Improvement of drought tolerance by overexpressing MdATG18a is mediated by modified antioxidant system and activated autophagy in transgenic apple

Strigolactones positively regulate HY5-dependent autophagy and the degradation of ubiquitinated proteins in response to cold stress in tomato

Autophagy, involved in protein degradation and amino acid recycling, plays a key role in plant development and stress responses. However, the relationship between autophagy and phytohormones remains unclear. We used diverse methods, including CRISPR/Cas9, ultra-performance liquid chromatography coupled with tandem mass spectrometry, chromatin immunoprecipitation, electrophoretic mobility shift assays, and dual-luciferase assays to explore the molecular mechanism of strigolactones in regulating autophagy and the degradation of ubiquitinated proteins under cold stress in tomato (Solanum lycopersicum). We show that cold stress induced the accumulation of ubiquitinated proteins. Mutants deficient in strigolactone biosynthesis were more sensitive to cold stress with increased accumulation of ubiquitinated proteins. Conversely, treatment with the synthetic strigolactone analog GR245DS enhanced cold tolerance in tomato, with elevated levels of accumulation of autophagosomes and transcripts of autophagy-related genes (ATGs), and reduced accumulation of ubiquitinated proteins. Meanwhile, cold stress induced the accumulation of ELONGATED HYPOCOTYL 5 (HY5), which was triggered by strigolactones. HY5 further trans-activated ATG18a transcription, resulting in autophagy formation. Mutation of ATG18a compromised strigolactone-induced cold tolerance, leading to decreased formation of autophagosomes and increased accumulation of ubiquitinated proteins. These findings reveal that strigolactones positively regulate autophagy in an HY5-dependent manner and facilitate the degradation of ubiquitinated proteins under cold conditions in tomato.

MdLRR-RLK1-MdATG3 module enhances the resistance of apples to abiotic stress via autophagy

Apple is an important economic species affected by abiotic stress, such as salt and drought. LRR-RLKs play a key role in plant responses to stress, although their physiological functions under abiotic stress are not yet fully understood. Autophagy is a highly conserved process in eukaryotes, which plays a vital role in drought and salt stress responses. In this study, overexpression of MdLRR-RLK1 in apple promoted plant growth and development and increased salt and drought stress tolerance. MdLRR-RLK1 interacts with MdATG3 in vivo and in vitro, and MdATG3 ubiquitinates and degrades MdLRR-RLK1. Intriguingly, MdLRR-RLK1 and MdATG3 enhance salt and drought tolerance through increasing autophagy. Moreover, MdATG3 interacts with MdATG8F and MdATG8I-like in apple. These findings reveal the interaction between MdLRR-RLK1 and MdATG3, suggesting mechanisms that regulate apple growth and resistance to abiotic stress.

γ-Aminobutyric acid mediated by MdCBF3- MdGAD1 mitigates low temperature damage in apple

Low temperatures can seriously affect apple yield and can also cause chilling injury to apple fruit. γ-aminobutyric acid (GABA) plays an important role in improving plant stress resistance. Some studies have reported that GABA can improve cold resistance in plants, only through exogenous treatment; however, the molecular mechanism of its resistance to low temperature is still unknown. This result suggested that exogenous GABA treatment of both apple seedlings and fruit could improve the resistance of apple to low temperatures. MdGAD1, a key gene involved in GABA synthesis, was overexpressed in tomato plants and apple callus to improve their cold tolerance. Both yeast one-hybrid and luciferase assay showed that MdCBF3 could bind to the MdGAD1 promoter to activate its expression and promote GABA synthesis. These results revealed a molecular mechanism utilizing the MdCBF3-MdGAD1 regulatory module that can enhance cold resistance by increasing endogenous GABA synthesis in apple.

Refinement and Enhancement of Agrobacterium-Mediated Transient Transformation for Functional Gene Examination in Mulberry (Morus L.)

Background: Mulberry (Morus L.), a vital perennial woody plant with significant economic importance, is utilized for silkworm rearing, human consumption and medicinal use. The availability of mulberry's whole-genome sequencing data has underscored the demand for an effective, user-friendly, and high-throughput protocol to facilitate the elucidation of gene functions. Methods and Results: In this investigation, we established a transient transformation approach using Agrobacterium tumefaciens-mediated sonication followed by vacuum infiltration in mulberry tissue culture seedlings. Simultaneously, we optimized the transformation conditions, including mulberry genotypes, A. tumefaciens strain, acetosyringone concentration, bacterial density, sonication time, and days after agroinfiltration. These optimizations aimed to achieve heightened transformation efficiency, employing GFP as a reporter gene to monitor transformation events. The optimized method included the use of an infiltration medium (10 mM MgCl2, 10 mM MES (2-(N-morpholino)ethanesulfonic acid sodium salt), 150 μM acetosyringone, and OD600 0.5 of A. tumefaciens LBA4404) supplemented with the surfactant 0.02% Silwet L-77, with 20 s sonication followed by 20 min vacuum infiltration (0.07 MPa). Among the four mulberry genotypes, 'Taiguo' was the most responsive genotype and produced the highest levels of GFP expression at 7 d after infiltration. Furthermore, the optimized transient transformation approach has been proven to be successfully applicable for transiently overexpressing MaANS and MaDFR in mulberry fruits of 'Taiguo', in vitro, which distinctly enhanced fruit coloring and significantly increased anthocyanin accumulation, respectively. Conclusions: In summary, we devised a dependable, stable and highly efficient transient transformation approach suitable for rapid gene function examination in mulberry leaves and fruits, in vitro.

The lowdown on breakdown: Open questions in plant proteolysis

Proteolysis, including post-translational proteolytic processing as well as protein degradation and amino acid recycling, is an essential component of the growth and development of living organisms. In this article, experts in plant proteolysis pose and discuss compelling open questions in their areas of research. Topics covered include the role of proteolysis in the cell cycle, DNA damage response, mitochondrial function, the generation of N-terminal signals (degrons) that mark many proteins for degradation (N-terminal acetylation, the Arg/N-degron pathway, and the chloroplast N-degron pathway), developmental and metabolic signaling (photomorphogenesis, abscisic acid and strigolactone signaling, sugar metabolism, and postharvest regulation), plant responses to environmental signals (endoplasmic-reticulum-associated degradation, chloroplast-associated degradation, drought tolerance, and the growth-defense trade-off), and the functional diversification of peptidases. We hope these thought-provoking discussions help to stimulate further research.

Identification of autophagy-related genes ATG18 subfamily genes in potato (Solanum tuberosum L.) and the role of StATG18a gene in heat stress

Autophagy is a highly conserved process in eukaryotes that is used to recycle the cellular components from the cytoplasm. It plays a crucial function in responding to both biotic and abiotic stress, as well as in the growth and development of plants. Autophagy-related genes (ATG) and their functions have been identified in numerous crop species. However, their specific tasks in potatoes (Solanum tuberosum L.), are still not well understood. This work is the first to identify and characterize the potato StATG18 subfamily gene at the whole-genome level, resulting in a total of 6 potential StATG18 subfamily genes. We analyzed the phylogenetic relationships, chromosome distribution and gene replication, conserved motifs and gene structure, interspecific collinearity relationship, and cis-regulatory elements of the ATG18 subfamily members using bioinformatics approaches. Furthermore, the quantitative real-time polymerase chain reaction (qRT-PCR) analysis suggested that StATG18 subfamily genes exhibit differential expression in various tissues and organs of potato plants. When exposed to heat stress, their expression pattern was observed in the root, stem, and leaf. Based on a higher expression profile, the StATG18a gene was further analyzed under heat stress in potatoes. The subcellular localization analysis of StATG18a revealed its presence in both the cytoplasm and nucleus. In addition, StATG18a altered the growth indicators, physiological characteristics, and photosynthesis of potato plants under heat stresses. In conclusion, this work offers a thorough assessment of StATG18 subfamily genes and provides essential recommendations for additional functional investigation of autophagy-associated genes in potato plants. Moreover, these results also contribute to our understanding of the potential mechanism and functional validation of the StATG18a gene's persistent tolerance to heat stress in potato plants.

