Interplay among NH₄+ uptake, rhizosphere pH and plasma membrane h+-ATPase determine the release of BNIs in sorghum roots - possible mechanisms and underlying hypothesis.

摘要

Aims and background: The ability to suppress soil nitrification through the release of nitrification inhibitors from plant roots is termed 'biological nitrification inhibition' (BNI). Earlier, we reported that sorghum roots release higher BNI-activity when grown with NH₄+, but not with NO₃- as N source. Also for BNI release, rhizosphere pH of <=5.0 is needed; beyond this, a negative effect on BNI release was observed with nearly 80% loss of BNI activity at pH >=7.0. This study is aimed at understanding the inter-functional relationships associated with NH₄+ uptake, rhizosphere-pH and plasma membrane H+-ATPase (PM H+-ATPase) activity in regulating the release of BNIs (biological nitrification inhibitors) from sorghum roots. Methods: Sorghum was grown hydroponically and root exudates were collected from intact plants using a pH-stat system to separate the secondary acidification effects by NH₄+ uptake on BNIs release. A recombinant luminescent Nitrosomonas europaea bioassay was used to determine BNI-activity. Root plasma membrane was isolated using a two-phase partitioning system. Hydrolytic H+-ATPase activity was determined. Split-root system setup was deployed to understand the localized responses to NH₄+, H+-ATPase-stimulator (fusicoccin) or H+-ATPase-inhibitor (vanadates) on BNI release by sorghum. Results: Presence of NH₄+ in the rhizosphere stimulated the expression of H+-ATPase activity and enhanced the release of BNIs from sorghum roots. Fusicoccin, which stimulates H+-ATPase activity, also stimulated BNIs release in the absence of NH₄+; vanadate, which suppresses H+-ATPase activity, also suppressed the release of BNIs. NH₄+ levels (in rhizosphere) positively influenced BNIs release and root H+-ATPase activity in the concentration range of 0-1.0 mM, indicating a close relationship between BNI release and root H+-ATPase activity with a possible involvement of carrier-mediated transport for the release of BNIs in sorghum. Conclusion: Our results suggest that NH₄+ uptake, PM H+-ATPase activity, and rhizosphere acidification are functionally inter-connected with BNI release in sorghum. Such knowledge is critical to gain insights into why BNI function is more effective in light-textured, mildly acidic soils compared to other soil types.