The release of biological nitrification inhibitors from sorghum root is regulated at the transcriptional level
Sorghum (Sorghum bicolor) roots release biological nitrification inhibitors (BNIs) to suppress soil-nitrifier activity and soil-nitrification. The presence of NH4+ in the rhizosphere stimulates the release of BNIs from roots and is hypothesized to be functionally linked to plasma membrane (PM) H+-ATPase activity. However, whether the H+-ATPase is regulated at the transcriptional level, and if so, which iso-forms of H+-ATPases are involved in BNIs release are not known. Also, the stimulation of BNI release, whether it is due to NH4+ uptake or its assimilation in roots, is unclear, and it would be subsequently addressed by this study.
NH4+ concentrations up to 1.0 mM positively stimulated both PM H+-ATPase activity and BNI release from sorghum roots; but at higher concentrations (>1.0 mM), NH4+ did not further increase BNI release and a decline in PM H+-ATPase activity was observed (Fig. 1a, b). Vanadate, an inhibitor of H+-ATPases, suppresses BNI release from intact sorghum plants (Fig. 1c). Twelve PM H+-ATPase genes (iso-forms, designated as SbA1 to SbA12) were identified in sorghum genome; however, only five H+-ATPase genes were stimulated by NH4+ in the rhizosphere. They have a similar expression pattern and is consistent with the observed variation in H+-ATPase activity (Fig. 2). Methyl-ammonium (MeA), a non-metabolizable analogue of NH4+, had no significant effect on BNI release, H+-ATPase activity, or the expression of H+-ATPase genes (Fig. 3). These results suggest that the functional link between PM H+-ATPase activity and BNI release is operational only at NH4+ concentrations of ≤1.0 mM in the rhizosphere. The variation in PM H+-ATPase activity by NH4+ is due to transcriptional regulation of five iso-forms of H+-ATPases. The stimulatory effect of NH4+ on BNI release is functionally associated with NH4+ assimilation and not from NH4+ uptake alone.
A mechanistic understanding of BNI release in sorghum helps in choosing suitable agro-ecological niche production systems where BNI function is expressed to its genetic potential for controlling soil nitrification. In addition, the use of slow-release N-fertilizers can allow soil ammonium levels ≤1.0 mM. This, coupled with the development of genetically modified crops with accelerated PM H+-ATPase activity, can further improve BNI release from sorghum root systems to make production systems low-nitrifying and low-N2O emitting with improved nitrogen-use efficiency, which in turn will be ultimately beneficial to human society and the environment.
Fig. 2. The expression of six sorghum PM H+-ATPase genes in response to NH4+ (AM) nutrition
Fig. 3. The effect of methyl-ammonium (MeA), a non-metabolizable analogue to NH4+, on BNI release (a), the H+-ATPase activity (b), and the expression of the H+-ATPase genes in sorghum roots (c)
Japan International Research Center for Agricultural Sciences Crop, Livestock and Environment Division
Subbarao Guntur Venkata ( 生産環境・畜産領域 )
Zeng H. ( 南京農業大学 )
T. Di ( 南京農業大学 )
Zhu Y. ( 南京農業大学 )
Zeng，H.et al. (2015) Plant and Soil (Online First)https://doi.org/10.1007/s11104-015-2675-2