Genotype-Specific Activation of Autophagy during Heat Wave in Wheat

Recycling of unnecessary or dysfunctional cellular structures through autophagy plays a critical role in cellular homeostasis and environmental resilience. Therefore, the autophagy trait may have been unintentionally selected in wheat breeding programs for higher yields in arid climates. This hypothesis was tested by measuring the response of three common autophagy markers, ATG7, ATG8, and NBR1, to a heat wave under reduced soil moisture content in 16 genetically diverse spring wheat landraces originating from different geographical locations. We observed in the greenhouse trials that ATG8 and NBR1 exhibited genotype-specific responses to a 1 h, 40 °C heat wave, while ATG7 did not show a consistent response. Three genotypes from Uruguay, Mozambique, and Afghanistan showed a pattern consistent with higher autophagic activity: decreased or stable abundance of both ATG8 and NBR1 proteins, coupled with increased transcription of ATG8 and NBR1. In contrast, three genotypes from Pakistan, Ethiopia, and Egypt exhibited elevated ATG8 protein levels alongside reduced or unaltered ATG8 transcript levels, indicating a potential suppression or no change in autophagic activity. Principal component analysis demonstrated a correlation between lower abundance of ATG8 and NBR1 proteins and higher yield in the field trials. We found that (i) the combination of heat and drought activated autophagy only in several genotypes, suggesting that despite being a resilience mechanism, autophagy is a heat-sensitive process; (ii) higher autophagic activity correlates positively with greater yield; (iii) the lack of autophagic activity in some high-yielding genotypes suggests contribution of alternative stress-resilient mechanisms; and (iv) enhanced autophagic activity in response to heat and drought was independently selected by wheat breeding programs in different geographic locations.

Jasmonate enhances cold acclimation in jojoba by promoting flavonol synthesis

Jojoba is an industrial oil crop planted in tropical arid areas, and its low-temperature sensitivity prevents its introduction into temperate areas. Studying the molecular mechanisms associated with cold acclimation in jojoba is advantageous for developing breeds with enhanced cold tolerance. In this study, metabolomic analysis revealed that various flavonols accumulate in jojoba during cold acclimation. Time-course transcriptomic analysis and weighted correlation network analysis (WGCNA) demonstrated that flavonol biosynthesis and jasmonates (JAs) signaling pathways played crucial roles in cold acclimation. Combining the biochemical and genetic analyses showed that ScMYB12 directly activated flavonol synthase gene (ScFLS). The interaction between ScMYB12 and transparent testa 8 (ScTT8) promoted the expression of ScFLS, but the negative regulator ScJAZ13 in the JA signaling pathway interacted with ScTT8 to attenuate the transcriptional activity of the ScTT8 and ScMYB12 complex, leading to the downregulation of ScFLS. Cold acclimation stimulated the production of JA in jojoba leaves, promoted the degradation of ScJAZ13, and activated the transcriptional activity of ScTT8 and ScMYB12 complexes, leading to the accumulation of flavonols. Our findings reveal the molecular mechanism of JA-mediated flavonol biosynthesis during cold acclimation in jojoba and highlight the JA pathway as a promising means for enhancing cold tolerance in breeding efforts.

Genome-wide identification of ATG genes and their expression profiles under biotic and abiotic stresses in Fenneropenaeus chinensis

BACKGROUND

Autophagy is a conserved catabolic process in eukaryotes that contributes to cell survival in response to multiple stresses and is important for organism fitness. Extensive research has shown that autophagy plays a pivotal role in both viral infection and replication processes. Despite the increasing research dedicated to autophagy, investigations into shrimp autophagy are relatively scarce.

RESULTS

Based on three different methods, a total of 20 members of the ATGs were identified from F. chinensis, all of which contained an autophagy domain. These genes were divided into 18 subfamilies based on their different C-terminal domains, and were found to be located on 16 chromosomes. Quantitative real-time PCR (qRT-PCR) results showed that ATG genes were extensively distributed in all the tested tissues, with the highest expression levels were detected in muscle and eyestalk. To clarify the comprehensive roles of ATG genes upon biotic and abiotic stresses, we examined their expression patterns. The expression levels of multiple ATGs showed an initial increase followed by a decrease, with the highest expression levels observed at 6 h and/or 24 h after WSSV injection. The expression levels of three genes (ATG1, ATG3, and ATG4B) gradually increased until 60 h after injection. Under low-salt conditions, 12 ATG genes were significantly induced, and their transcription abundance peaked at 96 h after treatment.

CONCLUSIONS

These results suggested that ATG genes may have significant roles in responding to various environmental stressors. Overall, this study provides a thorough characterization and expression analysis of ATG genes in F. chinensis, laying a strong foundation for further functional studies and promising potential in innate immunity.

Maize Autophagy-Related Protein ZmATG3 Confers Tolerance to Multiple Abiotic Stresses

Abiotic stresses pose a major increasing problem for the cultivation of maize. Autophagy plays a vital role in recycling and re-utilizing nutrients and adapting to stress. However, the role of autophagy in the response to abiotic stress in maize has not yet been investigated. Here, ZmATG3, which is essential for ATG8-PE conjugation, was isolated from the maize inbred line B73. The ATG3 sequence was conserved, including the C-terminal domains with HPC and FLKF motifs and the catalytic domain in different species. The promoter of the ZmATG3 gene contained a number of elements involved in responses to environmental stresses or hormones. Heterologous expression of ZmATG3 in yeast promoted the growth of strain under salt, mannitol, and low-nitrogen stress. The expression of ZmATG3 could be altered by various types of abiotic stress (200 mM NaCl, 200 mM mannitol, low N) and exogenous hormones (500 µM ABA). GUS staining analysis of ZmATG3-GUS transgenic Arabidopsis revealed that GUS gene activity increased after abiotic treatment. ZmATG3-overexpressing Arabidopsis plants had higher osmotic and salinity stress tolerance than wild-type plants. Overexpression of ZmATG3 up-regulated the expression of other AtATGs (AtATG3, AtATG5, and AtATG8b) under NaCl, mannitol and LN stress. These findings demonstrate that overexpression of ZmATG3 can improve tolerance to multiple abiotic stresses.

Autophagy receptor ZmNBR1 promotes the autophagic degradation of ZmBRI1a and enhances drought tolerance in maize

Drought stress is a crucial environmental factor that limits plant growth, development, and productivity. Autophagy of misfolded proteins can help alleviate the damage caused in plants experiencing drought. However, the mechanism of autophagy-mediated drought tolerance in plants remains largely unknown. Here, we cloned the gene for a maize (Zea mays) selective autophagy receptor, NEXT TO BRCA1 GENE 1 (ZmNBR1), and identified its role in the response to drought stress. We observed that drought stress increased the accumulation of autophagosomes. RNA sequencing and reverse transcription-quantitative polymerase chain reaction showed that ZmNBR1 is markedly induced by drought stress. ZmNBR1 overexpression enhanced drought tolerance, while its knockdown reduced drought tolerance in maize. Our results established that ZmNBR1 mediates the increase in autophagosomes and autophagic activity under drought stress. ZmNBR1 also affects the expression of genes related to autophagy under drought stress. Moreover, we determined that BRASSINOSTEROID INSENSITIVE 1A (ZmBRI1a), a brassinosteroid receptor of the BRI1-like family, interacts with ZmNBR1. Phenotype analysis showed that ZmBRI1a negatively regulates drought tolerance in maize, and genetic analysis indicated that ZmNBR1 acts upstream of ZmBRI1a in regulating drought tolerance. Furthermore, ZmNBR1 facilitates the autophagic degradation of ZmBRI1a under drought stress. Taken together, our results reveal that ZmNBR1 regulates the expression of autophagy-related genes, thereby increasing autophagic activity and promoting the autophagic degradation of ZmBRI1a under drought stress, thus enhancing drought tolerance in maize. These findings provide new insights into the autophagy degradation of brassinosteroid signaling components by the autophagy receptor NBR1 under drought stress.

Glucose uptake from the rhizosphere mediated by MdDOF3-MdHT1.2 regulates drought resistance in apple

In plants under drought stress, sugar content in roots increases, which is important for drought resistance. However, the molecular mechanisms for controlling the sugar content in roots during response to drought remain elusive. Here, we found that the MdDOF3-MdHT1.2 module-mediated glucose influx into the root is essential for drought resistance in apple (Malus × domestica). Drought induced glucose uptake from the rhizosphere and up-regulated the transcription of hexose transporter MdHT1.2. Compared with the wild-type plants, overexpression of MdHT1.2 promoted glucose uptake from the rhizosphere, thereby facilitating sugar accumulation in root and enhancing drought resistance, whereas silenced plants showed the opposite phenotype. Furthermore, ATAC-seq, RNA-seq and biochemical analysis demonstrated that MdDOF3 directly bound to the promoter of MdHT1.2 and was strongly up-regulated under drought. Overexpression of MdDOF3 in roots improved MdHT1.2-mediated glucose transport capacity and enhanced plant resistance to drought, but MdDOF3-RNAihr apple plants showed the opposite phenotype. Moreover, overexpression of MdDOF3 in roots did not attenuate drought sensitivity in MdHT1.2-RNAi plants, which was correlated with a lower glucose uptake capacity and glucose content in root. Collectively, our findings deciphered the molecular mechanism through which glucose uptake from the rhizosphere is mediated by MdDOF3-MdHT1.2, which acts to modulate sugar content in root and promote drought resistance.

