ABSTRACT
BACKGROUND: 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. RESULTS: 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. CONCLUSIONS: 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.
ABSTRACT
BACKGROUND: Iron is an important micronutrient for all living organisms. Almost 25% of the world population is affected by iron deficiency, a leading cause of anemia. In plants, iron deficiency leads to chlorosis and reduced yield. Both animals and plants may suffer from iron deficiency when their diet or environment lacks bioavailable iron. A sustainable way to reduce iron malnutrition in humans is to develop staple crops with increased content of bioavailable iron. Knowledge of where and how iron accumulates in seeds of crop plants will increase the understanding of plant iron metabolism and will assist in the production of staples with increased bioavailable iron. RESULTS: Here we reveal the distribution of iron in seeds of three Phaseolus species including thirteen genotypes of P. vulgaris, P. coccineus, and P. lunatus. We showed that high concentrations of iron accumulate in cells surrounding the provascular tissue of P. vulgaris and P. coccineus seeds. Using the Perls' Prussian blue method, we were able to detect iron in the cytoplasm of epidermal cells, cells near the epidermis, and cells surrounding the provascular tissue. In contrast, the protein ferritin that has been suggested as the major iron storage protein in legumes was only detected in the amyloplasts of the seed embryo. Using the non-destructive micro-PIXE (Particle Induced X-ray Emission) technique we show that the tissue in the proximity of the provascular bundles holds up to 500 microg g(-1) of iron, depending on the genotype. In contrast to P. vulgaris and P. coccineus, we did not observe iron accumulation in the cells surrounding the provascular tissues of P. lunatus cotyledons. A novel iron-rich genotype, NUA35, with a high concentration of iron both in the seed coat and cotyledons was bred from a cross between an Andean and a Mesoamerican genotype. CONCLUSIONS: The presented results emphasize the importance of complementing research in model organisms with analysis in crop plants and they suggest that iron distribution criteria should be integrated into selection strategies for bean biofortification.
