Functional Identification of MhPYL4 Involved in Iron-Deficiency Stress in Malus Halliana Koehne

The PYL protein family are crucial sensors of the core signals of abscisic acid (ABA) and significantly influence the plant's response to ABA-mediated abiotic stresses as well as its growth and development. However, research on the role of the <i>MhPYL4</i> gene in iron (Fe) deficiency in apple trees is limited. Studies have shown that the <i>MhPYL4</i> gene, when exposed to Fe-deficiency stress, exhibits more rapid transcriptional upregulation than other genes' quickly elevated transcription. However, the precise mechanism by which it alleviates this stress remains unclear. The <i>MhPYL4</i> gene (ID:103432868), isolated from <i>Malus halliana</i>, was analyzed to elucidate its function. <i>Arabidopsis</i> plants engineered to overexpress the <i>MhPYL4</i> gene exhibited increased leaf chlorosis and slower growth in response to Fe stress compared to the unmodified controls. The transgenic plants also exhibited elevated levels of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) activities, as well as ferric chelate reductase (FCR) activities. Levels of malondialdehyde (MDA), hydrogen peroxide (H₂O₂), and superoxide anion (O₂^-) were increased. In addition, these transgenic plants had lower concentrations of proline (Pro) and Fe²⁺, which indicated that their stress tolerance was reduced. Similarly, the overexpression of <i>MhPYL4</i> in apple calli resulted in inhibited growth and increased susceptibility under Fe stress conditions. Physiological evaluations indicated that the overexpression of <i>MhPYL4</i> in <i>Arabidopsis</i> reduced its Fe stress tolerance by inhibiting chlorophyll synthesis. In apple calli, it altered pH levels, antioxidant enzyme activity, and Fe-reducing capabilities under the same stress conditions. In summary, the elevated expression of the <i>MhPYL4</i> gene reduced the tolerance of both <i>Arabidopsis</i> and apple calli to Fe stress, suggesting that <i>MhPYL4</i> acts as a negative regulator in response to Fe deficiency.