The ZmbHLH47-ZmSnRK2.9 Module Promotes Drought Tolerance in Maize

Drought stress globally poses a significant threat to maize (Zea mays L.) productivity and the underlying molecular mechanisms of drought tolerance remain elusive. In this study, we characterized ZmbHLH47, a basic helix-loop-helix (bHLH) transcription factor, as a positive regulator of drought tolerance in maize. ZmbHLH47 expression was notably induced by both drought stress and abscisic acid (ABA). Transgenic plants overexpressing ZmbHLH47 displayed elevated drought tolerance and ABA responsiveness, while the zmbhlh47 mutant exhibited increased drought sensitivity and reduced ABA sensitivity. Mechanistically, it was revealed that ZmbHLH47 could directly bind to the promoter of ZmSnRK2.9 gene, a member of the subgroup III SnRK2 kinases, activating its expression. Furthermore, ZmSnRK2.9-overexpressing plants exhibited enhanced ABA sensitivity and drought tolerance, whereas the zmsnrk2.9 mutant displayed a decreased sensitivity to both. Notably, overexpressing ZmbHLH47 in the zmsnrk2.9 mutant closely resembled the zmsnrk2.9 mutant, indicating the importance of the ZmbHLH47-ZmSnRK2.9 module in ABA response and drought tolerance. These findings provided valuable insights and a potential genetic resource for enhancing the environmental adaptability of maize.

Comprehensive Analysis of Autophagy-Related Genes in Rice Immunity against Magnaporthe oryzae

Rice blast disease, caused by the fungus Magnaporthe oryzae, is a significant threat to rice production. Resistant cultivars can effectively resist the invasion of M. oryzae. Thus, the identification of disease-resistant genes is of utmost importance for improving rice production. Autophagy, a cellular process that recycles damaged components, plays a vital role in plant growth, development, senescence, stress response, and immunity. To understand the involvement of autophagy-related genes (ATGs) in rice immune response against M. oryzae, we conducted a comprehensive analysis of 37 OsATGs, including bioinformatic analysis, transcriptome analysis, disease resistance analysis, and protein interaction analysis. Bioinformatic analysis revealed that the promoter regions of 33 OsATGs contained cis-acting elements responsive to salicylic acid (SA) or jasmonic acid (JA), two key hormones involved in plant defense responses. Transcriptome data showed that 21 OsATGs were upregulated during M. oryzae infection. Loss-of-function experiments demonstrated that OsATG6c, OsATG8a, OsATG9b, and OsATG13a contribute to rice blast resistance. Additionally, through protein interaction analysis, we identified five proteins that may interact with OsATG13a and potentially contribute to plant immunity. Our study highlights the important role of autophagy in rice immunity and suggests that OsATGs may enhance resistance to rice blast fungus through the involvement of SA, JA, or immune-related proteins. These findings provide valuable insights for future efforts in improving rice production through the identification and utilization of autophagy-related genes.

Oxalic acid secretion alleviates saline-alkali stress in alfalfa by improving photosynthetic characteristics and antioxidant activity

Saline-alkali stress significantly affects the growth and yield of alfalfa (Medicago sativa L.). Organic acid secretion is crucial in alleviating abiotic stress-induced damage in plants. In this study, we evaluated the contents of the major organic acids secreted by the roots of tolerant (ZD) and sensitive (LYL) varieties of alfalfa under saline-alkali stress and investigated the effects of these organic acids on the growth, and physiological functions of alfalfa. Our results indicated that the oxalic acid (OA) content was the highest among the organic acids secreted from alfalfa roots under saline-alkali stress, and oxalic acid content was the most significantly different between the two varieties, ZD and LYL, compared to the contents of the other organic acids. Oxalic acid alleviated the inhibition of alfalfa growth caused by saline-alkali stress, improved photosynthetic characteristics, reduced the accumulation of reactive oxygen species, and increased the activity of antioxidant enzymes and content of osmoregulatory substances. Furthermore, oxalic acid resulted in significantly increased expression of genes involved in photosynthesis and antioxidant system in alfalfa under saline-alkali stress. This study revealed the effects of oxalic acid secreted by the root system on stress-related physiological processes, providing valuable insights into the functions of root secretions in plant saline-alkali resistance.

Melatonin-mediated regulation of autophagy is independent of ABA under drought stress in sensitive variety of Gossypium hirsutum L

Autophagy is a highly conserved process that plays a crucial role in adaptation of plants to stress conditions. Melatonin and abscisic acid (ABA) share an antagonistic relationship; however, both are reported to elevate autophagy individually. Here, we report that melatonin alleviates drought stress effects like wilting and stunted growth in 18-day-old plants of drought-sensitive variety of cotton (Gossypium hirsutum L.) and improves the plant growth, chlorophyll content, photosynthetic efficiency, and sugar metabolism and transport. Melatonin priming increased the endogenous melatonin content (5.02-times) but decreased the ABA (2.63-times) by reducing NCED3 expression as compared to unprimed plants under drought. Also, elevated expression of ATG8c and ATG8f correlated with higher lipidated-ATG8 levels and modulation of RAPTOR1 suggesting a higher occurrence of autophagy and regulation of plant growth in primed stressed plants. Additionally, decreased TPS63 and increased TPP22 expression could have lowered the accumulation of trehalose-6-P (T6P) in primed stressed plants thus contributing to autophagy progression. Priming also enhanced the expression of MAPK6 and RAF18, and increased the transcript/protein levels of SnRK2.6 and KIN10, which is pointing towards melatonin's beneficial effect on autophagy under drought. Despite higher ABA content, elevated TPS63 and downregulated TPP22 could have hindered autophagy induction in unprimed stressed plants. Although fluridone treatment reduced the ABA content, the expression of SnRK2.6 and KIN10 remained unaltered in fluridone-treated and untreated primed plants indicating the ABA-independent expression. These results suggest that the melatonin-mediated activation of MAPK contributes to the ABA-independent activation of SnRK2, consequently, SnRK1 and autophagy under drought.

The apple autophagy-related gene MdATG10 improves drought tolerance and water use efficiency in transgenic apple plants

The Loess Plateau is the main apple production area in China; low precipitation is one of the most important factors limiting apple production here. Autophagy is a conserved process in eukaryotes that recycles cell contents or damaged macromolecules. Previously, we identified an autophagy-related gene MdATG10 from apple plants, which was involved in the responses to stressed conditions. In this study, we found that MdATG10 improved the drought tolerance and water use efficiency (WUE) of transgenic apple plants. MdATG10-overexpressing (OE) apple plants were more tolerant of short-term drought stress, as evidenced by their fewer drought-related injuries, compared with wild-type (WT) apple plants. In addition, the WUE of OE plants was higher than that of WT plants under long-term moderate water deficit conditions. The growth rate, biomass accumulation, photosynthetic efficiency, and stomatal aperture were higher in OE plants than in WT plants under long-term moderate drought conditions. During the process of adapting to drought, the expressions of genes involved in the abscisic acid (ABA) pathway were reduced in OE plants to decrease the synthesis of ABA, which helped maintain the stomatal opening for gas exchange. Furthermore, autophagic activity was higher in OE plants than in WT plants, as evidenced by the higher expressions of ATG genes and the greater number of autophagy bodies. In sum, our results suggested that overexpression of MdATG10 improved drought tolerance and WUE in apple plants, possibly by regulating stomatal movement and enhancing autophagic activity, which then enhanced the photosynthetic efficiency and reduced damage, as well as the reactive oxygen species (ROS) accumulation in apple plants.

VvATG6 contributes to copper stress tolerance by enhancing the antioxidant ability in transgenic grape calli

Autophagy, a conserved degradation and reuse process, plays a crucial role in plant cellular homeostasis during abiotic stress. Although numerous autophagy-related genes (ATGs) that regulate abiotic stress have been identified, few functional studies have shown how they confer tolerance to copper (Cu) stress. Here, we cloned a novel Vitis vinifera ATG6 gene (VvATG6) which was induced by 0.5 and 10 mM Cu stress based on transcriptomic data, and transgenic Arabidopsis thaliana, tobacco (Nicotiana tabacum), and grape calli were successfully obtained through Agrobacterium-mediated genetic transformation. The overexpression of VvATG6 enhanced the tolerance of transgenic lines to Cu. After Cu treatment, the lines that overexpressed VvATG6 grew better and increased their production of biomass compared with the wild-type. These changes were accompanied by higher activities of antioxidant enzymes and a lower accumulation of deleterious malondialdehyde and hydrogen peroxide in the transgenic plants. The activities of superoxide dismutase, peroxidase, and catalase were enhanced owing to the elevation of corresponding antioxidant gene expression in the VvATG6 overexpression plants under Cu stress, thereby promoting the clearance of reactive oxygen species (ROS). Simultaneously, there was a decrease in the levels of expression of RbohB and RbohC that are involved in ROS synthesis in transgenic plants under Cu stress. Thus, the accelerated removal of ROS and the inhibition of its synthesis led to a balanced ROS homeostasis environment, which alleviated the damage from Cu. This could benefit from the upregulation of other ATGs that are necessary for the production of autophagosomes under Cu stress. To our knowledge, this study is the first to demonstrate the protective role of VvATG6 in the Cu tolerance of plants.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12298-024-01415-y.

