Inoculation insensitive promoters for cell type enriched gene expression in legume roots and nodules

Establishment and maintenance of mutualistic plant-microbial interactions in the rhizosphere and within plant roots involve several root cell types. The processes of host-microbe recognition and infection require complex signal exchange and activation of downstream responses. These molecular events coordinate host responses across root cell layers during microbe invasion, ultimately triggering changes of root cell fates. The progression of legume root interactions with rhizobial bacteria has been addressed in numerous studies. However, tools to globally resolve the succession of molecular events in the host root at the cell type level have been lacking. To this end, we aimed to identify promoters exhibiting cell type enriched expression in roots of the model legume Lotus japonicus, as no comprehensive set of such promoters usable in legume roots is available to date.

Here, we use promoter:GUS fusions to characterize promoters stemming from Arabidopsis, tomato (Lycopersicon esculentum) or L. japonicus with respect to their expression in major cell types of the L. japonicus root differentiation zone, which shows molecular and morphological responses to symbiotic bacteria and fungi. Out of 24 tested promoters, 11 showed cell type enriched activity in L. japonicus roots. Covered cell types or cell type combinations are epidermis (1), epidermis and cortex (2), cortex (1), endodermis and pericycle (2), pericycle and phloem (4), or xylem (1). Activity of these promoters in the respective cell types was stable during early stages of infection of transgenic roots with the rhizobial symbiont of L. japonicus, Mesorhizobium loti. For a subset of five promoters, expression stability was further demonstrated in whole plant transgenics as well as in active nodules.

11 promoters from Arabidopsis (10) or tomato (1) with enriched activity in major L. japonicus root and nodule cell types have been identified. Root expression patterns are independent of infection with rhizobial bacteria, providing a stable read-out in the root section responsive to symbiotic bacteria. Promoters are available as cloning vectors. We expect these tools to help provide a new dimension to our understanding of signaling circuits and transcript dynamics in symbiotic interactions of legumes with microbial symbionts.