Food Security in Nutrient-Stressed Environments: Exploiting Plants’ Genetic Capabilities  201-214 (2002)
edited by J.J. Adu-Gyamfi

23. Root exudates as mediators of mineral acquisition in low-nutrient environments
F. D. Dakora & D. A. Phillips
This paper previously appeared in Plant and Soil 245, 2002. pp. 35-47.

Introduction

Plant developmental processes are controlled by internal molecular signals that depend on the adequate supply of mineral nutrients by soil to roots. Changes in the levels of certain nutrients are reported to alter tissue concentrations of cytokinins and abscisic acid (Moorby and Besford, 1983), two major signals controlling plant growth. Consequently, seedling development has a high nutrient demand not only for growth of individual organs, synthesis of new cytoplasm as well as sub-cellular organelles and cell walls, but also for both cell division and expansion (Moorby and Besford, 1983). Nitrogen, a major component of DNA and proteins in all cells, is together with Mg also the key elemental constituent of chlorophyll, the light-harvesting pigment of photosynthetic plants (Lehninger, 1970). In addition, specific concentrations of N and other nutrient elements (P, S, Ca, Mg, Fe and Cu) elicit the production of isoflavonoids in plants, and these molecules function as signals to mutualistic soil microbes and/or phytoalexins against infecting pathogens (Dakora and Phillips, 1996). Calcium is involved in the closure of guard cells and also mediates the response of plants to ethylene, a gaseous plant hormone that controls important physiological processes including seedling development, flowering, fruit ripening and senescence (Raz and Fluhr, 1992). Besides their involvement in signalling, some elements also play crucial physiological roles in plant life. Low concentrations of nutrients such as K+, Na+ and Mg++ also readily stimulate the activity of major enzymes of the glycolytic pathway, namely phosphofructokinase and pyruvate kinase, which together regulate glycolysis in plant cells (Plaxton, 1996). Osmoregulation, the opening and closure of stomatal guard cells, and daily changes in leaf orientation are controlled by K+ mobility in plant cells (Hsiao and Lauchli, 1986), Individual micronutrients are similarly important components of major enzymes, which regulate all biological processes in plants.

It is clear from these considerations that tow nutrient availability can constrain plant growth in many environments of the world, especially the tropics where soils are extremely deficient in nutrients. Yet, the tendency in modern agriculture has been to select crop species for high soil fertility. This has resulted in the wide use of crop varieties that require high doses of applied fertilizer in order to meet optimal plant growth and grain yield. Plant species growing in naturally fertile soils also tend to respond to nutrient supply in a manner similar to agricultural cultivars. Because of the high concentrations of plant-available nutrients in fertilized or naturally fertile soils,, root uptake rates are also high. This is in sharp contrast to nutrient-poor sites where root uptake rates are usually low due to low or poor nutrient availability (Marschner, 1995).

Plant uptake of nutrients from soil is more marked in the 'rhizosphere' surrounding the root than outside this zone (Darrah, 1993). Root exudation of various chemical molecules into the rhizosphere is largely dependent on the nutritional status of the plant. with some species exuding organic acid anions in response to P and Fe deficiency or phytosiderophores due to Fe and Zn deficiency (Haynes, 1990; Jones and Darrah, 1994). Consequently, the released compounds can cause sonic nutrient elements to be relatively more available for uptake by plants. The rate of exudation itself is increased by the presence of microbes in the rhizosphere (Gardner et al., 1983) and promoted by the uptake and assimilation of certain nutrient elements. As a result, the composition of root exudates can he complex. and often ranges from mucilage, root border cells, extracellular enzymes, simple and complex sugars, phenolics. amino acids, vitamins, organic acids, nitrogenous macromolecules such as purines and nucleosides to inorganic or gaseous molecules such as HCO3-, OH-, H+, CO2 and H2, (Marschner, 1995; Rovira, 1969: Uren and Reisennuer, 1988: also see Table I). Many of these organic substrates excreted into the rhizosphere, particularly amino acids. organic acids, proteins, carbohydrates and vitamins, promote microbial biosynthesis of ethylene (Arshad and Frankenherger, 1990), a powerful plant signal controlling development. That apart, these components all play different roles that ultimately affect nutrient acquisition by plants.

