The WNK (with no lysine kinase) protein kinase gene family may be involved in regulating ion homeostasis and other physiological processes in mammals. WNK-like genes have also been identified in plants, including nine in Arabidopsis, designated AtWNK1-AtWNK9. However, it is not clear if there are further plant WNK genes, and what the evolutionary relationships are among these genes, nor if these genes have functions other than their roles in regulating circadian rhythms and vacuolar H(+)-ATPase. In the present study, we found a tenth Arabidopsis WNK gene, designated AtWNK10. Further phylogenetic analysis of Arabidopsis and rice WNK genes suggests that the most recent common ancestor of monocots and eudicots had 4-8 WNK genes, and that the WNK gene family has experienced duplication events and possible gene losses. Semi-quantitative RT-PCR revealed that all Arabidopsis WNK genes except AtWNK6 are expressed in organs from the seedling to the flowering plant. T-DNA knockout mutations in the AtWNK2, AtWNK5 and AtWNK8 genes in different phylogenetic clades caused early flowering. In contrast, a T-DNA knockout wnk1 mutant had a much delayed flowering time. In addition, the transcript levels of several genes in the photoperiod pathway for flowering, such as ELF4, TOC1, CO and FT, were altered in atwnk mutants. Taken together, our results suggest that the Arabidopsis WNK gene family regulates flowering time by modulating the photoperiod pathway.