CONCLUSIONS

Our results revealed that 'Dangshan Su' pear began to form a large number of stone cells and produce lignin after 7 DAP and mainly accumulated materials from 0 to 7 DAP. PbCCR1 is mainly involved in the biosynthesis of lignin in 'Dangshan Su' pear and plays a positive role in lignin biosynthesis.

Functional characterization of LkERF-B2 for improved salt tolerance ability in Arabidopsis thaliana

The ethylene response factors have been reported to play critical roles in developmental and environmental responses in plants. In the present study, an ERF transcription factor gene was aimed to be identified from Larix kaempferi. Molecular characteristics and function of this gene were further explored. The result showed that a 1344 bp ERF transcription factor gene containing initiation and termination codon was obtained by RT-PCR and named LkERF-B2. LkERF-B2 gene encoded 447 amino acids containing a typical AP2/ERF domain. Alignment of predicted amino acid sequence of LkERF-B2 in various plant species showed that this ERF transcription factor was highly homologous (79.0%) with that of Picea sitchensi. To elucidate the function of LkERF-B2, LkERF-B2 overexpression vector was successfully constructed and transformed to Arabidopsis thaliana via dip flower. Compared with control plant, LkERF-B2 overexpressed transgenic A. thaliana showed a significantly higher survival rate under cold, heat, NaCl and drought stresses. NaCl stress analysis revealed that control and transgenic Arabidopsis were both flowering earlier under 100 and 150 mM/L NaCl treatment. While under 200-300 mM/L NaCl treatment, the growth of control plant was significantly inhibited compared with transgenic A. thaliana. Salt injury rate and salt injury index of transgenic Arabidopsis were lower than those of the control. Further investigation showed that transgenic Arabidopsis exhibited much higher content of chloroplast pigments under different NaCl concentration. Meanwhile, the activity of SOD and POD was also enhanced in transgenic A. thaliana. These results suggested that LkERF-B2 was a key transcription factor and could lead to enhanced salt stress tolerance.

Allantoin accumulation through overexpression of ureide permease1 improves rice growth under limited nitrogen conditions

In legumes, nitrogen (N) can be stored as ureide allantoin and transported by ureide permease (UPS) from nodules to leaves where it is catabolized to release ammonium and assimilation to amino acids. In non-leguminous plants especially rice, information on its roles in N metabolism is scarce. Here, we show that OsUPS1 is localized in plasma membranes and are highly expressed in vascular tissues of rice. We further evaluated an activation tagging rice overexpressing OsUPS1 (OsUPS1OX ) under several N regimes. Under normal field conditions, panicles from OsUPS1OX plants (14 days after flowering (DAF)) showed significant allantoin accumulation. Under hydroponic system at the vegetative stage, plants were exposed to N-starvation and measured the ammonium in roots after resupplying with ammonium sulphate. OsUPS1OX plants displayed higher ammonium uptake in roots compared to wild type (WT). When grown under low-N soil supplemented with different N-concentrations, OsUPS1OX exhibited better growth at 50% N showing higher chlorophyll, tiller number and at least 20% increase in shoot and root biomass relative to WT. To further confirm the effects of regulating the expression of OsUPS1, we evaluated whole-body-overexpressing plants driven by the GOS2 promoter (OsUPS1GOS2 ) as well as silencing plants (OsUPS1RNAi ). We found significant accumulation of allantoin in leaves, stems and roots of OsUPS1GOS2 while in OsUPS1RNAi allantoin was significantly accumulated in roots. We propose that OsUPS1 is responsible for allantoin partitioning in rice and its overexpression can support plant growth through accumulation of allantoin in sink tissues which can be utilized when N is limiting.

