Boron deficiency alters root growth and development and interacts with auxin metabolism by influencing the expression of auxin synthesis and transport genes

Boron (B) deficiency inhibits and disturbs root growth and development by interacting with auxin (indole-3-acetic acid, IAA). However, the underlying mechanism of this interaction is still poorly understood. This study found that plants in a long-term boron deprivation treatment (∼0.25 µg L⁻¹) had inhibited elongation of trifoliate orange roots, enlarged root tips and severe necrosis as well as a significant decrease in soluble boron and IAA content in seedling root tips. The results of a short-term boron deprivation treatment showed that a significant decrease in soluble boron and IAA content occurred after 3 hours of treatment (HOT) and 1 day of treatment (DOT), respectively. Moreover, the expression of IAA synthetic genes (TAA1, TAR2, YUC3 and YUC8) was strongly induced as early as 3 HOT and was then significantly reduced. The expression of rootward IAA transport genes (AUX1, PIN1 and PIN4) decreased significantly in the boron deprivation treatment, but the expression levels of shootward IAA transport genes (LAX1, ABCB1 and PIN3) were significantly increased. Taken together, the increase in IAA content before 1 DOT may be due to increased IAA synthesis caused by the induction of TAA1, TAR2, YUC3 and YUC8 expression; the subsequent decrease and the significantly lower final IAA content compared to the +B treatment (0.25 mg L⁻¹ B) may be due to the reduced expression of IAA synthetic genes and rootward IAA transport genes and the increased expression of shootward IAA transport genes. Abbreviations: DAT: days after treatment; DOT: day of treatment; FW: fresh weight; HOT: hour of treatment; isB: insoluble boron; LC: liquid chromatography; LSD: least-significant difference; MS: mass spectrometry; PAT: polar auxin transport; qRT-PCR: quantitative real-time polymerase chain reaction; sB: soluble boron