MdHB7-like positively modulates apple salt tolerance by promoting autophagic activity and Na+ efflux

Salt stress adversely affects the yield and quality of crops and limits their geographical distribution. Studying the functions and regulatory mechanisms of key genes in the salt stress response is important for breeding crops with enhanced stress resistance. Autophagy plays an important role in modulating the tolerance of plants to various types of abiotic stressors. However, the mechanisms underlying salt-induced autophagy are largely unknown. Cation/Ca2+ exchanger proteins enhance apple salt tolerance by inhibiting Na+ accumulation but the mechanism underlying the response to salt stress remains unclear. Here, we show that the autophagy-related gene MdATG18a modulated apple salt tolerance. Under salt stress, the autophagic activity, proline content, and antioxidant enzyme activities were higher and Na+ accumulation was lower in MdATG18a-overexpressing transgenic plants than in control plants. The use of an autophagy inhibitor during the salt treatment demonstrated that the regulatory function of MdATG18a depended on autophagy. The yeast-one-hybrid assay revealed that the homeodomain-leucine zipper (HD-Zip) transcription factor MdHB7-like directly bound to the MdATG18a promoter. Transcriptional regulation and genetic analyses showed that MdHB7-like enhanced salt-induced autophagic activity by promoting MdATG18a expression. The analysis of Na+ efflux rate in transgenic yeast indicated that MdCCX1 expression significantly promoted Na+ efflux. Promoter binding, transcriptional regulation, and genetic analyses showed that MdHB7-like promoted Na+ efflux and apple salt tolerance by directly promoting MdCCX1 expression, which was independent of the autophagy pathway. Overall, our findings provide insight into the mechanism underlying MdHB7-like-mediated salt tolerance in apple through the MdHB7-like-MdATG18a and MdHB7-like-MdCCX1 modules. These results will aid future studies on the mechanisms underlying stress-induced autophagy and the regulation of stress tolerance in plants.

Populus euphratica plant cadmium resistance 2 mediates Cd tolerance by root efflux of Cd ions in poplar

KEY MESSAGE

Populus euphratica PePCR2 increases Cd resistance by functioning as a Cd extrusion pump and by mediating the expression of genes encoding other transporters. Cadmium (Cd) is a non-essential, toxic metal that negatively affects plant growth. Plant cadmium resistance (PCR) proteins play key roles in the response to heavy metal stress. In this study, we isolated the gene PePCR2 encoding a plant PCR from Populus euphratica. PePCR2 gene transcription was induced by Cd, and its transcript level peaked at 24 h after exposure, at a level approximately 18-fold higher than that at 0 h. The PePCR2 protein was localized to the plasma membrane. Compared with yeast cells harboring the empty vector, yeast cells expressing PePCR2 showed enhanced Cd tolerance and a lower Cd content. Compared with wild-type (WT) plants, poplar overexpressing PePCR2 showed higher Cd resistance. Net Cd2+ efflux measurements showed that Cd2+ efflux from the roots was 1.5 times higher in the PePCR2-overexpressing plants than in WT plants. Furthermore, compared with WT plants, the PePCR2-overexpressing plants showed increased transcript levels of ABCG29, HMA5, PDR2, YSL7, and ZIP1 and decreased transcript levels of NRAMP6, YSL3, and ZIP11 upon exposure to Cd. These data show that PePCR2 increased Cd resistance by acting as a Cd extrusion pump and/or by regulating other Cd2+ transporters to decrease Cd toxicity in the cytosol. The results of this study identify a novel plant gene with potential applications in Cd removal, and provide a theoretical basis for reducing Cd toxicity and protecting food safety.

MdNAC104 positively regulates apple cold tolerance via CBF-dependent and CBF-independent pathways

Low temperature is the main environmental factor affecting the yield, quality and geographical distribution of crops, which significantly restricts development of the fruit industry. The NAC (NAM, ATAF1/2 and CUC2) transcription factor (TF) family is involved in regulating plant cold tolerance, but the mechanisms underlying these regulatory processes remain unclear. Here, the NAC TF MdNAC104 played a positive role in modulating apple cold tolerance. Under cold stress, MdNAC104-overexpressing transgenic plants exhibited less ion leakage and lower ROS (reactive oxygen species) accumulation, but higher contents of osmoregulatory substances and activities of antioxidant enzymes. Transcriptional regulation analysis showed that MdNAC104 directly bound to the MdCBF1 and MdCBF3 promoters to promote expression. In addition, based on combined transcriptomic and metabolomic analyses, as well as promoter binding and transcriptional regulation analyses, we found that MdNAC104 stimulated the accumulation of anthocyanin under cold conditions by upregulating the expression of anthocyanin synthesis-related genes, including MdCHS-b, MdCHI-a, MdF3H-a and MdANS-b, and increased the activities of the antioxidant enzymes by promoting the expression of the antioxidant enzyme-encoding genes MdFSD2 and MdPRXR1.1. In conclusion, this study revealed the MdNAC104 regulatory mechanism of cold tolerance in apple via CBF-dependent and CBF-independent pathways.

Autophagy during maize endosperm development dampens oxidative stress and promotes mitochondrial clearance

The selective turnover of macromolecules by autophagy provides a critical homeostatic mechanism for recycling cellular constituents and for removing superfluous and damaged organelles, membranes, and proteins. To better understand how autophagy impacts seed maturation and nutrient storage, we studied maize (Zea mays) endosperm in its early and middle developmental stages via an integrated multiomic approach using mutants impacting the core macroautophagy factor AUTOPHAGY (ATG)-12 required for autophagosome assembly. Surprisingly, the mutant endosperm in these developmental windows accumulated normal amounts of starch and Zein storage proteins. However, the tissue acquired a substantially altered metabolome, especially for compounds related to oxidative stress and sulfur metabolism, including increases in cystine, dehydroascorbate, cys-glutathione disulfide, glucarate, and galactarate, and decreases in peroxide and the antioxidant glutathione. While changes in the associated transcriptome were mild, the proteome was strongly altered in the atg12 endosperm, especially for increased levels of mitochondrial proteins without a concomitant increase in mRNA abundances. Although fewer mitochondria were seen cytologically, a heightened number appeared dysfunctional based on the accumulation of dilated cristae, consistent with attenuated mitophagy. Collectively, our results confirm that macroautophagy plays a minor role in the accumulation of starch and storage proteins during maize endosperm development but likely helps protect against oxidative stress and clears unneeded/dysfunctional mitochondria during tissue maturation.

Integrated metabolomics and transcriptomics analysis reveals that the change of apoplast metabolites contributes to adaptation to winter freezing stress in Euonymus japonicus

Euonymus japonicus, a common urban street tree, can withstand winter freezing stress in temperate regions. The apoplast is the space outside the plasma membrane, and the changes of metabolites in apoplast may be involved in plant adaptation to adverse environments. To reveal the molecular mechanism underlying the winter freezing stress tolerance in E. japonicus, the changes in physiological and biochemical indexes, apoplast metabolites, and gene expression in the leaves of E. japonicus in early autumn and winter were analyzed. A total of 300 differentially accumulated metabolites were identified in apoplast fluids in E. japonicus, which were mainly related to flavone and flavonol biosynthesis, and galactose metabolism, amino acid synthesis, and unsaturated fatty acid synthesis. Integrated metabolomics and transcriptomics analysis revealed that E. japonicus adjust apoplast metabolites including flavonoids such as quercetin and kaempferol, and oligosaccharides such as raffinose and stachyose, to adapt to winter freezing stress through gene expression regulation. In addition, the regulation of ABA and SA biosynthesis and signal transduction pathways, as well as the activation of the antioxidant enzymes, also played important roles in the adaptation to winter freezing stress in E. japonicus. The present study provided essential data for understanding the molecular mechanism underlying the adaptation to winter freezing stress in E. japonicus.

Alfalfa MsATG13 Confers Cold Stress Tolerance to Plants by Promoting Autophagy

Autophagy is a conserved cellular process that functions in the maintenance of physiological and metabolic balance. It has previously been demonstrated to improve plant tolerance to abiotic stress. Numerous autophagy-related genes (ATGs) that regulate abiotic stress have been identified, but there have been few functional studies showing how ATGs confer cold stress tolerance. The cold transcriptome data of the crown buds that experienced overwintering of the alfalfa (Medicago sativa L.) showed that MsATG13 is upregulated in response to cold stress. In the present study, we found that MsATG13 transgenic tobacco enhanced cold tolerance compared to wild-type (WT) plants. Transmission electron microscopy demonstrated that transgenic tobacco overexpressing MsATG13 formed more autophagosomes than WT plants in response to cold stress conditions. The transgenic tobacco increased autophagy levels due to upregulation of other ATGs that were necessary for autophagosome production under cold stress conditions. MsATG13 transgenic tobacco also increased the proline contents and antioxidant enzyme activities, enhancing the antioxidant defense capabilities under cold stress conditions. Furthermore, MsATG13 overexpression decreased levels of superoxide anion radicals and hydrogen peroxide under cold stress conditions. These findings demonstrate the role of MsATG13 in enhancing plant cold tolerance through modulation of autophagy and antioxidant levels.

