Exogenous treatment with melatonin enhances waterlogging tolerance of kiwifruit plants.

Waterlogging stress has an enormous negative impact on the kiwifruit yield and quality. The protective role of exogenous melatonin on water stress has been widely studied, especially in drought stress. However, the research on melatonin-induced waterlogging tolerance is scarce. Here, we found that treatment with exogenous melatonin could effectively alleviate the damage on kiwifruit plants in response to waterlogging treatment. This was accompanied by higher antioxidant activity and lower ROS accumulation in kiwifruit roots during stress period. The detection of changes in amino acid levels of kiwifruit roots during waterlogging stress showed a possible interaction between melatonin and amino acid metabolism, which promoted the tolerance of kiwifruit plants to waterlogging. The higher levels of GABA and Pro in the roots of melatonin-treated kiwifruit plants partly contributed to their improved waterlogging tolerance. In addition, some plant hormones were also involved in the melatonin-mediated waterlogging tolerance, such as the enhancement of ACC accumulation. This study discussed the melatonin-mediated water stress tolerance of plants from the perspective of amino acid metabolism for the first time.

Comparative transcriptome provides insight into responding mechanism of waterlogging stress in Actinidia valvata Dunn.

Kiwifruit is one of the most popular fruits, and the area of its cultivation in China has grown rapidly over the last decade. However, kiwifruit vines are vulnerable to waterlogging, especially in the extensive areas of south China where it is grown. This has become an important factor limiting yields. Therefore, it is necessary to clarify the responses of kiwifruit to waterlogging. Here, we have selected Actinidia valvata Dunn which is able to withstand waterlogging conditions and the waterlogging-susceptible Actinidia deliciosa to perform the RNA-seq of roots under waterlogging stress. Seedling roots of Actinidia valvata Dunn and Actinidia deliciosa presented distinct root phenotypes after waterlogging treatments. Genome mapping showed a large genome difference between Actinidia valvata Dunn and Actinidia deliciosa. Transcription factors MYB, MYB-related, AP2-EREBP, bHLH, WRKY, and NAC were identified as the key genes involved in the response to waterlogging stress of kiwifruit. Meanwhile, the MAPK signaling pathway and the glycolysis/gluconeogenesis pathway were identified as the vital pathways involved in the response to waterlogging, and key genes were identified from these two pathways. These results will broaden our understanding of transcriptional response of waterlogging stress and will provide new insights into the molecular mechanisms associated with waterlogging stress. Furthermore, identification of the genes responsible will assist in the breeding of kiwifruit tolerant of waterlogging.

The protective role of MdATG10-mediated autophagy in apple plant under cadmium stress.

Autophagy is a conserved degradation pathway in plants, which plays an important role in plant cellular homeostasis during abiotic stress. Although various abiotic stressors have been reported to induce autophagic activity in plants, the specific role of autophagy in plant cadmium (Cd) tolerance remains undiscovered. In this study, we treated three MdATG10-overexpressing apple lines with hydroponic Cd stress and found the enhanced Cd tolerance in transgenic plants. Transgenic apple plants exhibited less growth limitation and reduced Cd damage on the photosynthetic system. That was accompanied by higher antioxidant enzymes activity and lower harmful ROS accumulation in apple leaves under Cd stress. The higher autophagic activity led to a more active metabolic system of Pro, His, and Arg in transgenic plants under Cd stress, which was closely related to the plant Cd tolerance. In addition, the transcriptional activities of several Cd transport and detoxification-related genes were regulated by MdATG10-overexpression in response to Cd stress. This study is the first to demonstrate the protective role of autophagy in the Cd tolerance of plants.

MdATG5a induces drought tolerance by improving the antioxidant defenses and promoting starch degradation in apple.

