Organisms must respond to environmental changes if they are to survive. As a result, species have evolved numerous intracellular and intercellular regulatory systems that often reflect an organism’s environment. In bacteria, one of the most important regulatory systems is the stringent response (Cashel et al., 1996). Signaling via this response is mediated by guanosine 5’-triphosphate 3’-diphosphate (pppGpp) and guanosine 5’-diphosphate 3’diphosphate (ppGpp), which function as second messengers. The stringent response was first discovered over 40 years ago in Escherichia coli. When E. coli cells are grown under nutrient-rich conditions but then transferred to a nutrient-limited environment, intracellular levels of pppGpp and ppGpp ((p)ppGpp) rapidly increase (Cashel et al., 1996). (p)ppGpp controls many vital cellular processes, including transcription and translation. For example, (p)ppGpp directly binds RNA polymerase and alters its promoter-binding affinity (Chatterji et al., 1998; Toulokhonov et al., 2001; Artsimovitch et al., 2004). When nutrient availability changes, therefore, the stringent response simultaneously adjusts the level of transcription for many genes. In E. coli, synthesis and degradation of (p)ppGpp are catalyzed by two enzymes RelA and SpoT (Cashel et al., 1996). Deficiencies in iron, phosphate, nitrogen, or carbon each represent environmental stresses that trigger (p)ppGpp accumulation (Cashel et al., 1996). For photosynthetic bacteria, sunlight is also an important “nutrient”. Characterization of a SpoT homolog in the purple photosynthetic bacterium, Rhodobacter capsulatus, showed that the stringent response also regulates photosynthesis (Masuda & Bauer, 2004). Genes that encode (p)ppGpp synthases and hydrolases are highly conserved in plants (van der Biezen et al., 2000; Kasai et al., 2002; Yamada et al., 2003; Givens et al., 2004; Tozawa et al., 2007; Masuda et al., 2008a; Kim et al., 2009) and are called RSHs (RelA/SpoT homologs). All known plant RSHs are targeted to chloroplasts, suggesting that they may control chloroplast function. Here we summarize our current understanding of the stringent response in phototrophs. For details concerning the mechanisms of the stringent response itself, several recent reviews are available (Magnusson et al., 2005; Braeken et al., 2006; Jain et al., 2006; Ochi, 2007; Potrykus & Cashel, 2008; Srivatsan & Wang, 2008).