Summary: N⁶‐methyladenosine (m⁶A) reader protein plays an important role in trichome morphology, developmental timing and morphogenesis in Arabidopsis. However, the function of m⁶A readers in plant‐microbe interaction remains unclear. Here, a Malus YTH‐domain family protein MhYTP2 was initially characterized as an m⁶A reader. MhYTP2 overexpression increased mRNA m⁶A modification level and translation efficiency. The m⁶A in the exon regions appeared to destabilize the mRNAs, whereas m⁶A in the untranslated regions positively correlated with the associated mRNA abundance. MhYTP2 overexpression enhanced apple powdery mildew resistance, possibly by rapidly degrading the bound mRNAs of MdMLO19 and MdMLO19‐X1 and improving the translation efficiency of the antioxidant genes. To conclude, the results shed light on the apple m⁶A profile, the effect of MhYTP2 on m⁶A profile, and the m⁶A roles in MdMLO19 and MdMLO19‐X1 mRNAs stability and glutamate dehydrogenase 1‐like MdGDH1L mRNA translation efficiency.

Drought seriously affects the yield and quality of apples. γ-aminobutyric acid (GABA) plays an important role in the responses of plants to various stresses. However, the role and possible mechanism of GABA in the drought response of apple seedlings remain unknown. To explore the effect of GABA on apple seedlings under drought stress, seedlings of Malus hupehensis were treated with seven concentrations of GABA, and the response of seedlings under 15-day drought stress was observed. The results showed that 0.5 mM GABA was the most effective at relieving drought stress. Treatment with GABA reduced the relative electrical conductivity and MDA content of leaves induced by drought stress and significantly increased the relative water content of leaves. Exogenous GABA significantly decreased the stomatal conductance and intercellular carbon dioxide concentration and transpiration rate, and it significantly increased the photosynthetic rate under drought. GABA also reduced the accumulation of superoxide anions and hydrogen peroxide in leaf tissues under drought and increased the activities of POD, SOD, and CAT and the content of GABA. Exogenous treatment with GABA acted through the accumulation of abscisic acid (ABA) in the leaves to significantly decrease stomatal conductance and increase the stomatal closure rate, and the levels of expression of ABA-related genes PYL4, ABI1, ABI2, HAB1, ABF3, and OST1 changed in response to drought. Taken together, exogenous GABA can enhance the drought tolerance of apple seedlings.

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.

Glomerella leaf spot (GLS), caused by the fungal pathogen Colletotrichum fructicola , significantly threatens apple production. Some resistances to plant disease are mediated by the accumulation of nucleotide‐binding site and leucine‐rich repeat (NBS‐LRR) proteins that are encoded by a major class of plant disease resistance genes (R genes). However, the R genes that confer resistance to GLS in apple remain largely unclear. Malus hupehensis YT521‐B homology domain‐containing protein 2 (MhYTP2) was identified as an N ⁶ ‐methyladenosine RNA methylation (m ⁶ A) modified RNA reader in our previous study. However, whether MhYTP2 binds to mRNAs without m ⁶ A RNA modifications remains unknown. In this study, we discovered that MhYTP2 exerts both m ⁶ A‐dependent and ‐independent functions by analysing previously obtained RNA immunoprecipitation sequencing results. The overexpression of MhYTP2 significantly reduced the resistance of apple to GLS and down‐regulated the transcript levels of some R genes whose transcripts do not contain m ⁶ A modifications. Further analysis indicated that MhYTP2 binds to and reduces the stability of MdRGA2L mRNA. MdRGA2L positively regulates resistance to GLS by activating salicylic acid signalling. Our findings revealed that MhYTP2 plays an essential role in the regulation of resistance to GLS and identified a promising R gene, MdRGA2L , for use in developing apple cultivars with GLS resistance.