Drought occurrence seriously affects the productivity and quality of apple crop worldwide. Autophagy, a conserved process for the degradation and recycling of unwanted cellular components, is considered to positively regulate the tolerance of various abiotic stresses in plants. In the current study, we isolated two ATG5 homologs genes, namely, MdATG5a and MdATG5b, from apple, demonstrating their responsiveness to drought and oxidative stresses. In addition to having the same cellular localization in the nucleus and cytoplasm, both MdATG5a and MdATG5b could interact with MdATG12. Transgenic apple plants overexpressing MdATG5a exhibited an improved drought tolerance, as indicated by less drought-related damage and higher photosynthetic capacities compared to wild-type (WT) plants under drought stress. The overexpression of MdATG5a improved antioxidant defenses in apple when exposed to drought via elevating both antioxidant enzyme activities and the levels of beneficial antioxidants. Furthermore, under drought stress, the overexpression of MdATG5a promoted the mobilization of starch to accumulate greater levels of soluble sugars, contributing to osmotic adjustments and supporting carbon skeletons for proline synthesis. Such changes in physiological responses may be associated with increased autophagic activities in the transgenic plants upon exposure to drought. Our results demonstrate that MdATG5a-mediated autophagy enhances drought tolerance of apple plants via improving antioxidant defenses and metabolic adjustments.

Enhanced Autophagic Activity Improved the Root Growth and Nitrogen Utilization Ability of Apple Plants under Nitrogen Starvation.

Autophagy is a conserved degradation pathway for recycling damaged organelles and aberrant proteins, and its important roles in plant adaptation to nutrient starvation have been generally reported. Previous studies found that overexpression of autophagy-related (ATG) gene MdATG10 enhanced the autophagic activity in apple roots and promoted their salt tolerance. The MdATG10 expression was induced by nitrogen depletion condition in both leaves and roots of apple plants. This study aimed to investigate the differences in the growth and physiological status between wild type and MdATG10-overexpressing apple plants in response to nitrogen starvation. A hydroponic system containing different nitrogen levels was used. The study found that the reduction in growth and nitrogen concentrations in different tissues caused by nitrogen starvation was relieved by MdATG10 overexpression. Further studies demonstrated the increased root growth and the higher nitrogen absorption and assimilation ability of transgenic plants. These characteristics contributed to the increased uptake of limited nitrogen nutrients by transgenic plants, which also reduced the starvation damage to the chloroplasts. Therefore, the MdATG10-overexpressing apple plants could maintain higher photosynthetic ability and possess better growth under nitrogen starvation stress.

Overexpression of MdATG8i Enhances Drought Tolerance by Alleviating Oxidative Damage and Promoting Water Uptake in Transgenic Apple.

Water deficit adversely affects apple (Malus domestica) productivity on the Loess Plateau. Autophagy plays a key role in plant responses to unfavorable environmental conditions. Previously, we demonstrated that a core apple autophagy-related protein, MdATG8i, was responsive to various stresses at the transcript level. Here, we investigated the function of this gene in the response of apple to severe drought and found that its overexpression (OE) significantly enhanced drought tolerance. Under drought conditions, MdATG8iOE apple plants exhibited less drought-related damage and maintained higher photosynthetic capacities compared with the wild type (WT). The accumulation of ROS (reactive oxygen species) was lower in OE plants under drought stress and was accompanied by higher activities of antioxidant enzymes. Besides, OE plants accumulated lower amounts of insoluble or oxidized proteins but greater amounts of amino acids and flavonoid under severe drought stress, probably due to their enhanced autophagic activities. Particularly, MdATG8iOE plants showed higher root hydraulic conductivity than WT plants did under drought conditions, indicating the enhanced ability of water uptake. In summary, the overexpression of MdATG8i alleviated oxidative damage, modulated amino acid metabolism and flavonoid synthesis, and improved root water uptake, ultimately contributing to enhanced drought tolerance in apple.

Overexpression of MdATG8i improves water use efficiency in transgenic apple by modulating photosynthesis, osmotic balance, and autophagic activity under moderate water deficit.

