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SUMMARY: Drought stress is one of the dominating challenges to the growth and productivity in crop plants. Elucidating the molecular mechanisms of plants responses to drought stress is fundamental to improve fruit quality. However, such molecular mechanisms are poorly understood in apple (Malus domestica Borkh.). In this study, we explored that the BTB‐BACK‐TAZ protein, MdBT2, negatively modulates the drought tolerance of apple plantlets. Moreover, we identified a novel Homeodomain‐leucine zipper (HD‐Zip) transcription factor, MdHDZ27, using a yeast two‐hybrid (Y2H) screen with MdBT2 as the bait. Overexpression of MdHDZ27 in apple plantlets, calli, and tomato plantlets enhanced their drought tolerance by promoting the expression of drought tolerance‐related genes [responsive to dehydration 29A (MdRD29A) and MdRD29B]. Biochemical analyses demonstrated that MdHDZ27 directly binds to and activates the promoters of MdRD29A and MdRD29B. Furthermore, in vitro and in vivo assays indicate that MdBT2 interacts with and ubiquitinates MdHDZ27, via the ubiquitin/26S proteasome pathway. This ubiquitination results in the degradation of MdHDZ27 and weakens the transcriptional activation of MdHDZ27 on MdRD29A and MdRD29B. Finally, a series of transgenic analyses in apple plantlets further clarified the role of the relationship between MdBT2 and MdHDZ27, as well as the effect of their interaction on drought resistance in apple plantlets. Collectively, our findings reveal a novel mechanism by which the MdBT2‐MdHDZ27 regulatory module controls drought tolerance, which is of great significance for enhancing the drought resistance of apple and other plants. Significance Statement: Collectively, our findings clearly decipher a novel regulatory network for drought tolerance in the apple plant, which centers on the regulatory module MdBT2‐MdHDZ27 and its roles in transcriptional regulation and post‐transcriptional modification, which will be useful for improving the drought tolerances of apple and other economically important plant species. [ABSTRACT FROM AUTHOR]

Summary: Changes in carbohydrates and organic acids largely determine the palatability of edible tissues of horticulture crops. Elucidating the potential molecular mechanisms involved in the change in carbohydrates and organic acids, and their temporal and spatial crosstalk are key steps in understanding fruit developmental processes. Here, we used apple (Malus domestica Borkh.) as research materials and found that MdbHLH3, a basic helix–loop–helix transcription factor (bHLH TF), modulates the accumulation of malate and carbohydrates. Biochemical analyses demonstrated that MdbHLH3 directly binds to the promoter of MdcyMDH that encodes an apple cytosolic NAD‐dependent malate dehydrogenase, activating its transcriptional expression, thereby promoting malate accumulation in apple fruits. Additionally, MdbHLH3 overexpression increased the photosynthetic capacity and carbohydrate levels in apple leaves and also enhanced the carbohydrate accumulation in fruits by adjusting carbohydrate allocation from sources to sinks. Overall, our findings provide new insights into the mechanism of how the bHLH TF MdbHLH3 modulates the fruit quality. It directly regulates the expression of cytosolic malate dehydrogenase MdcyMDH to coordinate carbohydrate allocation and malate accumulation in apple. [ABSTRACT FROM AUTHOR]

Background: The roles in photosystem I (PSI) assembly of the nucleus-encoded thylakoid protein Y3IP1 who interacts with the plastid-encoded Ycf3 protein that has been well-characterized in plants. However, its function and potential mechanisms in other aspects remain poorly understood. Results: We identified the apple MdY3IP1 gene, which encodes a protein highly homologous to the Arabidopsis Y3IP1 (AtY3IP1). Ectopic expression of MdY3IP1 triggered early-flowering and enhanced salt tolerance in Arabidopsis plants. MdY3IP1 controlled floral transition by accelerating sugar metabolism process in plant cells, thereby influencing the expression of flowering-associated genes. The increase in salt stress tolerance in MdY3IP1-expressing plants correlated with reduced reactive oxygen species (ROS) accumulation, and an increase in lateral root development by regulating both auxin biosynthesis and transport, as followed by enhancement of salt tolerance in Arabidopsis. Overall, these findings provide new evidences for additional functions of Y3IP1-like proteins and their underlying mechanisms of which Y3IP1 confers early-flowering and salt tolerance phenotypes in plants. Conclusions: These observations suggest that plant growth and stress resistance can be affected by the regulation of the MdY3IP1 gene. Further molecular and genetic approaches will accelerate our knowledge of MdY3IP1 functions in PSI complex formation and plants stress resistance, and inform strategies for creating transgenic crop varieties with early maturity and high-resistant to adverse environmental conditions. [ABSTRACT FROM AUTHOR]

Malate, the predominant organic acid in many fruits, is a crucial component of the organoleptic quality of fruit, including taste and flavor. The genetic and environmental mechanisms affecting malate metabolism in fruit cells have been studied extensively. However, the transcriptional regulation of malate-metabolizing enzymes and vacuolar transporters remains poorly understood. Our previous studies demonstrated that Md MYB1 modulates anthocyanin accumulation and vacuolar acidification by directly activating vacuolar transporters, including Md VHA-B1, Md VHA-E, Md VHP1 and Mdt DT. Interestingly, we isolated and identified a MYB transcription factor, Md MYB73, a distant relative of Md MYB1 in this study. It was subsequently found that Md MYB73 protein bound directly to the promoters of Md ALMT9 (aluminum-activated malate transporter 9), Md VHA-A (vacuolar ATPase subunit A) and Md VHP1 (vacuolar pyrophosphatase 1), transcriptionally activating their expression and thereby enhancing their activities. Analyses of transgenic apple calli demonstrated that Md MYB73 influenced malate accumulation and vacuolar pH. Furthermore, Md CIb HLH1 interacted with Md MYB73 and enhanced its activity upon downstream target genes. These findings help to elucidate how Md MYB73 directly modulates the vacuolar transport system to affect malate accumulation and vacuolar pH in apple. [ABSTRACT FROM AUTHOR]

Vacuolar H + -ATPase (V-H + -ATPase) is highly conserved, multisubunit proton pumps that play key roles in adaptation to abiotic stress in plants. Function of apple V-ATPase subunit B1 ( MdVHA-B1 ) has been identified in drought tolerance, however, it remains unclear whether MdVHA-B1 is involved in salt tolerance. In this work, we characterized the function of MdVHA-B1 gene in salt stress response. GUS and Western blotting assays showed that MdVHA-B1 was highly induced by salt stress. Yeast two-hybrid (Y2H) and bimolecular fluorescence complementation (BiFC) assays demonstrated that MdVHA-B1 interacts with MdSOS2L1, a Arabidopsis SOS2-like protein kinase, in apple. Compared with wild-type (WT) apple calli, V-ATPase activities in the transgenic calli were significantly enhanced under salt stress condition. Furthermore, Na + and K + were accumulated much higher in transgenic tomato plants than the WT control. These results suggest that MdVHA-B1 may confer salt tolerance through salt overly sensitive (SOS) signal pathway in apple to regulate the ion balance. [ABSTRACT FROM AUTHOR]