The rice ethylene response factor OsERF83 positively regulates disease resistance to Magnaporthe oryzae

Rice blast caused by Magnaporthe oryzae is one of the most destructive diseases of rice (Oryza sativa) worldwide. Here, we report the identification and functional characterization of a novel ethylene response factor (ERF) gene, OsERF83, which was expressed in rice leaves in response to rice blast fungus infection. OsERF83 expression was also induced by treatments with methyl jasmonate, ethephon, and salicylic acid, indicating that multiple phytohormones could be involved in the regulation of OsERF83 expression under biotic stress. Subcellular localization and transactivation analyses demonstrated that OsERF83 is a nucleus-localized transcriptional activator. A gel-shift assay using recombinant OsERF83 protein indicated that, like other ERFs, it binds to the GCC box. Transgenic rice plants overexpressing OsERF83 exhibited significantly suppressed lesion formation after rice blast infection, indicating that OsERF83 positively regulates disease resistance in rice. Genes encoding several classes of pathogenesis-related (PR) proteins, including PR1, PR2, PR3, PR5, and PR10, were upregulated in the OsERF83ox plants. Taken together, our findings show that OsERF83 is a novel ERF transcription factor that confers blast resistance by regulating the expression of defense-related genes in rice.

Overexpression of OsARD1 Improves Submergence, Drought, and Salt Tolerances of Seedling Through the Enhancement of Ethylene Synthesis in Rice

Acireductone dioxygenase (ARD) is a metal-binding metalloenzyme and involved in the methionine salvage pathway. In rice, OsARD1 binds Fe²⁺ and catalyzes the formation of 2-keto-4-methylthiobutyrate (KMTB) to produce methionine, which is an initial substrate in ethylene synthesis pathway. Here, we report that overexpression of OsARD1 elevates the endogenous ethylene release rate, enhances the tolerance to submergence stress, and reduces the sensitivity to drought, salt, and osmotic stresses in rice. OsARD1 is strongly induced by submergence, drought, salinity, PEG6000, and mechanical damage stresses and exhibits high expression level in senescent leaves. Transgenic plants overexpressing OsARD1 (OsARD1-OE) display fast elongation growth to escape submergence stress. The ethylene content is significantly maximized in OsARD1-OE plants compared with the wide type. OsARD1-OE plants display increased shoot elongation and inhibition of root elongation under the submergence stress and grow in dark due to increase of ethylene. The elongation of coleoptile under anaerobic germination is also significantly promoted in OsARD1-OE lines due to the increase of ethylene content. The sensitivity to drought and salt stresses is reduced in OsARD1-OE transgenic lines. Water holding capacity is enhanced, and the stomata and trichomes on leaves increase in OsARD1-OE lines. Drought and salt tolerance and ethylene synthesis-related genes are upregulated in OsARD1-OE plants. Subcellular localization shows that OsARD1 displays strong localization signal in cell nucleus, suggesting OsARD1 may interact with the transcription factors. Taken together, the results provide the understanding of the function of OsARD1 in ethylene synthesis and abiotic stress response in rice.

Overexpression of OsTF1L, a rice HD-Zip transcription factor, promotes lignin biosynthesis and stomatal closure that improves drought tolerance

Drought stress seriously impacts on plant development and productivity. Improvement of drought tolerance without yield penalty is a great challenge in crop biotechnology. Here, we report that the rice (Oryza sativa) homeodomain-leucine zipper transcription factor gene, OsTF1L (Oryza sativa transcription factor 1-like), is a key regulator of drought tolerance mechanisms. Overexpression of the OsTF1L in rice significantly increased drought tolerance at the vegetative stages of growth and promoted both effective photosynthesis and a reduction in the water loss rate under drought conditions. Importantly, the OsTF1L overexpressing plants showed a higher drought tolerance at the reproductive stage of growth with a higher grain yield than nontransgenic controls under field-drought conditions. Genomewide analysis of OsTF1L overexpression plants revealed up-regulation of drought-inducible, stomatal movement and lignin biosynthetic genes. Overexpression of OsTF1L promoted accumulation of lignin in shoots, whereas the RNAi lines showed opposite patterns of lignin accumulation. OsTF1L is mainly expressed in outer cell layers including the epidermis, and the vasculature of the shoots, which coincides with areas of lignification. In addition, OsTF1L overexpression enhances stomatal closure under drought conditions resulted in drought tolerance. More importantly, OsTF1L directly bound to the promoters of lignin biosynthesis and drought-related genes involving poxN/PRX38, Nodulin protein, DHHC4, CASPL5B1 and AAA-type ATPase. Collectively, our results provide a new insight into the role of OsTF1L in enhancing drought tolerance through lignin biosynthesis and stomatal closure in rice.

