A detailed analysis of the expression of a chimeric gene, consisting of the upstream region of the nuclear photosynthetic gene ST-LS1, encoding a component of the water-oxidizing complex of photosystem II, fused to the coding sequence of beta-glucuronidase (GUS) as a reporter, is described. The expression of this chimeric gene at the cellular level was detected by histochemical methods and shows that the expression of this gene is correlated with the presence of chloroplasts. Interestingly, the GUS activity was not only detected in typical photosynthetic tissues, e.g. leaves and stems, but also in green roots containing chloroplasts. In contrast no activity was detected in neighbouring white root tissue which was devoid of chloroplasts. One can therefore separate the relative importance of the (morphological) differentiation steps responsible for the formation of tissues normally involved in photosynthesis, from the importance of the developmental stage (characterized by the presence of chloroplasts), for the expression of this nuclear photosynthetic gene. Our data strongly suggest that the developmental stage of the plastids is the primary determinant for the activity of this nuclear photosynthetic gene, although they do not yet allow the exclusion of the reverse type of control, i.e. control of the differentiation of the plastid by the expression of certain nuclear genes. A chimeric gene, consisting of the promoter of the 35S cauliflower mosaic virus (CaMV) gene and the GUS coding sequence, was used as a control throughout the experiments, confirming that the observed differential ST-LS1-GUS gene expression reflects the particular transcriptional regulation impacted on this gene by its cis-acting regulatory sequences.