Fruit crops combating drought: Physiological responses and regulatory pathways

Drought is a common stress in agricultural production. Thus, it is imperative to understand how fruit crops respond to drought and to develop drought-tolerant varieties. This paper provides an overview of the effects of drought on the vegetative and reproductive growth of fruits. We summarize the empirical studies that have assessed the physiological and molecular mechanisms of the drought response in fruit crops. This review focuses on the roles of calcium (Ca2+) signaling, abscisic acid (ABA), reactive oxygen species signaling, and protein phosphorylation underlying the early drought response in plants. We review the resulting downstream ABA-dependent and ABA-independent transcriptional regulation in fruit crops under drought stress. Moreover, we highlight the positive and negative regulatory mechanisms of microRNAs in the drought response of fruit crops. Lastly, strategies (including breeding and agricultural practices) to improve the drought resistance of fruit crops are outlined.

Role of autophagy-related proteins ATG8f and ATG8h in the maintenance of autophagic activity in Arabidopsis roots under phosphate starvation

Nutrient starvation-induced autophagy is a conserved process in eukaryotes. Plants defective in autophagy show hypersensitivity to carbon and nitrogen limitation. However, the role of autophagy in plant phosphate (Pi) starvation response is relatively less explored. Among the core autophagy-related (ATG) genes, ATG8 encodes a ubiquitin-like protein involved in autophagosome formation and selective cargo recruitment. The Arabidopsis thaliana ATG8 genes, AtATG8f and AtATG8h, are notably induced in roots under low Pi. In this study, we show that such upregulation correlates with their promoter activities and can be suppressed in the phosphate response 1 (phr1) mutant. Yeast one-hybrid analysis failed to attest the binding of the AtPHR1 transcription factor to the promoter regions of AtATG8f and AtATG8h. Dual luciferase reporter assays in Arabidopsis mesophyll protoplasts also indicated that AtPHR1 could not transactivate the expression of both genes. Loss of AtATG8f and AtATG8h leads to decreased root microsomal-enriched ATG8 but increased ATG8 lipidation. Moreover, atg8f/atg8h mutants exhibit reduced autophagic flux estimated by the vacuolar degradation of ATG8 in the Pi-limited root but maintain normal cellular Pi homeostasis with reduced number of lateral roots. While the expression patterns of AtATG8f and AtATG8h overlap in the root stele, AtATG8f is more strongly expressed in the root apex and root hair and remarkably at sites where lateral root primordia develop. We hypothesize that Pi starvation-induction of AtATG8f and AtATG8h may not directly contribute to Pi recycling but rely on a second wave of transcriptional activation triggered by PHR1 that fine-tunes cell type-specific autophagic activity.

The m6A reader MhYTP2 regulates the stability of its target mRNAs contributing to low nitrogen tolerance in apple (Malus domestica)

Studies have shown that the m⁶A reader primarily affects genes expression by participating in the regulation of mRNA localization, splicing, degradation, translation, and other metabolic processes. Previously, we discovered that the apple (Malus domestica) m⁶A reader MhYTP2 bound with and destabilized m⁶A-modified MdMLO19 mRNA. In addition, it enhanced the translation efficiency of m⁶A-modified mRNA of MdGDH1L, encoding a glutamate dehydrogenase, which confers resistance to powdery mildew. In this study, we report the function of MhYTP2 in the regulation of resistance to low nitrogen (N). The overexpression of MhYTP2 enhances the resistance of apple to low N. We show that MhYTP2 binds with and stabilizes the mRNAs of MdALN, which participates in the allantoin catabolic process and cellular response to N starvation in apple; MdPIDL, which participates in root hair elongation; MdTTG1, which is involved in the differentiation process of trichomes; and MdATG8A, which is a core participant in the regulation of autophagy. In addition, MhYTP2 accelerates the degradation of MdRHD3 mRNA, which regulates root development. RNA immunoprecipitation-seq and electrophoretic mobility shift assays show that the mRNAs of MdALN, MdATG8A, MdPIDL, MdTTG1, and MdRHD3 are the direct targets of MhYTP2. Overexpressing or knocking down the above genes in MhYTP2 overexpressing plants dismisses the function of MhYTP2 under low N, suggesting the role of MhYTP2 is dependent on those genes. Together, these results demonstrate that MhYTP2 enhances the resistance of apple to N deficiency by affecting the stability of the bound mRNAs.

γ-Aminobutyric acid plays a key role in alleviating Glomerella leaf spot in apples

The fungal disease Glomerella leaf spot (GLS) seriously impacts apple production. As a nonprotein amino acid, γ-aminobutyric acid (GABA) is widely involved in biotic and abiotic stresses. However, it is not clear whether GABA is involved in a plant's response to GLS, nor is its molecular mechanism understood. Here, we found that exogenous GABA could significantly alleviate GLS, reduce lesion lengths, and increase antioxidant capacity. MdGAD1 was identified as a possible key gene for GABA synthesis in apple. Further analysis indicated that MdGAD1 promoted antioxidant capacity to improve apple GLS resistance in transgenic apple calli and leaves. Yeast one-hybrid analysis identified the transcription factor MdWRKY33 upstream of MdGAD1. Electrophoretic mobility shift assay, β-glucuronidase activity, and luciferase activity further supported that MdWRKY33 bound directly to the promoter of MdGAD1. The content of GABA and the transcription level of MdGAD1 in the MdWRKY33 transgenic calli were higher than that of the wild type. When MdWRKY33 transgenic calli and leaves were inoculated with GLS, MdWKRY33 positively regulated resistance to GLS. These results explained the positive regulatory effects of GABA on apple GLS and provided insight into the metabolic regulatory network of GABA.

Comparative Physiological and Transcriptome Analysis Reveals Potential Pathways and Specific Genes Involved in Waterlogging Tolerance in Apple Rootstocks

Apple (Malus × domestica Borkh.) is one of the most cultivated fruit crops in China. Apple trees frequently encounter waterlogging stress, mainly due to excess rainfall, soil compaction, or poor soil drainage, results in yellowing leaves and declined fruit quality and yield in some regions. However, the mechanism underlying the response to waterlogging has not been well elucidated. Therefore, we performed a physiological and transcriptomic analysis to examine the differential responses of two apple rootstocks (waterlogging-tolerant M. hupehensis and waterlogging-sensitive M. toringoides) to waterlogging stress. The results showed that M. toringoides displayed more severe leaf chlorosis during the waterlogging treatment than M. hupehensis. Compared with M. hupehensis, the more severe leaf chlorosis induced by waterlogging stress in M. toringoides was highly correlated with increased electrolyte leakage and superoxide radicals, hydrogen peroxide accumulation, and increased stomata closure. Interestingly, M. toringoides also conveyed a higher ethylene production under waterlogging stress. Furthermore, RNA-seq revealed that a total of 13,913 common differentially expressed genes (DEGs) were differentially regulated between M. hupehensis and M. toringoides under waterlogging stress, especially those DEGs involved in the biosynthesis of flavonoids and hormone signaling. This suggests a possible link of flavonoids and hormone signaling to waterlogging tolerance. Taken together, our data provide the targeted genes for further investigation of the functions, as well as for future molecular breeding of waterlogging-tolerant apple rootstocks.

The ubiquitin-binding protein MdRAD23D1 mediates drought response by regulating degradation of the proline-rich protein MdPRP6 in apple (Malus domestica)

RAD23 (RADIATION SENSITIVE23) proteins are a group of UBL-UBA (ubiquitin-like-ubiquitin-associated) proteins that shuttle ubiquitylated proteins to the 26S proteasome for breakdown. Drought stress is a major environmental constraint that limits plant growth and production, but whether RAD23 proteins are involved in this process is unclear. Here, we demonstrated that a shuttle protein, MdRAD23D1, mediated drought response in apple plants (Malus domestica). MdRAD23D1 levels increased under drought stress, and its suppression resulted in decreased stress tolerance in apple plants. Through in vitro and in vivo assays, we demonstrated that MdRAD23D1 interacted with a proline-rich protein MdPRP6, resulting in the degradation of MdPRP6 by the 26S proteasome. And MdRAD23D1 accelerated the degradation of MdPRP6 under drought stress. Suppression of MdPRP6 resulted in enhanced drought tolerance in apple plants, mainly because the free proline accumulation is changed. And the free proline is also involved in MdRAD23D1-mediated drought response. Taken together, these findings demonstrated that MdRAD23D1 and MdPRP6 oppositely regulated drought response. MdRAD23D1 levels increased under drought, accelerating the degradation of MdPRP6. MdPRP6 negatively regulated drought response, probably by regulating proline accumulation. Thus, "MdRAD23D1-MdPRP6" conferred drought stress tolerance in apple plants.