MdMYB88 and MdMYB124 Enhance Drought Tolerance by Modulating Root Vessels and Cell Walls in Apple

Water deficit is one of the main limiting factors in apple (Malus × domestica Borkh.) cultivation. Root architecture plays an important role in the drought tolerance of plants; however, research efforts to improve drought tolerance of apple trees have focused on aboveground targets. Due to the difficulties associated with visualization and data analysis, there is currently a poor understanding of the genetic players and molecular mechanisms involved in the root architecture of apple trees under drought conditions. We previously observed that MdMYB88 and its paralog MdMYB124 regulate apple tree root morphology. In this study, we found that MdMYB88 and MdMYB124 play important roles in maintaining root hydraulic conductivity under long-term drought conditions and therefore contribute toward adaptive drought tolerance. Further investigation revealed that MdMYB88 and MdMYB124 regulate root xylem development by directly binding MdVND6 and MdMYB46 promoters and thus influence expression of their target genes under drought conditions. In addition, MdMYB88 and MdMYB124 were shown to regulate the deposition of cellulose and lignin root cell walls in response to drought. Taken together, our results provide novel insights into the importance of MdMYB88 and MdMYB124 in root architecture, root xylem development, and secondary cell wall deposition in response to drought in apple trees.

A Zinc Finger Transcriptional Repressor Confers Pleiotropic Effects on Rice Growth and Drought Tolerance by Down-Regulating Stress-Responsive Genes

The C2H2-type zinc finger proteins (ZFPs) are involved in a wide range of plant development and stress responses. Many studies have shown the positive roles of ZFP genes in stress tolerance. However, overexpression of ZFP genes usually leads to the side effect of growth retardation. Here we report a new member of the ZFP family, Oryza sativa drought-responsive zinc finger protein 1 (OsDRZ1), positively regulating both stress tolerance and plant architecture in rice (Oryza sativa L.). OsDRZ1 was expressed throughout all tissues examined and could be induced by multiple abiotic stresses. OsDRZ1 protein was localized mostly in the nucleus. Unlike most reported rice ZFPs functioning as transcriptional activators, OsDRZ1 is a transcriptional repressor. Overexpression of OsDRZ1 in rice increased seedling drought tolerance and the transgenic plants appeared to accumulate more free proline and fewer reactive oxygen species (ROS), and elevate the activities of antioxidant enzymes. In contrast, RNA interference (RNAi) of OsDRZ1 led to lower activities of antioxidative response and more sensitivity to drought. RNA sequencing analysis revealed that the genes down-regulated by OsDRZ1 were mostly down-regulated by drought, implying the critical role of OsDRZ1 in modulating drought-responsive gene expression. A cupin gene OsGLP1 (germin-like protein1) was identified as one of the potential target genes of OsDRZ1, as suggested by real-time PCR and transient expression analysis in rice protoplasts. Moreover, overexpression of OsDRZ1 did not lead to growth inhibition but the promotion of rice growth, implying the potential application prospective of OsDRZ1 in engineering drought-tolerant crops.

Genome-wide identification of the AP2/ERF transcription factor family in pepper (Capsicum annuum L.)

