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SUMMARY: 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. [ABSTRACT FROM AUTHOR]

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/Ca 2+ 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., (© 2023 Society for Experimental Biology and John Wiley & Sons Ltd.)

Low temperature affects the yield and quality of crops. Inducer of CBF expression 1 (ICE1) plays a positive role in plant cold tolerance by promoting the expression of CRT binding factor (CBF) and cold-responsive (COR) genes. Several ICE1-interacting transcription factors (TFs) that regulate plant cold tolerance have been identified. However, how these TFs affect the function of ICE1 and CBF expression under cold conditions remains unclear. Here, we identified the MYC-type TF MdbHLH4, a negative regulator of cold tolerance in Arabidopsis (Arabidopsis thaliana) and apple (Malus domestica) plants. Under cold conditions, MdbHLH4 inhibits the expression of MdCBF1 and MdCBF3 by directly binding to their promoters. It also interacts with MdICE1L, a homolog of AtICE1 in apple, and inhibits the binding of MdICE1L to the promoters of MdCBF1/3 and thus their expression. We showed that MdCAX3L-2, a Ca2+/H+ exchanger (CAX) family gene that negatively regulates plant cold tolerance, is also a direct target of MdbHLH4. MdbHLH4 reduced apple cold tolerance by promoting MdCAX3L-2 expression. Moreover, overexpression of either MdCAX3L-2 or MdbHLH4 promoted the cold-induced ubiquitination and degradation of MdICE1L. Overall, our results reveal that MdbHLH4 negatively regulates plant cold tolerance by inhibiting MdCBF1/3 expression and MdICE1L promoter-binding activity, as well as by promoting MdCAX3L-2 expression and cold-induced MdICE1L degradation. These findings provide insights into the mechanisms by which ICE1-interacting TFs regulate CBF expression and ICE1 function and thus plant cold tolerance., (© American Society of Plant Biologists 2022. All rights reserved. For permissions, please email: journals.permissions@oup.com.)