Generation and possible roles of NO in plant roots and their apoplastic space

In recent years, three different enzymatic pathways and a few non-enzymatic reactions have been proposed for the generation of NO in plant roots. Two of the enzymatic pathways are located in the cytosol of the plant cells, whereas the third is exclusively located in the root plasma membrane facing the apoplast from where it seems to interact with nitrate metabolism by producing signals. A response of the NO pathways to external nitrate concentrations by preventing excess nitrate nutrition, particularly in the apoplast, as well as a regulatory role in root morphogenesis of NO in interaction with plant hormones is suggested. Other functions of NO, those in stimulating plant defence reactions against pathogens and against abiotic stress are reported. In addition to enzymatic NO formation by the plant, sources of NO in the soil, and hence in the rhizosphere from bacterial nitrification and denitrification, are discussed in view of their possible interaction with the plant roots. A synoptical perspective is given on the assumed roles of apoplastic NO in plant roots, based upon known facts and with some assumptions about the gaps in current knowledge.