Exogenous Glucose Mediates the Regulation of Root Morphology and Carbon–Nitrogen Metabolism by Indole-3-Acetic Acid (IAA) in Malus baccata (L.) Borkh. in Soil with Low Organic Carbon Content

The effects of an exogenous organic carbon source (i.e., glucose) and indole-3-acetic acid (IAA) on root architecture and carbon–nitrogen (C–N) metabolism of apple rootstock were evaluated under soil organic matter (SOM)-restricted conditions. Malus baccata (L.) Borkh. seedlings were exposed to 1.5 g kg⁻¹ glucose (GLC), 0.09 g kg⁻¹ IAA, 0.06 g kg⁻¹ 2,3,5-triiodobenzoic acid (TIBA, a widely used auxin polar transport inhibitor), GLC + IAA, or GLC + TIBA for 30 days. Both GLC and IAA promoted root architecture and growth by regulating SHY2, SHR, ALF4, and LBD11. They also enhanced C–N metabolism, and accelerated nitrate transformation to amino acids. In contrast, TIBA reduced endogenous IAA content in root and inhibited plant growth and C–N metabolism by downregulating expression of auxin polar transport genes, although auxin biosynthesis genes were induced. These adverse effects could be alleviated in GLC + TIBA, which exhibited higher endogenous IAA content in root than TIBA-treated seedlings alone, due to the upregulated expression of auxin biosynthesis genes (YUCCA8, TAR2, TAA1, and CYP79B3) and polar transport genes (PIN1, AUX1, and LAX2). In addition, the enhanced transcription and activities of enzymes involved in C metabolism (PEPC, NADP-ME, and NADP-ICDH) could provide more organic acids, adenosine triphosphate (ATP), and energy charge for N metabolism in roots under GLC + TIBA than in roots under TIBA. The induced NR, GS, NADH-GDH, NADH-GOGAT activities and mRNA levels of genes involved in N metabolism indicated the higher N assimilation ability in roots under GLC + TIBA than in roots under TIBA alone. In conclusion, exogenous glucose-mediated IAA biosynthesis and polar transport regulates root architecture and C–N metabolism of M. baccata (L.) Borkh. under low-SOM conditions.