eIF4A is a highly conserved RNA-stimulated ATPase and helicase involved in the initiation of mRNA translation. Previously, we found that eukaryotic initiation factor (eIF) 4A interacts with cyclin dependent kinase A (CDKA), the plant ortholog of mammalian CDK1. Here we show that this interaction occurs only in proliferating cells where the two proteins co-associate with 5'-cap-binding protein complexes, eIF4F or the plant specific eIFiso4F. CDKA phosphorylates eIF4A on a conserved threonine residue (T164) within the RNA-binding motif 1b "TPGR". In vivo, a phospho-null (APGR) variant of the Arabidopsis eIF4A1 protein retains the ability to functionally complement a mutant (eif4a1) plant line lacking eIF4A1, whereas a phosphomimetic (EPGR) variant fails to complement. The phospho-null variant (APGR) rescues the slow growth rate of roots and rosettes, together with the ovule abortion and late flowering phenotypes. In vitro, wild type recombinant eIF4A1 and its phospho-null variant both support translation in cell free wheat germ extracts dependent upon eIF4A, but the phosphomimetic variant does not support translation and was also deficient in ATP hydrolysis and helicase activity. These observations suggest a mechanism whereby CDK phosphorylation has the potential to down regulate eIF4A activity and thereby affect translation.