Cell cycle regulatory kinase that is not rate-limiting for cycle progression under normal growth conditions. Transcriptionally activated upon DNA stress or damage in an ATR- or ATM-dependent manner. Once activated, inhibits plant growth by arresting dividing cells in the G2 phase before proceeding into mitosis. Down-regulates CDKA-1 and CDKD-2 by tyrosine phosphorylation. May target principally CDKA-1. ATP + L-tyrosyl-[protein] = ADP + H(+) + O-phospho-L-tyrosyl-[protein] Interacts with CDKA-1, but not with CDKB1-1. Expressed in shoot apex, vasculatures tissues of roots and leaves, and developing flowers. By replication blocking agents (hydroxyurea and aphidicolin). Plants show no obvious cell division or endoreduplication phenotype when grown under nonstress conditions, but are hypersensitive to agents that impair DNA replication. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family. WEE1 subfamily.