ABA plays essential roles in regulating root growth by interacting with auxin and MAPK signaling pathways and cell-cycle machinery in rice seedlings

In this study, we examined the relationship between abscisic acid (ABA), auxin, and mitogen-activated protein kinase (MAPK) signaling and the cell cycle in rice (Oryza sativa L. cv. Zhonghua No. 11) roots. Root system growth was significantly promoted by 0.1 nM ABA but markedly inhibited by 0.3 mM Tungstate (TS, an ABA biosynthesis inhibitor). Detection of plants treated with DRB (an RNA synthesis inhibitor, 5,6-dichlorobenzimidazole 1-β-d-ribofuranoside), TIBA (2,3,5-triiodobenzoic acid, an inhibitor of polar auxin transport), or BFA (brefeldin, a protein transport inhibitor) using DR5-GUS staining revealed that ABA regulates the distribution of auxin via transcription and transport pathways. The expression of some auxin- and cell cycle-related genes, as well as several ABA or MAPK genes, was differentially regulated in roots by ABA and TS at 7 or 11 days, indicating that a certain level of ABA action may primarily target specific genes involved in MAPK and auxin signaling and the cell cycle at a given developmental stage. The results suggest that an appropriate level of ABA determines auxin homeostasis through controlling gene expression and the distribution of auxin, and that ABA regulates the auxin and MAPK signaling pathways and influences cell-cycle progression. Overall, ABA plays positive roles in regulating root system growth by modulating MAPK and auxin signaling and the cell cycle. Our findings help elucidate the integrative effects of ABA, auxin, and MAPK signaling and the cell cycle on root growth. We propose a working model for the role of ABA in root system growth.