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Nitrate signalling on the nitrate reductase (Nia1) gene promoter from Chlamydomonas reinhardtii has been studied by using a construct of the Nia1 promoter transcriptionally fused to the Chlamydomonas arylsulphatase gene as a reporter in strains bearing different sets of nitrate/nitrite transport genes. The high‐affinity nitrate transport (HANT) system I is required for efficient signalling by nitrate, even at submicromolar concentrations of the anion. In addition, the autogenous regulation of nitrate reductase has been found to depend on the presence of system I. The low‐affinity nitrate transport system III promoted signalling optimally on the promoter at millimolar nitrate concentrations. The HANT system IV, which is insensitive to ammonium and active at low CO₂, allowed nitrate signalling at micromolar concentrations even in the presence of ammonium, suggesting that the balance of these two effectors controls …