Description

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.

Figure, table

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   Fig. 2. The expression of six sorghum PM H⁺-ATPase genes in response to NH₄⁺ (AM) nutrition

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   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)