Essential for recycling GMP and indirectly, cGMP. Essential for chloroplast differentiation at early stage of leaf development. May not be involved in the synthesis and maintenance of the organellar DNA during leaf development. ATP + GMP = ADP + GDP Monomer. No visible phenotype under normal growth conditions at the permissive temperature of 30 degrees Celsius, but mutant seedlings develop chlorotic leaves with aberrant chloroplasts under the restrictive temperature of 20 degrees Celsius. Belongs to the guanylate kinase family.