Cyclic guanosine 3',5'-monophosphate (cGMP) is an important second messenger in animals, and is emerging as a player in regulatory functions in plants. In this study, we investigated the role of cGMP in seed germination in Arabidopsis thaliana (Col-0). We demonstrated that both, a membrane-permeant analogue of cGMP (8-Br-cGMP) and the cyclic nucleotide phosphodiesterase (PDE) inhibitor Tadalafil promoted A. thaliana seed germination, whereas the guanylate cyclase inhibitor LY 83583 (6-anilino-5,8-quinolinedione; LY) inhibited it. LY blocked gibberellic acid (GA)-induced seed germination, whereas GA and 8-Br-cGMP co-treatment increased the germination rate and more effectively overcame LY-inhibition than 8-Br-cGMP alone. The gibberellin biosynthesis inhibitor paclobutrazol (PAC) also blocked 8-Br-cGMP and Tadalafil promotion of seed germination. Furthermore, 8-Br-cGMP and Tadalafil decreased abscisic acid (ABA) sensitivity during seed germination. These findings highlight that cGMP is a positive regulator and plays a crucial role in Arabidopsis seed germination. Furthermore, both GA and cGMP are required for seed germination. (C) 2010 Elsevier GmbH. All rights reserved.