Overexpression of the FERONIA receptor kinase MdMRLK2 enhances apple cold tolerance

Cold is one of the main abiotic stresses in temperate fruit crops, affecting the yield and fruit quality of apple in China and European countries. The plant receptor-like kinase FERONIA is widely reported to be involved in abiotic stresses. However, its function in apple cold resistance remains unknown. Modification of cell wall components and accumulation of soluble sugars and amino acids are important strategies by which plants cope with cold. In this study, expression of the apple FERONIA receptor-like kinase gene MdMRLK2 was rapidly induced by cold. Apple plants overexpressing MdMRLK2 (35S:MdMRLK2) showed enhanced cold resistance relative to the wild type. Under cold conditions, 35S:MdMRLK2 apple plants had higher amounts of water insoluble pectin, lignin, cellulose, and hemicellulose, which may have resulted from reduced activities of polygalacturonase, pectinate lyase, pectinesterase, and cellulase. More soluble sugars and free amino acids and less photosystem damage were also observed in 35S:MdMRLK2 apple plants. Intriguingly, MdMRLK2 interacted with the transcription factor MdMYBPA1 and promoted its binding to MdANS and MdUFGT promoters, leading to more anthocyanin biosynthesis, particularly under cold conditions. These findings complemented the function of apple FERONIA MdMRLK2 responding to cold resistance.

MdbZIP74 negatively regulates osmotic tolerance and adaptability to moderate drought conditions of apple plants

Drought is the most prominent threat to global agricultural production. The basic leucine zipper (bZIP) family is related to the response to a series of abiotic stress. In this case, apple calli and the seedlings of MdbZIP74-RNAi transgenic lines were obtained. Under osmotic stress and moderate drought conditions, the content of malondialdehyde, relative water content and other stress-related assays were measured. MdbZIP74 was found to negatively regulate the osmotic tolerance of apple callus. The growth of MdbZIP74-RNAi calli enhanced resistance without significant production loss. The silencing of MdbZIP74 contributes to redox balance and the adaptability of apple seedlings to moderate drought conditions. Four related differentially expressed genes in the biosynthesis of cytokinin and catabolic pathway were identified through a transcriptome analysis of MdbZIP74-RNAi seedlings under moderate drought conditions. MdLOG8 was further identified as the target of MdbZIP74 involved in the drought adaptability of apple plants using a dual experiment. Further confirmation showed MdLOG8 was maintained in the MdbZIP74-RNAi seedlings presumably acting as the growth regulator to enhance drought adaptability. It was concluded that the correct regulation of cytokinin level under moderate drought conditions maintains the redox balance and avoids the situation of plants surviving with the minimal resources.

Abiotic stress tolerance in plants: a fascinating action of defense mechanisms

Climate fluctuation mediated abiotic stress consequences loss in crop yields. These stresses have a negative impact on plant growth and development by causing physiological and molecular changes. In this review, we have attempted to outline recent studies (5 years) associated with abiotic stress resistance in plants. We investigated the various factors that contribute to coping with abiotic challenges, such as transcription factors (TFs), microRNAs (miRNAs), epigenetic changes, chemical priming, transgenic breeding, autophagy, and non-coding RNAs. Stress responsive genes are regulated mostly by TFs, and these can be used to enhance stress resistance in plants. Plants express some miRNA during stress imposition that act on stress-related target genes to help them survive. Epigenetic alterations govern gene expression and facilitate stress tolerance. Chemical priming enhances growth in plants by modulating physiological parameters. Transgenic breeding enables identification of genes involved in precise plant responses during stressful situations. In addition to protein coding genes, non-coding RNAs also influence the growth of the plant by causing alterations at gene expression levels. For achieving sustainable agriculture for a rising world population, it is crucial to develop abiotic-resistant crops with anticipated agronomical traits. To achieve this objective, understanding the diverse mechanisms by which plants protect themselves against abiotic stresses is imperative. This review emphasizes on recent progress and future prospects for abiotic stress tolerance and productivity in plants.

MicroRNA156ab regulates apple plant growth and drought tolerance by targeting transcription factor MsSPL13

Drought stress substantially reduces the productivity of apple plants and severely restricts the development of the apple industry. Malus sieversii, a wild apple with excellent drought resistance, is a valuable wild resource for rootstock improvement of cultivated apple (Malus domestica). miRNAs and their targets play essential roles in plant growth and stress responses, but their roles in drought stress responses in apple are unknown. Here, we demonstrate that microRNA156ab is upregulated in M. sieversii in response to drought stress. Overexpressing msi-miR156ab promoted auxin accumulation, maintained the growth of apple plants, and increased plant resistance to osmotic stress. Antioxidant enzyme activities and proline contents were also increased in miR156ab-OE transgenic apple lines, which improved drought resistance. The SQUAMOSA PROMOTER BINDING PROTEIN-LIKE (SPL) transcription factor MsSPL13 is the target of msi-miR156ab, as demonstrated by 5-RACE and dual luciferase assays. Heterologous expression of MsSPL13 decreased auxin contents and inhibited growth in Arabidopsis (Arabidopsis thaliana) under normal and stress conditions. The activities of antioxidant enzymes were also suppressed in MsSPL13-OE transgenic Arabidopsis, reducing drought resistance. We showed that MsSPL13 regulates the expression of the auxin-related genes MsYUCCA5, PIN-FORMED7 (MsPIN7), and Gretchen Hagen3-5 (MsGH3-5) by binding to the GTAC cis-elements in their promoters, thereby regulating auxin metabolism. Finally, we demonstrated that the miR156ab-SPL13 module is involved in mediating the difference in auxin metabolism and stress responses between the M. sieversii and M26 (M. domestica) rootstocks. Overall, these findings reveal that the miR156ab-SPL13 module enhances drought stress tolerance in apples by regulating auxin metabolism and antioxidant enzyme activities.

Calcyclin-binding protein-promoted degradation of MdFRUCTOKINASE2 regulates sugar homeostasis in apple

Fructokinase (FRK) activates fructose through phosphorylation, which sends the activated fructose into primary metabolism and regulates fructose signaling capabilities in plants. The apple (Malus × domestica) FRK gene MdFRK2 shows especially high affinity to fructose, and its overexpression decreases fructose levels in the leaves of young plants. However, in the current study of mature plants, fruits of transgenic apple trees overexpressing MdFRK2 accumulated a higher level of fructose than wild-type (WT) fruits (at both young and mature stages). Transgenic apple trees with high mRNA MdFRK2 expression showed no significant differences in MdFRK2 protein abundance or FRK enzyme activity compared to WT in mature leaves, young fruits, and mature fruits. Immunoprecipitation-mass spectrometry analysis identified an skp1, cullin, F-box (SCF) E3 ubiquitin ligase, calcyclin-binding protein (CacyBP), that interacted with MdFRK2. RNA-sequencing analysis provided evidence for ubiquitin-mediated post-transcriptional regulation of MdFRK2 protein for the maintenance of fructose homeostasis in mature leaves and fruits. Further analyses suggested an MdCacyBP-MdFRK2 regulatory module, in which MdCacyBP interacts with and ubiquitinates MdFRK2 to facilitate its degradation by the 26S proteasome, thus decreasing the FRK enzyme activity to elevate fructose concentration in transgenic apple trees. This result uncovered an important mechanism underlying plant fructose homeostasis in different organs through regulating the MdFRK2 protein level via ubiquitination and degradation. Our study provides usable data for the future improvement of apple flavor and expands our understanding of the molecular mechanisms underlying plant fructose content and signaling regulation.

Exogenous Dopamine and MdTyDC Overexpression Enhance Apple Resistance to Fusarium solani

Fusarium solani, one of the main pathogenic fungi involved in apple replant disease (ARD), is a serious threat to apple growth and development. Dopamine and tyrosine decarboxylase (TyDC), a key enzyme in the dopamine synthesis pathway, have been reported to play an active role in plant responses to biotic and abiotic stresses, but little is known about the functions of dopamine and Malus domestica TyDC (MdTyDC) in the interaction between F. solani and apple roots. In this study, seedlings treated with exogenous dopamine and apple plants overexpressing MdTyDC were inoculated with F. solani; both treatments reduced the root system damage caused by F. solani. After inoculation with F. solani, exogenous dopamine increased dopamine content in the seedlings; alleviated the inhibition of biomass accumulation; increased root metabolic activity, photosynthetic efficiency, and antioxidant enzyme activities; reduced reactive oxygen species accumulation; and upregulated the expression of genes encoding chitinase, β-1,3-glucanase, and pathogenesis-related proteins. Similar results were observed in MdTyDC-overexpressing apple plants. In addition, the overexpression of MdTyDC increased tyramine content and the deposition of cell wall-bound amines in roots. Overall, our results reveal that exogenous dopamine and overexpression of MdTyDC enhance apple resistance to F. solani, which is an important application for the prevention of ARD.

