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.

中文翻译：

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外源葡萄糖通过 Indole-3-Acetic Acid (IAA) 调节 Malus baccata (L.) Borkh 的根形态和碳氮代谢。在有机碳含量低的土壤中

在土壤有机质 (SOM) 限制条件下评估外源有机碳源（即葡萄糖）和吲哚-3-乙酸（IAA）对苹果砧木根系结构和碳氮（C-N）代谢的影响使适应。Malus baccata (L.) Borkh。幼苗暴露于 1.5 g kg-1 葡萄糖 (GLC)、0.09 g kg-1 IAA、0.06 g kg-1 2,3,5-三碘苯甲酸（TIBA，一种广泛使用的生长素极性转运抑制剂）、GLC + IAA、或 GLC + TIBA 30 天。GLC 和 IAA 都通过调节 SHY2、SHR、ALF4 和 LBD11 来促进根结构和生长。它们还增强了 C-N 代谢，并加速了硝酸盐向氨基酸的转化。相比之下，TIBA 通过下调生长素极性转运基因的表达，降低根中内源性 IAA 含量并抑制植物生长和 C-N 代谢，尽管诱导了生长素生物合成基因。由于生长素生物合成基因（YUCCA8、TAR2、TAA1 和 CYP79B3）和极性转运基因的表达上调，GLC + TIBA 可以减轻这些不利影响，与单独 TIBA 处理的幼苗相比，根中内源 IAA 含量更高。 PIN1、AUX1 和 LAX2）。此外，在GLC+TIBA条件下，参与碳代谢的酶（PEPC、NADP-ME和NADP-ICDH）的转录和活性增强可以为根中的氮代谢提供更多的有机酸、三磷酸腺苷（ATP）和能量电荷比在 TIBA 下的根。诱导的 NR、GS、NADH-GDH、NADH-GOGAT 活性和参与 N 代谢的基因的 mRNA 水平表明，GLC + TIBA 下的根比单独 TIBA 下的根具有更高的 N 同化能力。综上所述，外源葡萄糖介导的 IAA 生物合成和极性转运调节 M. baccata (L.) Borkh 的根结构和 C-N 代谢。在低 SOM 条件下。