Possesses ppGpp (guanosine 3'-diphosphate 5'-diphosphate) synthetase activity in vitro and is able to functionally complement E.coli relA mutants. May be involved in a rapid plant ppGpp-mediated response to pathogens and other stresses. ATP + GTP = AMP + guanosine 3'-diphosphate 5'-triphosphate Expressed in roots, hypocotyls, shoots, cotyledons, rosette and cauline leaves, stems, petals, sepals, stamens, pistils and siliques. Circadian-regulation with a peak at noon. Induced by wounding, salt stress, 12-oxo-phytodienoic acid (OPDA) and abscisic acid (ABA). Belongs to the RelA/SpoT family.