Expression stability of selected reference genes as calculated by geNORM. The data sets used for the calculation of the average expression stability (M) values are provided in Additional file 2. 'All tested conditions' includes all probes tested (different cultivars, tissues and physiological conditions); 'Root tissue (100 mM salt stress)' includes all root samples of salt stress-treated plants; 'Root tissue' and 'Shoot tissue' include samples of non-stressed tissues of different cultivars. A lower M value indicates more stable expression. According to the geNORM manual, genes with M values >1.5 are not suitable reference genes for the selected conditions. Gene codes and primers used for the qRT-PCR experiments are given in Table 1.