Transcriptional response of plasma membrane H-ATPase genes to ammonium nutrition and its functional link to the release of biological nitrification inhibitors from sorghum roots.

Aims: Sorghum ( Sorghum bicolor) roots release biological nitrification inhibitors (BNIs) to suppress soil nitrification. Presence of NH in the rhizosphere stimulates BNIs release and it is hypothesized to be functionally associated with plasma membrane (PM) H-ATPase activity. However, whether the H-ATPase is regulated at the transcriptional level, and if so, which isoforms of the H-ATPases are involved in BNIs release are not known. Also, it is not clear whether the stimulation on BNIs release from roots is due to NH uptake or its assimilation, which are addressed in this study. Methods: Root exudates from intact sorghum plants were collected using aerated solutions of NH or methyl-ammonium (MeA); and the BNI-activity release was determined. PM vesicles were isolated from fresh roots using a two-phase partitioning system; and the hydrolytic H-ATPase activity was determined. All genes encoding PM H-ATPases were searched in sorghum genome, and their expression in response to NH or MeA were analyzed by quantitative RT-PCR in sorghum roots. Results: BNIs release and PM H-ATPase activity increased with NH concentration (≤1.0 mM) in the root-exudate collection solutions, but at higher concentrations, it did not respond further or declined in case of the PM H-ATPase activity. Twelve PM H-ATPase genes were identified in sorghum genome; and these isoforms were designated SbA1 to SbA12. Five H-ATPase genes were stimulated by NH in the rhizosphere, and have similar expression pattern, which is consistent with the variation in H-ATPase activity. MeA, a non-metabolizable analogue of NH, had no significant effects on BNIs release, H-ATPase activity, or expression of the H-ATPase genes. Conclusions: Our results suggest that the functional link between PM H-ATPase activity and BNIs release is evident only at NH levels of ≤1.0 mM in the rhizosphere. The variation in PM H-ATPase activity by NH is due to transcriptional regulation of five isoforms of the H-ATPases. The stimulatory effect of NH on BNIs release is functionally associated with NH assimilation and not just with NH uptake alone. [ABSTRACT FROM AUTHOR]