Limitations to the Potential of Transgenic Trifolium subterraneum L. Plants that Exude Phytase when Grown in Soils with a Range of Organic P Content

Abstract

Growth and P-nutrition of transgenic Trifolium subterraneum L. which express a chimeric fungal phytase gene (ex::phyA) was compared to azygous and wild-type controls in a range of soils that differed in organic P content. Shoot and root growth by plant lines were measured and effects of reducing the influence of soil microorganisms were investigated by pasteurising the soils. Plants that expressed phyA did not have better P-nutrition than control plants after 56 days growth, except in a soil that contained a large concentration of both total organic P and organic P that was amenable to hydrolysis by a plant-derived phytase. Pasteurisation had little effect on the relative P-nutrition of the various plant lines in any of the soils. Roots of transgenic plants that expressed ex::phyA were shorter than controls up to 21 days growth in a number of soils, which resulted in an initial greater P accumulation efficiency. However, greater P accumulation efficiency was only maintained in the soil where significant growth and P nutrition responses were also observed. Availability of inositol phosphates in soil is a major factor that limits the effectiveness of expressing fungal phytase genes in plants as a means to improve P-nutrition. Reducing the influence of rhizosphere microorganisms appeared to have little effect on the P-nutrition of plant lines, but the longer root system produced by control plants may have initially provided them with greater access to soil P resources. This research highlights the inherent difficulty in improving the P-nutrition of plants by the manipulation of single traits in isolation, but does provide some evidence that such approaches can be successful under certain edaphic conditions.

Effect of exogenous phosphatase and phytase activities on organic phosphate mobilization in soils with different phosphate adsorption capacities

“…7). These results support the hypothesis that the solubility of organic P is a limiting factor for mobilization in the soil, as inferred from results of other studies (George et al 2005a;Wasaki et al 2009). …”

“…For effective use of these forms, chelation by organic acids is important (George et al 2005a;Wasaki et al 2009). Adams and Pate (1992) reported that white lupin plants can use phytate-P in the soil.…”

Soil Microorganisms Mediating Phosphorus Availability Update on Microbial Phosphorus

“…Plants genetically modified to release an extracellular fungal phytase (from Aspergillus niger) from roots showed similar novel ability to acquire P directly from phytate . However, when evaluated in soil, these plants show significant improvement in P nutrition only in soils with higher substrate levels (as measured by enzyme lability assays), or in soils that were artificially amended with phytate (George et al, 2005a(George et al, , 2005b. In addition to the importance of substrate availability, rapid adsorption of phytase to the soil solid phase was observed with loss of activity being dependent both on enzyme characteristics and soil properties (George et al, 2007a;Giaveno et al, 2010).…”

“…Genetic manipulation of plants and microorganisms for key traits that are known to be associated with P mobilization or growth promotion (e.g. extracellular phytase and genes for synthesis and/or release of organic anions by plants and microorganisms; Gyaneshwar et al, 2002;George et al, 2005a;Rodriguez et al, 2006), along with generation of specific mutants in key target genes for particular traits (e.g. organic anion release in Pseudomonas spp., Miller et al, 2010), are useful for both elucidation of mechanisms and for quantifying their contribution to increased P availability in soil.…”

Novel approaches in plant breeding for rhizosphere-related traits

“…Transgenic Trifolium subterraneum transformed with a phytase gene from Aspergillus niger improved P uptake of plants supplied with phytate and grown in agar under sterile conditions (Richardson et al 2001), but this positive effect was not seen in soil grown plants, possibly because phytate was bound to soil particles and therefore not available (George et al 2005). Delhaize et al (2001) overexpressed a citrate synthase gene from Pseudomonas aeruginosa in tobacco (Nicotiana tabacum L.) and detected greatly enhanced citrate synthase protein levels in the transgenic plants.…”