Strigolactones enhance apple drought resistance via the MsABI5-MsSMXL1-MsNAC022 cascade

Drought stress limits plant growth, development, and yield in apple (<i>Malus</i>). Strigolactones (SLs) work with abscisic acid (ABA) to improve drought resistance in plants, but how this synergistic mechanism functions remains unclear. Here, we determined that SLs promote drought resistance in apple in an <i>ABSCISIC ACID INSENSITIVE5</i> (<i>MsABI5</i>)-related manner. During drought stress of a wild apple species (<i>Malus sieversii</i>), SLs enhanced the expression of <i>MsABI5</i>, encoding a major transcription factor involved in ABA signaling. MsABI5 bound to the promoter of the gene encoding delta-1-pyrroline-5-carboxylate synthase (<i>MsP5CS2.2</i>), upregulating its expression and thereby enhancing proline accumulation and drought resistance. In addition, MsABI5 suppressed the expression of <i>MsSMXL1</i>, encoding a major transcriptional repressor involved in SL signaling. MsSMXL1 interacted with MsNAC022 instead of MsABI5 to repress the transactivation activity of MsNAC022. <i>MsNAC022</i> was upregulated by MsABI5, and MsNAC022 directly promoted <i>MsP5CS2.2</i> expression to enhance proline accumulation and drought resistance. These findings suggest that MsSMXL1 and MsNAC022 comprise a regulatory node downstream of MsABI5 during drought stress in apple. Together, our findings suggest that in apple, SLs increase drought resistance by activating the MsABI5-MsSMXL1-MsNAC022 cascade.