The AP2/ERF family is one of the largest transcription factor families in the plant kingdom. AP2/ERF genes contributing to various processes including plant growth, development, and response to various stresses have been identified. In this study, 175 putative AP2/ERF genes were identified in the latest pepper genome database and classified into AP2, RAV, ERF, and Soloist subfamilies. Their chromosomal localization, gene structure, conserved motif, cis-acting elements within the promoter region, and subcellular locations were analyzed. Transient expression of CaAP2/ERF proteins in tobacco revealed that CaAP2/ERF064, CaAP2/ERF109, and CaAP2/ERF127 were located in the nucleus, while CaAP2/ERF171 was located in the nucleus and cytoplasm. Most of the CaAP2/ERF genes contained cis-elements within their promoter regions that responded to various stresses (HSE, LTR, MBS, Box-W1/W-box, and TC-rich repeats) and phytohormones (ABRE, CGTCA-motif, and TCA-element). Furthermore, RNA-seq analysis revealed that CaAP2/ERF genes showed differential expression profiles in various tissues as well as under biotic stresses. Moreover, qRT-PCR analysis of eight selected CaAP2/ERF genes also showed differential expression patterns in response to infection with Phytophthora capsici (HX-9) and in response to phytohormones (SA, MeJA, and ETH). This study will provide basic insights for further studies of the CaAP2/ERF genes involved in the interaction between pepper and P. capsici.

Natural Variation in OsLG3 Increases Drought Tolerance in Rice by Inducing ROS Scavenging

Improving the performance of rice (Oryza sativa) under drought stress has the potential to significantly affect rice productivity. Here, we report that the ERF family transcription factor OsLG3 positively regulates drought tolerance in rice. In our previous work, we found that OsLG3 has a positive effect on rice grain length without affecting grain quality. In this study, we found that OsLG3 was more strongly expressed in upland rice than in lowland rice under drought stress conditions. By performing candidate gene association analysis, we found that natural variation in the promoter of OsLG3 is associated with tolerance to osmotic stress in germinating rice seeds. Overexpression of OsLG3 significantly improved the tolerance of rice plants to simulated drought, whereas suppression of OsLG3 resulted in greater susceptibility. Phylogenetic analysis indicated that the tolerant allele of OsLG3 may improve drought tolerance in cultivated japonica rice. Introgression lines and complementation transgenic lines containing the elite allele of OsLG3^IRAT109 showed increased drought tolerance, demonstrating that natural variation in OsLG3 contributes to drought tolerance in rice. Further investigation suggested that OsLG3 plays a positive role in drought stress tolerance in rice by inducing reactive oxygen species scavenging. Collectively, our findings reveal that natural variation in OsLG3 contributes to rice drought tolerance and that the elite allele of OsLG3 is a promising genetic resource for the development of drought-tolerant rice varieties.

Using Biotechnology-Led Approaches to Uplift Cereal and Food Legume Yields in Dryland Environments

Drought and heat in dryland agriculture challenge the enhancement of crop productivity and threaten global food security. This review is centered on harnessing genetic variation through biotechnology-led approaches to select for increased productivity and stress tolerance that will enhance crop adaptation in dryland environments. Peer-reviewed literature, mostly from the last decade and involving experiments with at least two seasons' data, form the basis of this review. It begins by highlighting the adverse impact of the increasing intensity and duration of drought and heat stress due to global warming on crop productivity and its impact on food and nutritional security in dryland environments. This is followed by (1) an overview of the physiological and molecular basis of plant adaptation to elevated CO2 (eCO2), drought, and heat stress; (2) the critical role of high-throughput phenotyping platforms to study phenomes and genomes to increase breeding efficiency; (3) opportunities to enhance stress tolerance and productivity in food crops (cereals and grain legumes) by deploying biotechnology-led approaches [pyramiding quantitative trait loci (QTL), genomic selection, marker-assisted recurrent selection, epigenetic variation, genome editing, and transgene) and inducing flowering independent of environmental clues to match the length of growing season; (4) opportunities to increase productivity in C3 crops by harnessing novel variations (genes and network) in crops' (C3, C4) germplasm pools associated with increased photosynthesis; and (5) the adoption, impact, risk assessment, and enabling policy environments to scale up the adoption of seed-technology to enhance food and nutritional security. This synthesis of technological innovations and insights in seed-based technology offers crop genetic enhancers further opportunities to increase crop productivity in dryland environments.

