The pivotal role of N⁶-methyladenosine (m⁶A) demethylases in regulating plant stress responses has been widely explored; however, the function of apple m⁶A demethylases under heat stress and fixed-carbon starvation is unclear.  In this study, the apple RNA demethylase gene family was identified, and the demethylase gene MdALKBH1A was selected for further analysis.  Using liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis, we demonstrated that MdALKBH1A is the m⁶A demethylase of apple.  Moreover, transgenic ‘Micro Tom’ tomato plants overexpressing MdALKBH1A were more sensitive to high temperature, probably due to the decreased antioxidant ability, increased membrane lipid peroxidation and reduced plasma membrane stability.  However, these tomato plants overexpressing MdALKBH1A were more resistant to fixed-carbon starvation, as evidenced by the improved plasma membrane stability, enhanced photosynthetic rates and elevated autophagic activity.  In summary, our results highlight the crucial role played by MdALKBH1A in the response of apple plants to high-temperature stress and fixed-carbon starvation.

Apple replant disease (ARD) has led to severe yield and quality reduction in the apple industry.  Fusarium solani (F. solani) has been identified as one of the main microbial pathogens responsible for ARD.  Auxin (indole-3-acetic acid, IAA), an endogenous hormone in plants, is involved in almost all plant growth and development processes and plays a role in plant immunity against pathogens.  Gretchen Hagen3 (GH3) is one of the early/primary auxin response genes.  The aim of this study was to evaluate the function of MdGH3-2 and MdGH3-12 in the defense response of F. solani by treating MdGH3-2/12 RNAi plants with F. solani.  The results show that under F. solani infection, RNAi of MdGH3-2/12 inhibited plant biomass accumulation and exacerbated root damage.  After inoculation with F. solani, MdGH3-2/12 RNAi inhibited the biosynthesis of acid-amido synthetase.  This led to the inhibition of free IAA combining with amino acids, resulting in excessive free IAA accumulation.  This excessive free IAA altered plant tissue structure, accelerated fungal hyphal invasion, reduced the activity of antioxidant enzymes (SOD, POD and CAT), increased the reactive oxygen species (ROS) level, and reduced total chlorophyll content and photosynthetic ability, while regulating the expression of PR-related genes including PR1, PR4, PR5 and PR8.  It also changed the contents of plant hormones and amino acids, and ultimately reduced the resistance to F. solani.  In conclusion, these results demonstrate that MdGH3-2 and MdGH3-12 play an important role in apple tolerance to F. solani and ARD.