Malic acid modulates adventitious root elongation through auxin accumulation and MdTCP17-mediated regulation in apple

• Malic acid (MA) suppresses adventitious root (AR) number but promotes AR elongation in apple microshoots. • MdTCP17 -RNAi lines show enhanced AR elongation and greater tolerance to high MA stress. • RNA-seq reveals MA's effect is mediated through auxin signaling pathways. • MdTCP17 overexpression disrupts the MA/IAA pathway, inhibiting rooting despite high auxin. The formation of adventitious roots (AR) is a crucial and complex step in apple propagation. Malic acid (MA), an essential organic acid, significantly influences root development; nevertheless, its precise effects on AR formation in apples are poorly understood. This study investigates the impact of MA on AR formation in MdTCP17 -overexpressing (T-OE), MdTCP17 -RNAi (T-RNAi), and wild-type (WT) apple microshoots. MA treatments reduced AR numbers across genotypes but promoted AR elongation, particularly in T-RNAi microshoots in response to medium MA concentration, which also exhibited enhanced tolerance to high MA concentrations. Anatomical observations indicated that MA concentration stimulates AR primordia formation in WT and T-RNAi microshoots. Endogenous hormone analysis revealed elevated indole-3-acetic acid (IAA) levels in T-RNAi and T-OE microshoots as compared to WT. RNA-seq analysis identified differentially expressed genes (DEGs) enriched in plant hormone signaling pathways, particularly auxin. The findings indicate that medium MA concentrations facilitated AR elongation in T-RNAi microshoots by enhancing IAA accumulation and modulating auxin-related gene expression. While MdTCP17 overexpression disrupted this process, consistent with its previously established role in suppressing key regulators like MdWOX11 , thereby overriding the positive effects of elevated IAA levels. These findings reveal MdTCP17 as a central regulator of root architecture plasticity, modulating the hormonal response to MA concentration to determine the balance between root initiation and elongation.