Genome-Wide Identification of ATG Gene Family Members in Fagopyrum tataricum and Their Expression during Stress Responses

Abiotic stresses such as drought and salinity are major environmental factors limiting plant productivity. Autophagy-related genes are extensively involved in plant growth, development, and adverse stress responses, which have not yet been characterized in Tartary buckwheat (Fagopyrum tataricum, TB). In this study, we verified that drought stress could induce autophagy in TB roots. Next, 49 FtATGs in the whole genome of TB were identified. All FtATGs were randomly distributed in 8 known chromosomes, while 11 FtATGs were predictably segmental repeats. As the core component of autophagy, there were 8 FtATG8s with similar gene structures in TB, while FtATG8s showed high expression at the transcription level under drought and salt stresses. The cis-acting element analysis identified that all FtATG8 promoters contain light-responsive and MYB-binding elements. FtATG8s showed a cell-wide protein interaction network and strongly correlated with distinct stress-associated transcription factors. Furthermore, overexpression of FtATG8a and FtATG8f enhanced the antioxidant enzyme activities of TB under adverse stresses. Remarkably, FtATG8a and FtATG8f may be vital candidates functioning in stress resistance in TB. This study prominently aids in understanding the biological role of FtATG genes in TB.

Comparison of lauric acid and 12-hydroxylauric acid in the alleviation of drought stress in peach (Prunus persica (L.) Batsch)

Water shortage is a key factor that can restrict peach tree growth. Plants produce fatty acids and the fatty acid derivatives lauric acid (LA) and 12-hydroxylauric acid (LA-OH), which are involved in abiotic stress responses, but the underlying stress response mechanisms remain unclear. In this study, physiological examination revealed that in Prunus persica (L.) Batsch, pretreatment with 50 ppm LA-OH and LA reduced drought stress, efficiently maintained the leaf relative water content, and controlled the relative conductivity increase. Under drought stress, LA-OH and LA treatments prevented the degradation of photosynthetic pigments, increased the degree of leaf stomatal opening and enhanced the net photosynthetic rate. Compared with drought stress, LA-OH and LA treatment effectively increased the net photosynthetic rate by 204.55% and 115.91%, respectively, while increasing the Fv/Fm by 2.75% and 7.75%, respectively, but NPQ decreased by 7.67% and 37.54%, respectively. In addition, the level of reactive oxygen species increased under drought stress. The content of O₂ ^- in LA-OH and LA treatment decreased by 12.91% and 11.24% compared to CK-D, respectively, and the content of H₂O₂ decreased by 13.73% and 19.94%, respectively. At the same time, the content of malondialdehyde (MDA) decreased by 55.56% and 58.48%, respectively. We believe that the main reason is that LA-OH and LA treatment have improved the activity of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT). The application of exogenous LA increased the levels of soluble sugars, soluble proteins, proline and free amino acids under drought stress, and maintained the osmotic balance of cells. Compared with CK-D treatment, it increased by 24.11%, 16.89%, 29.3% and 15.04%, respectively. At the same time, the application of exogenous LA-OH also obtained similar results. In conclusion, exogenous LA-OH and LA can alleviate the damage to peach seedlings caused by drought stress by enhancing the photosynthetic and antioxidant capacities, increasing the activities of protective enzymes and regulating the contents of osmotic regulators, but the molecular mechanism is still in need of further exploration.

Methylation of a MITE insertion in the MdRFNR1-1 promoter is positively associated with its allelic expression in apple in response to drought stress

Miniature inverted-repeat transposable elements (MITEs) are widely distributed in the plant genome and can be methylated. However, whether DNA methylation of MITEs is associated with induced allelic expression and drought tolerance is unclear. Here, we identified the drought-inducible MdRFNR1 (root-type ferredoxin-NADP+ oxidoreductase) gene in apple (Malus domestica). MdRFNR1 plays a positive role in drought tolerance by regulating the redox system, including increasing NADP+ accumulation and catalase and peroxidase activities and decreasing NADPH levels. Sequence analysis identified a MITE insertion (MITE-MdRF1) in the promoter of MdRFNR1-1 but not the MdRFNR1-2 allele. MdRFNR1-1 but not MdRFNR1-2 expression was significantly induced by drought stress, which was positively associated with the MITE-MdRF1 insertion and its DNA methylation. The methylated MITE-MdRF1 is recognized by the transcriptional anti-silencing factors MdSUVH1 and MdSUVH3, which recruit the DNAJ domain-containing proteins MdDNAJ1, MdDNAJ2, and MdDNAJ5, thereby activating MdRFNR1-1 expression under drought stress. Finally, we showed that MdSUVH1 and MdDNAJ1 are positive regulators of drought tolerance. These findings illustrate the molecular roles of methylated MITE-MdRF1 (which is recognized by the MdSUVH-MdDNAJ complex) in induced MdRFNR1-1 expression as well as the drought response of apple and shed light on the molecular mechanisms of natural variation in perennial trees.

Autophagy in the Lifetime of Plants: From Seed to Seed

Autophagy is a highly conserved self-degradation mechanism in eukaryotes. Excess or harmful intracellular content can be encapsulated by double-membrane autophagic vacuoles and transferred to vacuoles for degradation in plants. Current research shows three types of autophagy in plants, with macroautophagy being the most important autophagic degradation pathway. Until now, more than 40 autophagy-related (ATG) proteins have been identified in plants that are involved in macroautophagy, and these proteins play an important role in plant growth regulation and stress responses. In this review, we mainly introduce the research progress of autophagy in plant vegetative growth (roots and leaves), reproductive growth (pollen), and resistance to biotic (viruses, bacteria, and fungi) and abiotic stresses (nutrients, drought, salt, cold, and heat stress), and we discuss the application direction of plant autophagy in the future.

Ethyl Phloretate and Ethyl p-Coumarate: Two Phytotoxins from Valsa mali and Their Pathogenic Activities

Valsa mali, the causal agent of apple Valsa canker, produces several phytotoxic metabolites to promote infection. Bioassay and ¹H nuclear magnetic resonance (NMR)-guided isolation from the culture filtrate of V. mali strain 03-8 led to the identification of seven compounds including three unreported ones, ethyl phloretate (1), ethyl p-coumarate (2), and 1-p-hydroxybenzoyl glycerol (3). Compounds 1 and 2 produced significant phytotoxicity, with average lesion areas of 6.22 and 3.74 mm², along with 2.96 and 3.47 mm² at 1 mg/ml on mature and tissue-cultured apple leaves, respectively, whereas compound 3 did not cause any symptoms on host plants. The necrotic lesion area of compounds 1 and 2 on tobacco leaves was 52.65 and 48.28 mm², respectively, compared with the negative control (0.46 mm²) at 1 mg/ml. At the same concentration, compounds 1 and 2 showed no significant influence on the germination rate of lettuce seeds while significantly decreasing the root length of lettuce seedlings to 6.74 and 4.67 mm, respectively, compared with that treated with sterile distilled water (22.01 mm). The discovery indicated that compounds 1 and 2 could be considered as non-host-specific toxins. Furthermore, compounds 1 and 2 could cause cell shrinkage, organelle damage, plasmolysis, and eventually ruptured protoplasmic membranes with cell death for their phytotoxicity in the host plants under optical microscopy and transmission electron microscopy. The results shed light on the mechanism for toxins 1 and 2 in V. mali-infected plants at the macroscopic and cellular levels.

miR164g-MsNAC022 acts as a novel module mediating drought response by transcriptional regulation of reactive oxygen species scavenging systems in apple

Under drought stress, reactive oxygen species (ROS) overaccumulate as a secondary stress that impairs plant performance and thus severely reduces crop yields. The mitigation of ROS levels under drought stress is therefore crucial for drought tolerance. MicroRNAs (miRNAs) are critical regulators of plant development and stress responses. However, the complex molecular regulatory mechanism by which they function during drought stress, especially in drought-triggered ROS scavenging, is not fully understood. Here, we report a newly identified drought-responsive miRNA, miR164g, in the wild apple species Malus sieversii and elucidate its role in apple drought tolerance. Our results showed that expression of miR164g is significantly inhibited under drought stress and it can specifically cleave transcripts of the transcription factor MsNAC022 in M. sieversii. The heterologous accumulation of miR164g in Arabidopsis thaliana results in enhanced sensitivity to drought stress, while overexpression of MsNAC022 in Arabidopsis and the cultivated apple line 'GL-3' (Malus domestica Borkh.) lead to enhanced tolerance to drought stress by raising the ROS scavenging enzymes activity and related genes expression levels, particularly PEROXIDASE (MsPOD). Furthermore, we showed that expression of MsPOD is activated by MsNAC022 in transient assays. Interestingly, Part1 (P1) region is the key region for the positive regulation of MsPOD promoter by MsNAC022, and the different POD expression patterns in M. sieversii and M. domestica is attributed to the specific fragments inserted in P1 region of M. sieversii. Our findings reveal the function of the miR164g-MsNAC022 module in mediating the drought response of M. sieversii and lay a foundation for breeding drought-tolerant apple cultivars.

