Plant and Soil 245: 59-70 (2002)
Genetic control of root exudation
Z. Rengel
Soil Science and Plant Nutrition, Faculty of Agriculture,
The University of Western Australia, Nedlands WA 6907,

Abstract

The literature on genetics of root exudation and on genotypic differences in qualitative and quantitative composition of root exudates in crop and native plant species was critically assessed. Differences in exudation have been reported for genotypes that differ in tolerance to nutrient deficiencies, ion toxicities, and pathogen attack. The exudation profile of a limited number of genotypes (frequently only two genotypes with the contrasting response tothe environmental stress) have been reported to date. Little is known about the variability in larger samples of the germplasm or about actual genetics behind differential qualitative and quantitative composition of root exudates. Changing the exudation profile of a given genotype may be achieved by manipulating the biosynthetic capacity and by increasing the capacity of the plasma membrane to transport the specific compound out into the rhizosphere. Overexpression of the bacterial citrate synthase gene in the cytoplasm of tobacco plants resulted in exudation of large quantities of citrate into the rhizosphere and partial alleviation of the aluminium (Al) toxicity stress. A similar strategy of transforming plants with citrate synthase gene is being tried as a way of improving plant capacity to extract phosphorus (P) from soils with notoriously low P availability.

More research into the genetic basis of qualitative and quantitative differences in root exudation is warranted. Understanding the genetic control of root exudation, followed by manipulation of qualitative and quantitative composition of root exudates, will result in better adaptation of plants to environmental conditions and a greater yield of crops.

Introduction

Plants exude a variety of organic compounds (carboxylate anions, phenolics, carbohydrates, amino acids, enzymes, other proteins, etc.) and inorganic ions (protons, phosphate, other nutrients, etc.) into the rhizosphere to change the chemistry and biology of the rhizosphere and enhance adaptation to a particular environment (Crowley and Rengel, 1999). Common conditions under which plants increase exudation of organic compounds into the rhizosphere encompass deficiency of various nutrients [e.g., phosphorus (P), iron (Fe), zinc (Zn), manganese (Mn); Azaizeh et al., 1995; Marschner and Römheld, 1994; Rengel, 1997, 1999; Rengel et al., 1998; Subbarao et al., 1997], ion toxicity (e.g., Al Basu et al., 1994, 1999; de la Fuente et al., 1997; Delhaize et al., 1993; Ma et al., 1997; Ma and Miyasaka, 1998; Ryan et al., 1995a, b; Ryan et al., 1997; Zheng et al., 1998a, b), pathogen attack (e.g., Mandal and Sinha, 1991; Mehta et al., 1992; Parashar et al., 1992; Stevenson et al., 1995), and nodulation of legume hosts (Bolanos-Vasquez and Werner, 1997; Gagnon and Ibrahim, 1998; Janczarek et al., 1997; Maxwell et al., 1989; Wojtaszek et al., 1993; Zuanazzi et al., 1998).

Understanding of the role that root exudates play in increasing plant adaptation to a given set of environmental conditions is sketchy at present. However, it is generally accepted that the type of exudates plants release into the rhizosphere has played a significant role in the distribution of plant species in various ecosystems. Calcicole plants exude mainly di- and tricarboxylic acids (with the former being good extractors of P and the latter good extractors of Fe and Mn from calcareous soils), while calcifuge plants exude mostly monocarboxylic acids (poor in mobilising P or Fe from calcareous soils) (Ström, 1997; Strömet al., 1994; Tyler and Ström, 1995). Similar distinctions also exist between crop plants (cf. Zhang et al., 1997).

Effective exudation of organic substances into the rhizosphere relies on at least three processes: 1, signalling sequence; 2, effective biosynthetic machinery producing a relatively large amount of potential exudate compounds, and 3, a membrane transporter that allows transfer of organic compounds into the rhizosphere.


CybeRose note: The rest of this paper deals with gene-splicing of crop plants, and is worth reading.