The rhizosphere encompasses the millimeters of soil surrounding a plant root where complex biological and ecological processes occur. This review describes recent advances in elucidating the role of root exudates in interactions between plant roots and other plants, microbes, and nematodes present in the rhizosphere. Evidence indicating that root exudates may take part in the signaling events that initiate the execution of these interactions is also presented. Various positive and negative plant-plant and plant-microbe interactions are highlighted and described from the molecular to the ecosystem scale. Furthermore, methodologies to address these interactions under laboratory conditions are presented. [ Addendum ]

Rapid, long-distance signaling in plants: A plant injured on one leaf by a nibbling insect can alert its other leaves to begin anticipatory defense responses. Working in the model plant Arabidopsis , Toyota et al. show that this systemic signal begins with the release of glutamate, which is perceived by glutamate receptor–like ion channels (see the Perspective by Muday and Brown-Harding). The ion channels then set off a cascade of changes in calcium ion concentration that propagate through the phloem vasculature and through intercellular channels called plasmodesmata. This glutamate-based long-distance signaling is rapid: Within minutes, an undamaged leaf can respond to the fate of a distant leaf. Science , this issue p. 1112 ; see also p. 1068

Here we present evidence that Centaurea maculosa (spotted knapweed), an invasive species in the western United States, displaces native plant species by exuding the phytotoxin (–)-catechin from its roots. Our results show inhibition of native species' growth and germination in field soils at natural concentrations of (–)-catechin. In susceptible species such as Arabidopsis thaliana , the allelochemical triggers a wave of reactive oxygen species (ROS) initiated at the root meristem, which leads to a Ca ^{2 +} signaling cascade triggering genome-wide changes in gene expression and, ultimately, death of the root system. Our results support a “novel weapons hypothesis” for invasive success.

Significance: This work documents a previously unreported plant-wide signaling system based on the rapid, long-distance transmission of Ca ²⁺ waves. In the root these waves move through the cortical and endodermal cell layers at speeds of up to 400 µm/s, i.e., traversing several cells per second. This Ca ²⁺ wave system correlates with the triggering of molecular responses in distant parts of the plant upon perception of localized (salt) stress. Such propagating Ca ²⁺ waves provide a new mechanism for the rapid integration of activities throughout the plant body.

ROS, calcium, and electric signals mediate rapid systemic signaling in plants.