Possesses calcium-dependent 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 Activated by calcium. Expressed in shoots. The calcium-binding sites of the 2 EF-hand domains are required for enzyme activity. Belongs to the RelA/SpoT family.