Water deficit is one of the major limiting factors for apple (Malus domestica) production on the Loess Plateau, a major apple cultivation area in China. The identification of genes related to the regulation of water use efficiency (WUE) is a crucial aspect of crop breeding programs. As a conserved degradation and recycling mechanism in eukaryotes, autophagy has been reported to participate in various stress responses. However, the relationship between autophagy and WUE regulation has not been explored. We have shown that a crucial autophagy protein in apple, MdATG8i, plays a role in improving salt tolerance. Here, we explored its biological function in response to long-term moderate drought stress. The results showed that MdATG8i-overexpressing (MdATG8i-OE) apple plants exhibited higher WUE than wild-type (WT) plants under long-term moderate drought conditions. Plant WUE can be increased by improving photosynthetic efficiency. Osmoregulation plays a critical role in plant stress resistance and adaptation. Under long-term drought conditions, the photosynthetic capacity and accumulation of sugar and amino acids were higher in MdATG8i-OE plants than in WT plants. The increased photosynthetic capacity in the OE plants could be attributed to their ability to maintain optimal stomatal aperture, organized chloroplasts, and strong antioxidant activity. MdATG8i overexpression also promoted autophagic activity, which was likely related to the changes described above. In summary, our results demonstrate that MdATG8i-OE apple lines exhibited higher WUE than WT under long-term moderate drought conditions because they maintained robust photosynthesis, effective osmotic adjustment processes, and strong autophagic activity.

MdHARBI1, a MdATG8i-interacting protein, plays a positive role in plant thermotolerance.

Autophagy is a major degradation pathway in plants for maintaining cellular homeostasis in response to various environmental stressors. ATG8 is one of a series of autophagy-related (ATG) proteins and plays a central role in both bulk and selective autophagy. Previously, we characterized MdATG8i in apple and demonstrated that it has a positive role in apple stress resistance. Although many ATG8-interacting proteins have been found in Arabidopsis, no protein has been reported to interact with MdATG8 in apple. Here, we identified MdHARBI1 as a MdATG8i-interacting protein in apple, however, the functions of HARBI1-like proteins have not been explored in plants. Expression analysis of MdHARBI1 and pro-MdHARBI1-GUS staining of transgenic Arabidopsis exposed to high temperature demonstrated that MdHARBI1 was significantly induced by heat stress. Moreover, heat-treated MdHARBI1-trangenic tomato plants maintained higher autophagic activity, accumulated fewer ROS, and displayed stronger chlorophyll fluorescence than wild-type plants. Because these phenotypes were consistent with those displayed by MdATG8i-overexpressing apple plants under high temperature, we concluded that the MdATG8i-interacting protein MdHARBI1 plays a critical role in the basal thermotolerance of plants, mainly by influencing autophagy pathways.

Overexpression of MdATG18a enhances alkaline tolerance and GABA shunt in apple through increased autophagy under alkaline conditions.

Soil alkalization affects apple production in northwest China. Autophagy is a highly conserved degradative protein pathway in eukaryotes. Autophagy in plants can be activated by various abiotic factors. We previously identified the positive role of the autophagy-related gene MdATG18a in drought, nitrogen deficiency and resistance to Diplocarpon mali infection in apple. However, it is still unclear whether ATG18a is related to alkaline stress. In this study, we used hydroponic culture to simulate alkaline stress and found that the overexpression of MdATG18a significantly improved the tolerance of apple to alkaline stress. The overexpression of MdATG18a increased biomass, photosynthetic rate and antioxidant capacity of transgenic plants compared with wild-type plants under alkaline stress. The overexpression of MdATG18a promoted γ-aminobutyric acid (GABA) shunt via an increase in glutamate (GABA precursor) and GABA contents and upregulation of GABA shunt-related genes. In addition, the overexpression of MdATG18a significantly upregulated the expression of other core ATG genes and increased the formation of autophagosomes under alkaline stress. In conclusion, these results suggest that the overexpression of MdATG18a in apple enhances alkaline tolerance and the GABA shunt, which may be owing to the increase in autophagic activity.

The Apple Autophagy-Related Gene MdATG9 Confers Tolerance to Low Nitrogen in Transgenic Apple Callus.

Autophagy is an efficient degradation system for maintaining cellular homeostasis when plants are under environmental stress. ATG9 is the only integral membrane protein within the core ATG machinery that provides a membrane source for autophagosome formation. In this study, we isolated an ATG9 homologs gene in apple, MdATG9, from Malus domestica. The analysis of its sequence, subcellular localization, promoter cis-elements, and expression patterns revealed the potential function of MdATG9 in response to abiotic stressors. Overexpression of MdATG9 in apple callus conferred enhanced tolerance to nitrogen depletion stress. During the treatment, other important MdATGs were expressed at higher levels in transgenic callus than in the wild type. Furthermore, more free amino acids and increased sucrose levels were found in MdATG9-overexpression apple callus compared with the wild type in response to nitrogen starvation, and the expression levels of MdNRT1.1, MdNRT2.5, MdNIA1, and MdNIA2 were all increased higher in transgenic lines. These data suggest that, as an important autophagy gene, MdATG9 plays an important role in the maintenance of amino acids and sugars in response to nutrient starvation in apple.