Enhancement of nutrient supply by root exudate effects on symbiotic microbes

The components of plant root exudates are many and complex (Table 1), and serve not only as a source of carbon substrate for microbial growth, but also contain chemical molecules that promote chemotaxis of soil microbes to the rhizosphere. Although the root exudates of N2-fixing legumes are known generally for their capacity to attract rhizobia to root hairs, it is in fact their individual chemical components such as flavonoids (Caetano-Anolles et al., 1988), aromatic acids (Parke et al., 1985). amino acids and dicarboxylic acids (Barbour et al., 1991) that function as specific chemoattractants for micro-organisms (Table 2). Once in the rhizosphere, many bacteria may multiply rapidly in response to growth stimulation by quercetin or other flavonoid molecules released by plants (Hartwig et al., 1991), and in turn promote further exudation of new or existing flavonoids into the rhizosphere (Dakora et al., l993a,b; Recourt et al., 1991). Besides their direct effects on rhizobia, root exudates can also attract pathogenic microbes (Morris and Ward, 1992) and promote the growth of plants, mutualistic fungi (Siqueira et al., 1991) and rhizobacteria antagonistic to these pathogenic micro-organisms (Cook et al., 1995). In nutrient poor soils, such rapidly intense colonization of the rhizosphere can itself lead to stiff competition between microbes and the plant nutrient resources.

In addition to serving as chemotactic signals and growth promoters of rhizosphere bacteria (Table 2), root exudates also control the N nutrition of symbiotic legumes. These nodulating plants routinely use flavonoid molecules in root exudates to induce transcriptionof nodulation (nod) genes in rhizobia, leading to nodule formation and N2 fixation. The compounds involved are typically flavonoid in nature, although roles for betaines (Phillips et al., 1992) and aldonic acids (Gagnon and Ibrahim, 1998) have also been reported (Table 3). The flavonoid nod gene inducers are eitherexuded directly by roots of N2-fixing legumes (Phillips, 1992) or by root border cells present in exudates (Zhu et al., 1998). It has been shown that root exudates of legumes growing in an extremely acidic medium cause reduced induction of nod genes (Richardson etal., 1988), clearly indicating that the quality of root exudate determines the level of nod gene transcription. In fact, legumes which are inhibited in the biosynthesis and exudation of phenolic nod-gene inducers, exhibit reduced nodulation and N2 fixation for their N nutrition (Phillips et al., 1994). However, because high soil N inhibits nodulation and N2 fixation, and low concentrations of certain nutrients stimulate the biosynthesis of isoflavone nod gene inducers in symbiotic legumes (Dakora and Phillips, 1996), low-nutrient soils would seem to be the ideal for stimulating N2 fixation and increased N nutrition in nodulated legumes.Whatever the case, root exudates contain the signals for transcription of nod genes in symbiotic rhizobia, and therefore control the extent of legume dependenceon N2 fixation for its N nutrition.

An important consideration is the persistence of these signals in soil. Depending on their stability, the activity of individual molecules could be short-lived in the rhizosphere due to rapid utilization as substrates by microbes (Barz, 1970). Also, the efficacy of both aglycones and their conjugates could be altered either following hydrolysis or chemical modification in the rhizosphere (Rao et al., 1991). For example, bacterial hydrolysis of the inactive compound luteolin7-O-glucoside can produce the active nod gene inducer luteolin (Hartwig and Phillips, 1991). However, the compounds released from decomposition of organic matter such as dead roots and nodules or fallen leaves could potentially supplement root exudates in maintaining a steady concentration of flavonoids and mineral nutrients in the rhizosphere.

