In rice, OsABA8ox encodes abscisic acid (ABA) 8′-hydroxylase, which catalyzes the committed step of ABA catabolism.

The contribution of ABA catabolism in root development remains unclear. We investigated the role of OsABA8ox2 in root growth and development and drought response. GUS staining results showed that OsABA8ox2 was expressed mainly in roots at seedling stage and was strongly expressed in the meristematic zone of the radicle. OsABA8ox2 expression in roots was markedly decreased after 0.5 h polyethylene glycol (PEG) treatment and increased after 0.5 h rehydration, implying that OsABA8ox2 is a drought-responsive gene. OsABA8ox2 knockout mediated by the CRISPR-Cas9 system increased drought-induced ABA and indole-3-acetic acid accumulation in roots, conferred increased ABA sensitivity, and promoted a more vertically oriented root system architecture (RSA) beneficial to drought tolerance. OsABA8ox2 overexpression suppressed root elongation and increased stomatal conductance and transpiration rate. Consequently, OsABA8ox2 knockout dramatically improved rice drought tolerance, whereas OsABA8ox2 overexpression seedlings were hypersensitive to drought stress, suggesting that OsABA8ox2 contributes to drought response in rice. Compared with wild type, functional leaves of OsABA8ox2 knockout seedlings showed higher ABA levels, whereas overexpression lines showed lower ABA levels, suggesting that OsABA8ox2, as an ABA catabolic gene, modulates ABA concentration through ABA catabolism. OsABA8ox2 and OsABA8ox3 were both localized in the endoplasmic reticulum. Together, these results indicate that OsABA8ox2 suppresses root elongation of rice seedlings, increases water transpiration, and contributes to drought response through ABA catabolism, and that OsABA8ox2 knockout dramatically improves rice drought tolerance. They highlight the key role of ABA catabolism mediated by OsABA8ox2 on root growth and development. OsABA8ox2, as a novel RSA gene, would be a potential genetic target for the improvement of rice drought tolerance.





Figure 4: Root morphology of WT, OsABA8ox2 KO1 and OE2 transgenic seedlings. (A) Twelve-day-old seedlings grown on germination bags after 5-day 2% PEG-6000 treatment. Seedlings grown on germination bags containing sterile distilled water were used as control. White frame indicates the altered length ratio of root to shoot of KO1 and OE2. (B) Root of 6-week-old seedlings under soil drought stress imposed by withholding irrigation for 2 weeks. Lateral root growth of WT and KO1 was clearly increased under drought stress (top panel). Total root length of OE2 was much lower than those of WT and KO1.