Required for tolerance to DNA-damaging and cross-linking agents such as UVB irradiation, gamma-radiation, aphidicolin, ionizing radiation and hydroxyurea (HU), cisplatin (CDDP) and mitomycin C (MMC) (PubMed:19619159, PubMed:19619158, PubMed:28556304). Involved in cell-cycle G2/M arrest in response to DNA damage (PubMed:19619159, PubMed:19619158). Required for aluminum-dependent gene regulation and root growth inhibition in an ATR-dependent manner by halting cell cycle progression and triggering loss of the quiescent center (QC) (PubMed:28556304). Forms multimers through the coiled-coil domain. Accumulates throughout the root tip. Becomes more restricted to the distal region of the root tip upon addition of aluminum (Al). Probably phosphorylated by ATR. Hypersensitivity to DNA-damaging agents including UVB, gamma-radiation, aphidicolin, ionizing radiation and hydroxyurea (HU) (PubMed:19619159, PubMed:19619158). Defective in cell-cycle G2/M arrest in response to DNA damage (PubMed:19619159, PubMed:19619158). Reversed extreme hypersensitivity to aluminum (Al) of the mutant als3-1, including Al-dependent terminal differentiation of the root tip and transition to endoreduplication. Increased tolerance to Al (PubMed:28556304). Belongs to the serpin family.