SUMMARY Jasmonate (JA) induces the biosynthesis of anthocyanin and proanthocyanidin. MdMYB9 is essential for modulating the accumulation of both anthocyanin and proanthocyanidin in apple, but the molecular mechanism for induction of anthocyanin and proanthocyanidin biosynthesis by JA is unclear. In this study, we discovered an apple telomere‐binding protein (MdTRB1) to be the interacting protein of MdMYB9. A series of biological assays showed that MdTRB1 acted as a positive modulator of anthocyanin and proanthocyanidin accumulation, and is dependent on MdMYB9. MdTRB1 interacted with MdMYB9 and enhanced the activation activity of MdMYB9 to its downstream genes. In addition, we found that the JA signaling repressor MdJAZ1 interacted with MdTRB1 and interfered with the interaction between MdTRB1 and MdMYB9, therefore negatively modulating MdTRB1‐promoted biosynthesis of anthocyanin and proanthocyanidin. These results show that the JAZ1–TRB1–MYB9 module dynamically modulates JA‐mediated accumulation of anthocyanin and proanthocyanidin. Taken together, our data further expand the functional study of TRB1 and provide insights for further studies of the modulation of anthocyanin and proanthocyanidin biosynthesis by JA.

Abscisic acid (ABA) induces chlorophyll degradation and leaf senescence; however, the molecular mechanism remains poorly understood, especially in woody plants. In this study, we found that MdABI5 plays an essential role in the regulation of ABA-triggered leaf senescence in Malus domestica (apple). Through yeast screening, three transcription factors, MdBBX22, MdWRKY40 and MdbZIP44, were found to interact directly with MdABI5 in vitro and in vivo. Physiological and biochemical assays showed that MdBBX22 delayed leaf senescence in two pathways. First, MdBBX22 interacted with MdABI5 to inhibit the transcriptional activity of MdABI5 on the chlorophyll catabolic genes MdNYE1 and MdNYC1, thus negatively regulating chlorophyll degradation and leaf senescence. Second, MdBBX22 interacted with MdHY5 to interfere with the transcriptional activation of MdHY5 on MdABI5, thereby inhibiting the expression of MdABI5, which also contributed to the delay of leaf senescence. MdWRKY40 and MdbZIP44 were identified as positive regulators of leaf senescence. They accelerated MdABI5-promoted leaf senescence through the same regulatory pathways, i.e., interacting with MdABI5 to enhance the transcriptional activity of MdABI5 on MdNYE1 and MdNYC1. Taken together, our results suggest that MdABI5 works with its positive or negative interaction partners to regulate ABA-mediated leaf senescence in apple, in which it acts as a core regulator. The antagonistic regulation pathways ensure that plants respond to external stresses flexibly and efficiently. Our results provide a concept for further study on the regulation mechanisms of leaf senescence.

Iron (Fe) is an essential element for plant growth, development and metabolism. Due to its lack of solubility and low bioavailability in soil, Fe levels are usually far below the optimum amount for most plants' growth and development. In apple production, excessive use of nitrogen fertilizer may cause iron chlorosis symptoms in the newly growing leaves, but the regulatory mechanisms underlying this phenomenon are unclear. In this study, low nitrate (NO 3 − , LN) application alleviated the symptoms of Fe deficiency and promoted lower rhizosphere pH, which was beneficial for root Fe acquisition. At the same time, LN treatment increased citrate and abscisic acid accumulation in roots, which promoted Fe transport from root to shoot and maintained Fe homeostasis. Moreover, qRT-PCR analysis showed that nitrate application caused differential expression of genes related to Fe uptake and transport, as well as transcriptional regulators. In summary, our data reveal that low nitrate alleviated Fe deficiency through multiple pathways, demonstrating a new option for minimizing Fe deficiency by regulating the balance between nutrients.Fe is an essential element for plant growth and development, due to its role in iron-sulphur (FeS) proteins, ferredoxins and various metabolic enzymes. It impacts various cellular processes, including photosynthesis, respiration, electron transfer reactions and others (Connorton, Balk, & Rodríguez-Celma, 2017). Excessive Fe causes yellow-brown spots on older leaves and represses plant growth, whereas Fe deficiency triggers chlorosis and reduces fruit yields

Nitrate is a major nitrogen resource for plant growth and development and acts as both a crucial nutrient and a signaling molecule for plants; hence, understanding nitrate signaling is important for crop production. Abscisic acid (ABA) has been demonstrated to be involved in nitrate signaling, but the underlying mechanism is largely unknown in apple. In this study, we found that exogenous ABA inhibited the transport of nitrate from roots to shoots in apple, and the transcription of the nitrate transporter MdNRT1.5/MdNPF7.3 was noticeably reduced at the transcriptional level by ABA, which inhibited the transport of nitrate from roots to shoots. Then, it was found that the ABA-responsive transcription factor MdABI5 bound directly to the ABRE recognition site of the MdNRT1.5 promoter and suppressed its expression. Overexpression of MdABI5 inhibited ABA-mediated transport of nitrate from roots to shoots. Overall, these results demonstrate that MdABI5 regulates the transport of nitrate from roots to shoots partially by mediating the expression of MdNRT1.5, illuminating the molecular mechanism by which ABA regulates nitrate transport in apple.

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