Expression of AhDREB1, an AP2/ERF Transcription Factor Gene from Peanut, Is Affected by Histone Acetylation and Increases Abscisic Acid Sensitivity and Tolerance to Osmotic Stress in Arabidopsis

Drought stress negatively affects plant growth and development. An increasing number of reports have revealed the involvement of APETALA2/Ethylene Responsive Factor (AP2/ERF) transcription factors (TFs) in biotic and abiotic stress regulation in plants. However, research on these TFs in the peanut plant (Arachis hypogaea) has been limited. Here, we isolated a full-length coding sequence (CDS) of the AP2/ERF family gene AhDREB1 from the peanut plant and showed that its expression was induced by Polyethylene Glycol (PEG) 6000 and exogenous abscisic acid (ABA) treatment. When overexpressed in Arabidopsis, AhDREB1 increased both ABA levels and ABA sensitivity, affected the ABA signaling pathway and increased the expression of downstream drought stress-related genes RD29A, P5CS1, P5CS2 and NCED1. These results demonstrate that AhDREB1 can improve tolerance to drought via the ABA-dependent pathway in Arabidopsis. In the peanut plant, the specific histone deacetylases (HDACs) inhibitor trichostatin A (TSA) promotes AhDREB1 transcription and the enrichment level of H3ac was increased in regions of the AhDREB1 gene during TSA and PEG treatment. In summary, histone acetylation can affect the expression of AhDREB1 under osmotic stress conditions, thereby improving plant drought resistance.

OsERF71 confers drought tolerance via modulating ABA signaling and proline biosynthesis

Plants have evolved multiple protective strategies to adapt to adverse environmental conditions. Upland rice (UR) has evolved as a "drought-resistant type". However, little is known about genes or mechanisms in UR that underlying drought tolerance at the molecular level. Here we report isolation and functional characterization of the ERF gene, OsERF71, from the UR variety, IRAT109. The expression of OsERF71 was induced by abscisic acid (ABA) and various abiotic stresses preferentially in IRAT109 under ABA, dehydration, and polyethyleneglycol (PEG) treatments. OsERF71 was verified as a nuclear-localized protein and had transcriptional activity in yeast cells. Overexpression of the OsERF71 in Nipponbare demonstrated a significant increase in tolerance to drought stress and a reduced rate of water loss. In contrast, OsERF71 interference lines were sensitive to drought stress and exhibited a higher rate of water loss. OsERF71-overexpressing lines also showed enhanced tolerance to high salinity. Moreover, OsERF71 regulated the expression of several ABA- responsive and proline biosynthesis genes under drought stress, resulting in enhanced sensitivity to exogenous ABA treatment and proline accumulation. Accordingly, we suggest that OsERF71 plays a positive role in drought stress tolerance by increasing the expression of genes associated with ABA signaling and proline biosynthesis under stress.

Overexpression of OsNAC14 Improves Drought Tolerance in Rice

Plants have evolved to have sophisticated adaptation mechanisms to cope with drought stress by reprograming transcriptional networks through drought responsive transcription factors. NAM, ATAF1-2, and CUC2 (NAC) transcription factors are known to be associated with various developmental processes and stress tolerance. In this study, we functionally characterized the rice drought responsive transcription factor OsNAC14. OsNAC14 was predominantly expressed at meiosis stage but is induced by drought, high salinity, ABA, and low temperature in leaves. Overexpression of OsNAC14 resulted in drought tolerance at the vegetative stage of growth. Field drought tests demonstrated that OsNAC14 overexpressing transgenic rice lines exhibited higher number of panicle and filling rate compared to non-transgenic plants under drought conditions. RNA-sequencing analysis revealed that OsNAC14 overexpression elevated the expression of genes for stress response, DNA damage repair, defense related, and strigolactone biosynthesis. In addition, chromatin immunoprecipitation analysis confirmed the direct interaction of OsNAC14 with the promoter of OsRAD51A1, a key component in homologous recombination in DNA repair system. Collectively, these results indicate that OsNAC14 mediates drought tolerance by recruiting factors involved in DNA damage repair and defense response resulting in improved tolerance to drought.