Increased autophagic activity in roots caused by overexpression of the autophagy-related gene MdATG10 in apple enhances salt tolerance.

Autophagy is a conserved pathway to degrade and recycle damaged proteins and organelles, which has generally been reported to play an important role in plant adaption to various abiotic stressors. Here, we isolated a new apple autophagy-related gene, MdATG10, from Malus domestica. Expression of MdATG10 was induced by salt stress, particularly in roots. To investigate the effects of increased autophagic activity on salt tolerance of apple, we generated three MdATG10-overexpressing apple lines and exposed them to salt stress. The transgenic apple plants exhibited enhanced salt tolerance, accompanied by slightly damaged photosynthetic ability and a milder growth limitation under the salt treatment. In addition, damage to growth and vitality of the root system caused by the salt treatment was alleviated by overexpressing MdATG10. Furthermore, reduced accumulation of Na+ and a lower Na+: K+ ratio was detected in the MdATG10-overexpressing apple lines under salt stress. The salt treatment induced expression of genes involved in ion homeostasis in transgenic apple roots. These results demonstrate a promoting role of autophagy in ion transport when plants encounter salty conditions.

MdATG18a overexpression improves basal thermotolerance in transgenic apple by decreasing damage to chloroplasts.

High temperature is an abiotic stress factor that threatens plant growth and development. Autophagy in response to heat stress involves the selective removal of heat-induced protein complexes. Previously, we showed that a crucial autophagy protein from apple, MdATG18a, has a positive effect on drought tolerance. In the present study, we treated transgenic apple (Malus domestica) plants overexpressing MdATG18a with high temperature and found that autophagy protected them from heat stress. Overexpression of MdATG18a in apple enhanced antioxidase activity and contributed to the production of increased beneficial antioxidants under heat stress. Transgenic apple plants exhibited higher photosynthetic capacity, as shown by the rate of CO2 assimilation, the maximum photochemical efficiency of photosystem II (PSII), the effective quantum yield, and the electron transport rates in photosystems I and II (PSI and PSII, respectively). We also detected elevated autophagic activity and reduced damage to chloroplasts in transgenic plants compared to WT plants. In addition, the transcriptional activities of several HSP genes were increased in transgenic apple plants. In summary, we propose that autophagy plays a critical role in basal thermotolerance in apple, primarily through a combination of enhanced antioxidant activity and reduced chloroplast damage.

Overexpression of MdATG18a in apple improves resistance to Diplocarpon mali infection by enhancing antioxidant activity and salicylic acid levels.

Marssonina apple blotch, caused by Diplocarpon mali, is one of the most serious diseases of apple. Autophagy plays a key role in pathogen resistance. We previously showed that MdATG18a has a positive influence on drought tolerance. Herein, we describe how overexpression (OE) of MdATG18a enhances resistance to D. mali infection, probably because less H2O2 but more salicylic acid (SA) is accumulated in the leaves of OE apple plants. Expression of chitinase, ß-1,3-glucanase, and SA-related marker genes was induced more strongly by D. mali in OE lines. Transcript levels of other important MdATG genes were also drastically increased by D. mali in OE plants, which indicated increased autophagy activities. Taken together, these results demonstrate that OE of MdATG18a enhances resistance to infection by D. mali and plays positive roles in H2O2-scavenging and SA accumulations. Our findings provide important information for designing strategies which could induce autophagy to minimize the impact of this disease on apple production.

MdATG18a overexpression improves tolerance to nitrogen deficiency and regulates anthocyanin accumulation through increased autophagy in transgenic apple.

