NH 4 + triggers the synthesis and release of biological nitrification inhibition compounds in Brachiaria humidicola roots
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The release of chemical compounds from plant roots that suppress soil nitrification is termed biological nitrification inhibition (BNI). Determining the environmental factors that control the synthesis and release of BNI-compounds from Brachiaria humidicola (Rendle) Schweick, a tropical pasture grass that thrives on acid soils, is the focus of this investigation. Because the BNI trait is related to the N status of the plant, we investigated the possibility that the expression of this trait would be related to the forms of N found in the root environment. Plants were grown with two sources of N, NH 4 + or NO 3 − for 60 days and the release of BNI-compounds monitored. Only plants grown with NH 4 + released BNI-compounds from roots. The presence of NH 4 + and possibly the secondary effect of its uptake (i.e., acidic pH) in the root environment significantly enhanced the release of BNI-compounds. Both the NH 4 + and NO 3 − grown plants responded to the stimulus from NH 4 + in the root environment. BNI-compounds found in root tissue and their release were nearly three times greater in NH 4 + grown than from NO 3 − grown plants. The BNI-compounds released from roots composed of at least three active components—Type-I (stable to pH changes from 3.0 to 10), Type-II (temporarily loses its inhibitory effect at a pH higher than a threshold pH of 4.5 and the inhibitory effect is reestablished when the root exudate pH is adjusted to <4.5) and Type-III (inhibitory effect is irreversibly lost if the pH of the root exudate reaches 10.0 or above). A major portion of BNI-compounds released in the presence of NH 4 + is of Type-I. In the absence of NH 4 + , mostly Type-II and Type-III BNI-compounds were released. The BNI-compounds inhibited the function of Nitrosomonas europaea through the blocking of both ammonia monooxygenase and hydroxylamino oxidoreductase pathways. These results indicate that the release of BNI-compounds from B. humidicola roots is a regulated function and that presence of NH 4 + in the root environment is necessary for the sustained synthesis and release of BNI.
KeywordsBiological nitrification inhibition BNI-compounds Bioluminescence assay Nitrogen forms
We are grateful to Dr Taro Iizumi (Kurita Water Industries Limited, Wakamtya, Japan) who has kindly provided us with the luminescent recombinant N. europaea strain for this research. We acknowledge the participation and help from our colleagues at CIAT (Cali, Colombia) during this study have also provided the seed material used for various experiments described in this manuscript. We are thankful to Dr Yiyong Zhu (Nanjing Agricultural University, Nanjing, China) for many thoughtful suggestions and inputs during the interpretation of the data. Also, we are grateful to Dr K.L. Sahrawat (ICRISAT, India), who has gone through our manuscript critically and offered many suggestions to improve the clarity of our presentation and interpretation of the data.
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