The viral 35S CaMV promoter is considered to be constitutive and is widely used to control expression of RNAi to suppress gene activity in order to study the role of plant genes involved in plant-microorganism interactions. In the interaction between Medicago truncatula and Sinorhizobium meliloti the activity of the 35S promoter appears differentially regulated, with a complete absence of activity in nodule bacteroid-containing cells. An alternative is the EF1alpha promoter of Arabidopsis, which has a high level of expression during all stages of nodule development. The cover image shows the visualisation of promoter activity in roots and nodules of M. truncatula by GUS staining or GFP fluorescence. For the article by Auriac and Timmers, see page 1040.