Nitrogen (N) availability is an essential factor for plant growth. Recycling and remobilization of N have strong impacts on crop yield and quality under N deficiency. Autophagy is a critical nutrient-recycling process that facilitates remobilization under starvation. We previously showed that an important AuTophaGy (ATG) protein from apple, MdATG18a, has a positive role in drought tolerance. In this study, we explored its biological role in response to low-N. Overexpression of MdATG18a in both Arabidopsis and apple improved tolerance to N-depletion and caused a greater accumulation of anthocyanin. The increased anthocyanin concentration in transgenic apple was possibly due to up-regulating flavonoid biosynthetic and regulatory genes (MdCHI, MdCHS, MdANS, MdPAL, MdUFGT, and MdMYB1) and higher soluble sugars concentration. MdATG18a overexpression enhanced starch degradation with up-regulating amylase gene (MdAM1) and up-regulated sugar metabolism related genes (MdSS1, MdHXKs, MdFK1, and MdNINVs). Furthermore, MdATG18a functioned in nitrate uptake and assimilation by up-regulating nitrate reductase MdNIA2 and 3 high-affinity nitrate transporters MdNRT2.1/2.4/2.5. MdATG18a overexpression also elevated other important MdATG genes expression and autophagosomes formation under N-depletion, which play key contributions to above changes. Together, these results demonstrate that overexpression of MdATG18a enhances tolerance to N-deficiencies and plays positive roles in anthocyanin biosynthesis through greater autophagic activity.

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

Autophagy is a major and conserved pathway for delivering and recycling unwanted proteins or damaged organelles to be degraded in the vacuoles. AuTophaGy-related (ATG) protein 18a has been established as one of the essential components for autophagy occurrence in Arabidopsis thaliana. We previously cloned the ATG18a homolog from Malus domestica (MdATG18a) and monitored its responsiveness to various abiotic stresses at the transcriptional level. However, it is still unclear what its function is under abiotic stress in apple. Here, we found that heterologous expression of MdATG18a in tomato plants markedly enhanced their tolerance to drought. Overexpression (OE) of that gene in apple plants improved their drought tolerance as well. Under drought conditions, the photosynthesis rate and antioxidant capacity were significantly elevated in OE lines when compared with the untransformed wild type (WT). Transcript levels of other important apple ATG genes were more strongly up-regulated in transgenic MdATG18a OE lines than in the WT. The percentage of insoluble protein in proportion to total protein was lower and less oxidized protein accumulated in the OE lines than in the WT under drought stress. This was probably due to more autophagosomes being formed in the former. These results demonstrate that overexpression of MdATG18a in apple plants enhances their tolerance to drought stress, probably because of greater autophagosome production and a higher frequency of autophagy. Those processes help degrade protein aggregation and limit the oxidation damage, thereby suggesting that autophagy plays important roles in the drought response.

Unveiling the mechanism of melatonin impacts on maize seedling growth: sugar metabolism as a case.

Melatonin regulates growth in many plants; however, the mechanism remains unclear. In this study, exogenous melatonin feeding resulted in both promotional (≤10 µm) and inhibitory (≥100 µm) effects on maize seedling growth. Initial analyses suggested positive correlations between the amount of melatonin and sucrose synthesis and hydrolysis-related gene expression, enzyme activities, and sucrose metabolites. However, assays of photosynthetic rate, hexokinase (HxK) activity, expression of photosynthetic marker genes, and HxK-related genes showed opposite effects under 10 µm (positive) and 100 µm (negative) melatonin treatments. Similarly, 10 µm melatonin accelerated starch catabolism at night, whereas 100 µm melatonin significantly decreased this process and led to starch accumulation in photosynthetic tissues. Furthermore, expression analysis of genes related to sucrose phloem loading resulted in a slight upregulation of sucrose transporters (SUT1 and SUT2) when seedlings were induced with 10 µm melatonin, while treatment with 100 µm melatonin resulted in significant downregulation of these sucrose transporter genes (SUT1 and SUT2), as well as tie-dyed2 (Tdy2) and sucrose export defective 1. Taken together, these results suggest that low doses of melatonin benefit maize seedling growth by promoting sugar metabolism, photosynthesis, and sucrose phloem loading. Conversely, high doses of melatonin inhibit seedling growth by inducing the excessive accumulation of sucrose, hexose and starch, suppressing photosynthesis and sucrose phloem loading.