Possesses calcium-dependent ppGpp (guanosine 3'-diphosphate 5'-diphosphate) synthetase activity in vitro and is able to functionally complement E.coli relA mutants. Plays an important role in the timing adjustment of pistil and pollen maturation required for successful pollination. 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, cotyledons, rosette and cauline leaves, stems, sepals, pistils and siliques. Circadian-regulation with a peak at midnight. The calcium-binding sites of the 2 EF-hand domains are required for enzyme activity. Plants silencing CRSH have abnormally small siliques with few seeds, due to altered timing of pistil and pollen maturation and unsuccessful pollination. Belongs to the RelA/SpoT family.