Root exudates also contain chemical molecules that govern the development of plant-fungal symbiosis (Table 3), as they provide powerful signals that alert the mycorrhizal fungi to the presence of a host plant. The molecules involved are the same group of flavonoid compounds used for signalling rhizobia. They induce spore germination and/or hyphal growth in vesicular-arbuscular fungi (Becard et al., 1992; Gianinazzi-Pearson et al., 1989). In alfalfa root exudates, quercetin and 4',7-dihydroxyfIavone are the specific molecules used to stimulate spore germination and fungal growth in Glomus and Gigaspora species (Tsai and Phillips, 1991). two mycorrhizal symbionts of this legume. Because of the increased growth of fungal hyphae which extend outwards into the soil environment and the network of plant and fungal nutrient-specific transporters, mycorrhizae promote increased acquisition of water and nutrients. especially P and N, by the host plant (Harrison, 1999). However, its efficacy in supplying nutrients, especially P, to the plant is more marked under nutrient-poor conditions than in fertile soils. The fynbos biome of the Western Cape in South Africa is characterized by P-deficient soils, consequently the dominant field legumes are infected with mychorrizae to enhance P nutrition (Allsop, 1992).

Table 1. Organic compounds and enzymes identified in root exudates of different plant speciesa

Amino
acids
Organic
acids
Sugars Vitamins Purines/
nucleosides
Enzymes Inorganic ions and
gaseous molecules
a-alanine citric glucose biotin adenine acid/alkaline-
phosphatase
HCO3-
b-alanine oxalic fructose thiamin guanine OH-
asparagine malic galactose niacin cytidine invertase H+
aspartate fumaric maltose pantothenate uridine amylase CO2
cystein succinic ribose riboflavin   protease H2
cystine acetic xylose        
glutamate butyric rhamnose        
glycine valeric arabinose        
isoleucine glycolic raffinose        
leucine piscidic desoxyribose        
lysine formic oligosaccharides        
methionine aconitic          
serine lactic          
threonine pyruvic          
proline glutaric          
valine malonic          
tryptophan aldonic          
ornithine erythronic          
histidine tetronic          
arginine            
homoserine            
phenylalanine            
g-Aminobutyric acid            
a-Aminoadipic acid            
aCompiled from West (1939), Fries and Forsman (1951), Rovira and Harris (1961), Vancura (1964), Vancura and Hovadik (1965), Boutleret al. (1966), Rovira (1969), Gardner et al. (1983), Lipton et al. (1987), Fox and Comerford (1990), Ae et al. (1990), Ohwaki and Hirata (1992), Hoffland et al. (1992) and Gagnon and Ibrahim (1998). The root exudates of plants studied include Bison and Novelty flax, barley, wheat, oat, cucumber, tomato, red pepper, turnip cabbage, pea, soybean, chickpea, peanut, lupin, alfalfa, slash pine, pigeon pea and rape.

Figure 1. Effects of root exudate components on nutrient availability and uptake by plants and rhizosphere
microbes. OA = organic acids; AA = amino acids including phytosiderophores, Phe = phenolic compounds.

Root exudate effects on rhizosphere pH and nutrient availability

Another nutritional effect that organic acids have in root exudates is acidification of the rhizosphere (Table 2). Root exudation of high concentrations of organic acid anions as a result of P deficiency (Hoffland et al., 1989) does lower rhizosphere pH, making P (Haynes, 1990; Jones and Darrah, 1994) and micronutrients such as Mn, Fe and Zn to be more available in calcareous soils (Dinkelaker et al., 1989). However, the relationship between organic acid exudation and rhizosphere acidification is not that simple as the extrusion of H+ would depend on the amounts of anions absorbed by roots relative to cations (Haynes, 1990; Jones and Darrah, 1994). Whatever the case, acidification below pH 5.5 can cause even major macronutrients to become limiting. Because micronutrients such as Mn, Fe and Al occur in high concentrations below pH 5.5 (Brady, 1990), any further acidification by organic acids below this level can result in phytotoxic effects on plant roots and beneficial microbes. Intriguingly, the white lupin can mobilize P from both acid and alkaline soils by using citric acid in its proteoid root exudates to acidify even the alkaline soil, and thus solubilize P as well as Fe, Mn, Cu and Zn for uptake by roots (Gardner et al., 1983).

Root excretion of inorganic ions (e.g. HCO3-, OH-, H+) is also important in the mineral nutrition of plants. Plant uptake of anions in excess of cations often causes the roots to secrete HCO3- in order to maintain electrical neutrality, a process that leads to increased rhizosphere pH. Conversely, the uptake of cations in excess of anions can cause roots to exude H+ and lower the rhizosphere pH. The change in pH with HCO3- extrusion tends to increase nutrient supply in acidic soils, as happens with H+ exudation in calcareous soils.

