Humic substances have been proven to stimulate plant growth and nutrient accumulation (for review see Vaughan and Malcolm, 1985; Chen and Aviad, 1990; Varanini and Pinton, 1995). Studies on uptake kinetics, use of protein synthesis inhibitors and variations in experimental conditions (e.g. temperature) suggest that the effects of humic substances on plant nutrition may be mediated by variations in the synthesis and functionality of membrane carriers. Moreover, stimulation of active proton extrusion from roots (Pinton et a!., 1997) and transmembrane potential hyperpolarization (Slesak and Jurek, 1988) indicated an involvement of the plasma membrane H+-ATPase (PM H+-ATPase) in the increased nutrient uptake due to the presence of humic substances. Direct proof of an interaction between humic molecules and PM H+-ATPase has been obtained by Varanini et a!. (1993) who demonstrated that low molecular weight (< 5 kDa) humic molecules can stimulate the phosphohydrolitic activity of this enzyme in isolated plasma membrane vesicles. Stimulation could also be observed in the presence of the detergent Brij 58, thus suggesting a possible direct interaction involving the scalar activity of the enzyme. On the other hand, experiments on the effects of humic substances on proton transport properties in plasma membrane vesicles showed that membrane permeability to ions was also affected by these molecules, which therefore appear to have a variety of membrane targets. PM H+-ATPase activity was also shown to be stimulated by a WEHS fraction after short-term (4 h) treatment of maize roots (Pinton et a!., 1999b); since no increase in protein amount was observed, this effect was attributed to an undefined post-translational mechanism. Canellas et a!. (2002) showed that a prolonged root treatment (up to 7 d) with humic acids extracted from earthworm compost increased PM H+-ATPase activities and the protein amount at steady-state. The possibility that humic substances exert an influence at the level of PM H+-ATPase gene(s) expression was investigated by Quaggiotti et a!. (2004), who showed an increase in transcript abundance of the PM H -ATPase (isoform MHA2) in maize roots treated for 48 h with an earthworm low molecular weight humic fraction. Taken together, these results strongly support the view that humic substances affect PM H+-ATPase, thus providing an increased proton-motive force for nutrient uptake. This effect can be of relevance for Fe nutrition, since increased PM H+-ATPase activity can contribute to:
• solubilisation of Fe in the apoplast and in the rhizosphere;
• maintenance of favourable conditions for the activity of Fe3+-chelate reductase (low apoplastic pH and transmembrane electrical potential homeostasis);
• uptake of free Fe2+ or Fe3+-complex (e.g. Fe-phytosiderophores). 5.2 Plant growth
Many authors have observed that plants treated with humic molecules have different growth and morphology as compared to control plants (Nardi et a!., 2002). Treatment with these substances modified root morphology, inducing a proliferation of root hairs in the subapical regions and a higher differentiation rate of root cells. Canellas et a!. (2002) showed that humic acids extracted from earthworm compost enhanced root growth of maize seedlings and caused a marked proliferation of sites of lateral root emergence. This kind of plant root responses to the treatment with humic substances has been interpreted in terms of a hormone-like activity of humic substances. Recent studies have shown that humic substances extracted from different sources could react to antibodies directed against indolacetic acid (Muscolo et a!., 1998; Pizzeghello et a!., 2001). Furthermore, the presence of exchangeable auxin groups in humic substances extracted from earthworm compost was revealed by means of gas chromatography-mass spectrometry (Canellas et a!., 2002). On the other hand, Chen et a!. (1994) failed to provide evidence of the presence of phytohormones in humic extracts. In an attempt to clarify whether humic molecules act as auxin-like substances, Schmidt et a!. (unpublished) analysed the response of an array of Arabidopsis auxin-related mutants to the presence of a water extractable humic fraction (WEHS). Root hair density was significantly increased by the presence of WEHS in the wild type. The phenotypes of the mutants, all exhibiting a reduced number of root hairs, were not rescued by the application of WEHS, suggesting that functional products of the auxin signalling cascade are required for translating the response of root cells to the humic molecules. In addition, mutants defective in root hair initiation rhds6, known to develop normal hairs in the presence of auxin, were not affected by a wide range of applied concentration of WEHS indicating that this humic fraction cannot substitute for the hormone. These results show that, at least for WEHS-like humic fractions, a modification of root hair density can be induced in a manner different from that exerted by exogenous auxin.
The increased root hair density, similar to that occurring at sub-optimal availability of immobile nutrients such as Fe (Müller and Schmidt, 2004), support the view that WEHS-like humic fractions induce a "nutrient acquisition response" that favours nutrient capture via an increase in the absorptive surface area.
A schematic representation of the possible interactions between soil humic substances and mechanisms related to Fe acquisition is shown in Figure 7-3.
Strategy II Strategy I
Strategy II Strategy I
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