Overexpression of OsERF48 causes regulation of OsCML16, a calmodulin-like protein gene that enhances root growth and drought tolerance

The AP2/ERF family is a plant-specific transcription factor family whose members have been associated with various developmental processes and stress tolerance. Here, we functionally characterized the drought-inducible OsERF48, a group Ib member of the rice ERF family with four conserved motifs, CMI-1, -2, -3 and -4. A transactivation assay in yeast revealed that the C-terminal CMI-1 motif was essential for OsERF48 transcriptional activity. When OsERF48 was overexpressed in an either a root-specific (ROX^OsERF48 ) or whole-body (OX^OsERF48 ) manner, transgenic plants showed a longer and denser root phenotype compared to the nontransgenic (NT) controls. When plants were grown on a 40% polyethylene glycol-infused medium under in vitro drought conditions, ROX^OsERF48 plants showed a more vigorous root growth than OX^OsERF48 and NT plants. In addition, the ROX^OsERF48 plants exhibited higher grain yield than OX^OsERF48 and NT plants under field-drought conditions. We constructed a putative OsERF48 regulatory network by cross-referencing ROX^OsERF48 root-specific RNA-seq data with a co-expression network database, from which we inferred the involvement of 20 drought-related genes in OsERF48-mediated responses. These included genes annotated as being involved in stress signalling, carbohydrate metabolism, cell-wall proteins and drought responses. They included, OsCML16, a key gene in calcium signalling during abiotic stress, which was shown to be a direct target of OsERF48 by chromatin immunoprecipitation-qPCR analysis and a transient protoplast expression assay. Our results demonstrated that OsERF48 regulates OsCML16, a calmodulin-like protein gene that enhances root growth and drought tolerance.

A novel AP2/ERF family transcription factor from Glycine soja, GsERF71, is a DNA binding protein that positively regulates alkaline stress tolerance in Arabidopsis

KEY MESSAGE

Here we first found that GsERF71, an ERF factor from wild soybean could increase plant alkaline stress tolerance by up-regulating H+-ATPase and by modifing the accumulation of Auxin. Alkaline soils are widely distributed all over the world and greatly limit plant growth and development. In our previous transcriptome analyses, we have identified several ERF (ethylene-responsive factor) genes that responded strongly to bicarbonate stress in the roots of wild soybean G07256 (Glycine soja). In this study, we cloned and functionally characterized one of the genes, GsERF71. When expressed in epidermal cells of onion, GsERF71 localized to the nucleus. It can activate the reporters in yeast cells, and the C-terminus of 170 amino acids is essential for its transactivation activity. Yeast one-hybrid and EMSA assays indicated that GsERF71 specifically binds to the cis-acting elements of the GCC-box, suggesting that GsERF71 may participate in the regulation of transcription of the relevant biotic and abiotic stress-related genes. Furthermore, transgenic Arabidopsis plants overexpressing GsERF71 showed significantly higher tolerance to bicarbonate stress generated by NaHCO3 or KHCO3 than the wild type (WT) plants, i.e., the transgenic plants had greener leaves, longer roots, higher total chlorophyll contents and lower MDA contents. qRT-PCR and rhizosphere acidification assays indicated that the expression level and activity of H+-ATPase (AHA2) were enhanced in the transgenic plants under alkaline stress. Further analysis indicated that the expression of auxin biosynthetic genes and IAA contents were altered to a lower extent in the roots of transgenic plants than WT plants under alkaline stress in a short-term. Together, our data suggest that GsERF71 enhances the tolerance to alkaline stress by up-regulating the expression levels of H+-ATPase and by modifying auxin accumulation in transgenic plants.