H+ extrusion is largely governed by cation/anion balance and is thus very much influenced by the N source. Plants growing on nitrate generally maintain electronic neutrality by releasing excess anions, including OH-, which cause an increase in rhizosphere pH and an enhanced nutrient (P, Mo, etc.) availability in acidic soils (Marschner, 1995). An exception is the study by Gahoonia et al. (1992) which showed no effect on P mobilization in a luvisol soil following nitrate-induced increase in rhizosphere pH. However, when ammonium is supplied, there is a large excess of cations and a resulting enhanced H+ extrusion and rhizosphere acidification, thereby making P, Mn, Fe, Cu and Zn more toxic in the acidic range, or readily available in the alkaline range (Aguilar and Van Diest, 1981; Gahoonia et al., 1992; Gardner et al., 1983; Gillespie and Pope, 1990; Runge and Rode, 1991). Sometimes, however, acidification from ammonium nutrition does not result in increased P mobilization, especially in acidic oxisols (Gahoonia et al., 1992). Furthermore, H+ extrusion occurs during N2 fixation by symbiotic legumes (Raven et al., 1990), and this can lead to rhizosphere acidification and increased availability of limiting nutrient elements like P, Mo and Fe (Aguilar and Van Diest, 1981; Gahoonia et al., 1992: Gillespie and Pope, 1990: Runge and Rode, 1991) which are much needed in diazotrophy. Additionally, there are many reports of enhanced H+ extrusion under Fe deficiency and P deficiency, both leading to acidification of localized areas around the root tips (Bienfait, 1985, 1988; Gardner et al., 1983; Hoffland et al., 1989: Römheld and Marschner, 1986) and a consequent improvement in the availability of these nutrients. Higher cation/anion uptake ratios as occurs during Zn deficiency (Cakmak and Marschner, 1990) can also acidify the rhizosphere and influence nutrient uptake by plants.

Environments which are naturally very acidic can pose a challenge to nutrient acquisition by plant roots, and threaten the survival of many beneficial microbes and that of the roots themselves (Runge and Rode, 1991). Under those conditions. rhizobia develop tolerance of low pH through expression of acid tolerance genes (Tiwari et al., 1996). As found with a transgenic plant (Degenhardt et al., 1998), a few species such as the Rooibos tea plant (Aspalathus linearis), actively modify their rhizosphere pH by extruding OH- and HCO3- (Muofhe and Dakora, 2000) to facilitate growth in low pH soils (pH 3-5). By raising the pH of the rhizosphere, major nutrients such as K, Ca, Mg, P, S and Mo that are low in Cederberg soils (Muofhe, 1997), become readily available for uptake by plant roots. With a favourable pH, soil N levels can also rise from increased activity of nitrifying bacteria. However, an elevation in rhizosphere pH caused by Aspalathus linearis plants growing in very acidic soils can also be a strategy for reducing the toxicity of Al, Mn and Fe (Runge and Rode, 1991). Whereas net alkalinization of the rhizosphere by nitrate assimilation in plants is understood, the mechanisms underlying rhizosphere alkalinization by root exudates of Aspalathus linearis are only now being clarified.

Increased soil acidification is generally accompanied by Al toxicity as both the concentration and activity of this element are increased with a lowering of pH in the rhizosphere (Gahoonia, 1993) often resulting in reduced Ca and Mg uptake and root growth (Kochian, 1995; Ryan et al., 1992 ). One of the ways for reducing Al toxicity is to increase rhizosphere pH. The findings of a recent study (Degenhardt et al., 1998) involving the use of transgenic plants revealed a twofold increase in H+ influx, which resulted in increased rhizosphere pH. So, in addition to internal or external Al detoxification by organic acids (Larsen et al., 1998; Zheng et al., 1998), an increase in rhizosphere pH can also reduce Al toxicity. Plant genotypes however differ in their handling of excess Al. Calba and Jaillard (1997) showed that a maize genotype that was most tolerant of Al toxicity caused a lesser rhizosphere acidification because NO3- was less affected than for the Al-sensitive genotype. …

Mineral Uptake