Essential for recycling GMP and indirectly, cGMP. Required for normal development of the gametophyte and embryo, in association with GK1. ATP + GMP = ADP + GDP Monomer. A number of isoforms are produced. According to EST sequences. Growth retardation, narrow and dark-green leaves with elongated petioles, root growth inhibition and reduced fertility. The double mutant atgk1 and atgk2 is lethal. Belongs to the guanylate kinase family.