CRISPR/Cas9 mediated gene-editing of GmHdz4 transcription factor enhances drought tolerance in soybean (Glycine max [L.] Merr.)

The HD-Zip transcription factors play a crucial role in plant development, secondary metabolism, and abiotic stress responses, but little is known about HD-Zip I genes in soybean. Here, a homeodomain-leucine zipper gene designated GmHdz4 was isolated. Chimeric soybean plants, GmHdz4 overexpressing (GmHdz4-oe), and gene-editing via CRISPR/Cas9 (gmhdz4) in hairy roots, were generated to examine the GmHdz4 gene response to polyethylene glycol (PEG)-simulated drought stress. Bioinformatic analysis showed GmHdz4 belonged to clade δ, and was closely related to other drought tolerance-related HD-Zip I family genes such as AtHB12, Oshox12, and Gshdz4. The GmHdz4 was located in the plant nucleus and showed transcriptional activation activity by yeast hybrid assay. Quantitative real-time PCR analysis revealed that GmHdz4 expression varied in tissues and was induced by PEG-simulated drought stress. The gmhdz4 showed promoted growth of aboveground parts, and its root system architecture, including the total root length, the root superficial area, and the number of root tips were significantly higher than those of GmHdz4-oe even the non-transgenic line (NT) on root tips number. The better maintenance of turgor pressure by osmolyte accumulation, and the higher activity of antioxidant enzymes to scavenge reactive oxygen species, ultimately suppressed the accumulation of hydrogen peroxide (H₂O₂), superoxide anion (O^2-), and malondialdehyde (MDA), conferring higher drought tolerance in gmhdz4 compared with both GmHdz4-oe and NT. Together, our results provide new insights for future research on the mechanisms by which GmHdz4 gene-editing via CRISPR/Cas9 system could promote drought stress and provide a potential target for molecular breeding in soybean.

Copper stress in grapevine: Consequences, responses, and a novel mitigation strategy using 5-aminolevulinic acid

Improper application of copper-based fungicides has made copper stress critical in viticulture, necessitating the need to identify substances that can mitigate it. In this study, leaves of 'Shine Muscat' ('SM') grapevine seedlings were treated with CuSO₄ solution (10 mM/L), CuSO₄ + 5-aminolevulinic acid (ALA) (50 mg/L), and distilled water to explore the mitigation effect of ALA. Physiological assays demonstrated that ALA effectively reduced malondialdehyde accumulation and increased peroxidase and superoxide dismutase activities in grapevine leaves under copper stress. Copper ion absorption, transport pathways, chlorophyll metabolism pathways, photosynthetic system, and antioxidant pathways play key roles in ALA alleviated-copper stress. Moreover, expression changes in genes, such as CHLH, ALAD, RCA, and DHAR, play vital roles in these processes. Furthermore, abscisic acid reduction caused by NCED down-regulation and decreased naringenin, leucopelargonidin, and betaine contents confirmed the alleviating effect of ALA. Taken together, these results reveal how grapevine responds to copper stress and the alleviating effects of ALA, thus providing a novel means of alleviating copper stress in viticulture.

Combining Physio-Biochemical Characterization and Transcriptome Analysis Reveal the Responses to Varying Degrees of Drought Stress in Brassica napus L

Brassica napus L. has become one of the most important oil-bearing crops, and drought stress severely influences its yield and quality. By combining physio-biochemical characterization and transcriptome analysis, we studied the response of B. napus plants to different degrees of drought stress. Some physio-biochemical traits, such as fresh weight (FW), dry weight (DW), abscisic acid (ABA) content, net photosynthetic rate (Pn), stomatal conductance (g_s), and transpiration rate (Tr), were measured, and the total content of the epidermal wax/cutin, as well as their compositions, was determined. The results suggest that both stomatal transpiration and cuticular transpiration are affected when B. napus plants are subjected to varying degrees of drought stress. A total of 795 up-regulated genes and 1050 down-regulated genes were identified under severe drought stress by transcriptome analysis. Gene ontology (GO) enrichment analysis of differentially expressed genes (DEGs) revealed that the up-regulated genes were mainly enriched in the stress response processes, such as response to water deprivation and abscisic acid, while the down-regulated genes were mainly enriched in the chloroplast-related parts affecting photosynthesis. Moreover, overexpression of BnaA01.CIPK6, an up-regulated DEG, was found to confer drought tolerance in B. napus. Our study lays a foundation for a better understanding of the molecular mechanisms underlying drought tolerance in B. napus.

The apple FERONIA receptor-like kinase MdMRLK2 negatively regulates Valsa canker resistance by suppressing defence responses and hypersensitive reaction

Valsa canker, caused by the fungus Valsa mali, is one of the most destructive diseases of apple trees in China and other East Asian countries. The plant receptor-like kinase FERONIA is involved in plant cell growth, development, and immunity. However, little is known about the function of FERONIA in apple defence against V. mali. In this study, we found that MdMRLK2 was highly induced by V. mali in twigs of V. mali-susceptible Malus mellana but not in those of the resistant species Malus yunnaensis. 35S:MdMRLK2 apple plants showed compromised resistance relative to wild-type (WT) plants. Further analyses indicated that 35S:MdMRLK2 apple plants had enhanced abscisic acid (ABA) levels and reduced salicylic acid (SA) levels relative to the WT on V. mali infection. MdMRLK2 overexpression also suppressed polyphenol accumulation and inhibited the activities of phenylalanine ammonia-lyase (PAL), β-1,3-glucanase (GLU), and chitinase (CHT) during V. mali infection. Moreover, MdMRLK2 interacted with MdHIR1, a hypersensitive-induced response protein, and suppressed the MdHIR1-mediated hypersensitive reaction (HR), probably by impairing MdHIR1 self-interaction. Collectively, these findings demonstrate that overexpression of MdMRLK2 compromises Valsa canker resistance, probably by (a) altering ABA and SA levels, (b) suppressing polyphenol accumulation, (c) inhibiting PAL, GLU, and CHT activities, and (d) blocking MdHIR1-mediated HR by disrupting MdHIR1 self-interaction.

Characterization of 24-epibrassinolide-mediated modulation of the drought stress responses: Morphophysiology, antioxidant metabolism and hormones in grapevine (Vitis vinifera L.)

Drought stress is one of the major abiotic stresses that limit grape growth and yield. Brassinosteroids (BRs) are a class of phytohormones essential for plant growth, development, and adaptation to environmental stress. This study aimed to reveal the physiological and biochemical mechanisms of exogenous BRs in alleviating the drought stress in grapevines. Two-year-old grape seedlings (Vitis vinifera L.) were sprayed with 24-epibrassinolide (EBR), a synthetic analog of BRs, and then subjected to drought treatment. The results showed that exogenous EBR significantly mitigated the reduction of photosynthetic pigment contents and photosystem II efficiency and decreased the damage to chloroplasts when grape seedlings were subjected to drought stress. Drought stress resulted in the accumulation of reactive oxidative species (ROS) and an increase in lipid peroxidation. A reduction in oxidative damage was observed in EBR-pretreated plants, which was probably due to the elevated antioxidant system. Exogenous EBR improved the activities of superoxide dismutase (14%), catalase (18%), peroxidase (17%), and ascorbate peroxidase (9%), and promoted the accumulation of ascorbic acid (10%) and glutathione (7%) under drought stress. EBR pretreatment also promoted autophagic activity, which contributed to the degradation of damaged chloroplasts. Moreover, EBR pretreatment increased the concentrations of abscisic acid, jasmonic acid, auxin, and gibberellic acid. Taken together, exogenous EBR could ameliorate the deleterious effects of drought stress by up-regulating photosynthetic capacity, antioxidant system, autophagic activity, and hormone concentrations.

Autophagy-Mediated Regulation of Different Meristems in Plants

Autophagy is a highly conserved cell degradation process that widely exists in eukaryotic cells. In plants, autophagy helps maintain cellular homeostasis by degrading and recovering intracellular substances through strict regulatory pathways, thus helping plants respond to a variety of developmental and environmental signals. Autophagy is involved in plant growth and development, including leaf starch degradation, senescence, anthers development, regulation of lipid metabolism, and maintenance of peroxisome mass. More and more studies have shown that autophagy plays a role in stress response and contributes to maintain plant survival. The meristem is the basis for the formation and development of new tissues and organs during the post-embryonic development of plants. The differentiation process of meristems is an extremely complex process, involving a large number of morphological and structural changes, environmental factors, endogenous hormones, and molecular regulatory mechanisms. Recent studies have demonstrated that autophagy relates to meristem development, affecting plant growth and development under stress conditions, especially in shoot and root apical meristem. Here, we provide an overview of the current knowledge about how autophagy regulates different meristems under different stress conditions and possibly provide new insights for future research.