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Root Exudation and Rhizosphere Biology

Travis S. Walker, Harsh Pal Bais1, Erich Grotewold4 and Jorge M. Vivanco

Plant Physiology May 2003 vol. 132 no. 1 44 - 51

The ability to secrete a vast array of compounds into the rhizosphere is one of the most remarkablemetabolic features of plant roots, with nearly 5% to 21% of all photosynthetically fixed carbon beingtransferred to the rhizosphere through root exudates.

Root exudates have traditionally been grouped into low and high compounds. Low compounds such asamino acids, organic acids, sugars, phenolics, and various other secondary metabolites are believed tocomprise the majority of root exudates, whereas high exudates primarily include mucilage (highpolysaccharides) and proteins.

Root-root and root-microbe communication can either be positive (symbiotic) to the plant, such as theassociation of epiphytes, mycorrhizal fungi, and nitrogen-fixing bacteria with roots; or negative to the

plant, including interactions with parasitic plants, pathogenic bacteria, fungi, and insects.

A large body of knowledge suggests that root exudates may act as messengers that communicate andinitiate biological and physical interactions between roots and soil organisms.

Allelopathy is mediated by the release of certain secondary metabolites by plant roots and plays animportant role in the establishment and maintenance of terrestrial plant communities. A secondarymetabolite secreted by the roots of knapweed (Centaurea maculosa) provides a classic example of rootexudates exhibiting negative root-root communication in the rhizosphere. Recently, identified ()-catechin as the root-secreted phytotoxin responsible for the invasive behavior of knapweed in therhizosphere. Interestingly, ()-catechin was shown to account for the allelochemical activity, whereas

(+)-catechin was inhibitory to soil-borne bacteria . In addition to racemic catechin being detected in theexudates of in vitro-grown plants, the compound was also detected in soil extracts from knapweed-invaded fields, which strongly supported the idea that knapweed's invasive behavior is due to theexudation of ()-catechin. Moreover, this study established the biological significance of the exudationof a racemic compound such as catechin, demonstrating that one enantiomer can be responsible for theinvasive nature of the plant, whereas the other enantiomer can contribute to plant defense.

In a more recent study, reported the characterization of fluorescent -carboline alkaloids from the rootexudates of O. tuberosa (oca). The main fluorescent compounds were identified as harmine (7-methoxy-1-methyl--carboline) and harmaline (3, 4-dihydroharmine); addition to their fluorescentnature, these alkaloids exhibit strong phototoxicity against a polyphagous feeder,Trichoplusia ni,suggesting their insecticidal activity may be linked to photoactivation. The Andean highlands, whereO. tuberosa is primarily cultivated, are subjected to a high incidence of UV radiation, and it wasobserved that the strongest fluorescence intensity occurred with oca varieties that showed resistance tothe larvae ofMycrotrypes spp., the Andean tuber weevil. These data suggest that UV light penetratingsoil layers could photoactivate fluorescent -carboline alkaloids secreted by oca roots to create aninsecticidal defense response.

In at least some plants, channels are likely to be involved in the secretion of organic acids normallypresent at high levels in the cytoplasm. A good example is provided by the exudation of citrate, malate,

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and related organic acids by maize and wheat (Triticum aestivum) in response to high Al3+concentrations. However, plants have the potential to express 100,000 compounds, primarily derivedfrom secondary metabolism.

The scarce information available suggests that the pattern of exudation is not homogeneous along theroot axis. Release of phytosiderophores in response to iron deficiencies appears to be concentrated in

the apical zones of the root. Release of organic anions would also follow a heterogeneous pattern alongthe root, which is consistent with the presence of a pH gradient from the tip to the base of the root.