Abstract
β-glucuronidase (GUS) is a useful reporter for the evaluation of promoter characteristics in transgenic plants. Here, we introduce an original technique to quantify the strength of promoters at subtissue resolution of cell clusters. The method combines cryotomy, laser microdissection, and improved fluorometric analysis of GUS activity using 6-chloro-4-methylumbelliferyl-β-D-glucuronide as an efficient fluorogenic substrate for kinetic studies in plants. The laser microdissection/6-chloro-4-methylumbelliferyl-β-D-glucuronide method is robust and reliable in a wide range of GUS expression levels and requires extremely low (few cells) tissue amounts. Suitability of the assay was demonstrated on rapeseed (Brassica napus) plants transformed with a P35S2::GUS construct. GUS expression patterns were visualized and quantified in approximately 30 tissues of vegetative and generative organs. Considerable differences in promoter activity within the tissues are discussed in relation to the cell type and developmental state.