The formation of bottlebrush-like root clusters in white lupin (Lupinus albus), as well as in members of the Proteaceae and some other plant species, is closely linked with intense chemical changes at the soil-root interface (rhizosphere) of these root structures, mediating the mobilization particularly of sparingly soluble forms of soil phosphorus (P) but also of iron and micronutrients. The whole range of known modifications comprises: (1) intense rhizosphere acidification, (2) increased reductive capacity, (3) release of large amounts of organic metal chelators (mainly citrate) and phenolics, (4) secretion of ectoenzymes (phosphatases involved in hydrolysis of organic P forms), and (5) high affinity P uptake. The paper describes the expression of these modifications as related to corresponding changes in root metabolism and presents a model for the putative physiological base of chemical nutrient mobilization in cluster roots. We are grateful that our scientific colleagues found this paper of interest in stimulating more detailed studies on the physiological and molecular base of plant adaptations to environments with limited availability of mineral nutrients.

The paper demonstrates the extreme importance of considering temporal and spatial variations in root activity for root-physiological studies. It presents some easy applicable methodological approaches for the analysis of root exudation and nutrient uptake with high spatial resolution and monitoring of metabolic alterations in the corresponding root zones.

The paper points out the potential of cluster roots in Lupinus albus as a well-characterized model system to study nutrient mobilization in the rhizosphere, plant-microbial interactions as related to root exudations, and root-induced chemical changes in the rhizosphere, as well as to define putative molecular targets for manipulation of crop plants with respect to improved nutrient efficiency, metal tolerance, and strategies for phytoremediation and phytomining.

I joined the rhizosphere working group of the late Prof. Horst Marschner at the Institute of Plant Nutrition at the University of Hohenheim, Stuttgart, Germany as a post doc in 1995. In 1996 we started a research collaboration with Enrico Martinoia´s group in Neuchâtel/Zürich, Switzerland to investigate the physiological mechanisms involved in the regulation of nutrient acquisition in cluster roots. The manuscript discussed in this section was the first basic publication arising from this collaboration.

Günter Neumann 
Institute of Plant Nutrition
University of Hohenheim
Stuttgart, Germany