Transcriptional Network Analysis Reveals Drought Resistance Mechanisms of AP2/ERF Transgenic Rice

This study was designed to investigate at the molecular level how a transgenic version of rice "Nipponbare" obtained a drought-resistant phenotype. Using multi-omics sequencing data, we compared wild-type rice (WT) and a transgenic version (erf71) that had obtained a drought-resistant phenotype by overexpressing OsERF71, a member of the AP2/ERF transcription factor (TF) family. A comprehensive bioinformatics analysis pipeline, including TF networks and a cascade tree, was developed for the analysis of multi-omics data. The results of the analysis showed that the presence of OsERF71 at the source of the network controlled global gene expression levels in a specific manner to make erf71 survive longer than WT. Our analysis of the time-series transcriptome data suggests that erf71 diverted more energy to survival-critical mechanisms related to translation, oxidative response, and DNA replication, while further suppressing energy-consuming mechanisms, such as photosynthesis. To support this hypothesis further, we measured the net photosynthesis level under physiological conditions, which confirmed the further suppression of photosynthesis in erf71. In summary, our work presents a comprehensive snapshot of transcriptional modification in transgenic rice and shows how this induced the plants to acquire a drought-resistant phenotype.

Rice OsERF71-mediated root modification affects shoot drought tolerance

Drought is the most serious problem that impedes crop development and productivity worldwide. Although several studies have documented the root architecture adaption for drought tolerance, little is known about the underlying molecular mechanisms. Our latest study demonstrated that overexpression of the OsERF71 in rice roots under drought conditions modifies root structure including larger aerenchyma and radial root growth, and thereby, protects the rice plants from drought stresses. The OsERF71-mediated root modifications are caused by combinatory overexpression of general stress-inducible, cell wall-associated and lignin biosynthesis genes that contribute to drought tolerance. Here we addressed that the OsERF71-mediated root modifications alter physiological capacity in shoots without evidence of developmental changes for drought tolerance. Thus, the OsERF71-mediated root modifications provide novel molecular insights into drought tolerance mechanisms.

Co-overexpression of the Constitutively Active Form of OsbZIP46 and ABA-Activated Protein Kinase SAPK6 Improves Drought and Temperature Stress Resistance in Rice

Drought is one of the major abiotic stresses threatening rice (Oryza sativa) production worldwide. Drought resistance is controlled by multiple genes, and therefore, a multi-gene genetic engineering strategy is theoretically useful for improving drought resistance. However, the experimental evidence for such a strategy is still lacking. In this study, a few drought-responsive genes from rice were assembled by a multiple-round site-specific assembly system, and the constructs were introduced into the rice cultivar KY131 via Agrobacterium-mediated transformation. The transgenic lines of the multi-gene and corresponding single-gene constructs were pre-evaluated for drought resistance. We found that the co-overexpression of two genes, encoding a constitutively active form of a bZIP transcription factor (OsbZIP46CA1) and a protein kinase (SAPK6) involved in the abscisic acid signaling pathway, showed significantly enhanced drought resistance compared with the single-gene transgenic lines and the negative transgenic plants. Single-copy lines of this bi-gene combination (named XL22) and the corresponding single-gene lines were further evaluated for drought resistance in the field using agronomical traits. The results showed that XL22 exhibited greater yield, biomass, spikelet number, and grain number under moderate drought stress conditions. The seedling survival rate of XL22 and the single-gene overexpressors after drought stress treatment also supported the drought resistance results. Furthermore, expression profiling by RNA-Seq revealed that many genes involved in the stress response were specifically up-regulated in the drought-treated XL22 lines and some of the stress-related genes activated in CA1-OE and SAPK6-OE were distinct, which could partially explain the different performances of these lines with respect to drought resistance. In addition, the XL22 seedlings showed improved tolerance to heat and cold stresses. Our results demonstrate that the multi-gene assembly in an appropriate combination may be a promising approach in the genetic improvement of drought resistance.