AUTHOR=Hu An Yong , Zheng Man Man , Sun Li Ming , Zhao Xue Qiang , Shen Ren Fang 

TITLE=Ammonium Alleviates Manganese Toxicity and Accumulation in Rice by Down-Regulating the Transporter Gene OsNramp5 Through Rhizosphere Acidification

JOURNAL=Frontiers in Plant Science

VOLUME=10

YEAR=2019

URL=https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2019.01194

DOI=10.3389/fpls.2019.01194

ISSN=1664-462X

ABSTRACT=<p>Ammonium (<inline-formula><mml:math display="inline" id="M1" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>N</mml:mi><mml:msubsup><mml:mi>H</mml:mi><mml:mn>4</mml:mn><mml:mo>+</mml:mo></mml:msubsup></mml:mrow></mml:math></inline-formula>) alleviates manganese (Mn) toxicity in various plant species, but the underlying mechanisms are still unclear. In this study, we compared the effects of <inline-formula><mml:math display="inline" id="M2" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>N</mml:mi><mml:msubsup><mml:mi>H</mml:mi><mml:mn>4</mml:mn><mml:mo>+</mml:mo></mml:msubsup></mml:mrow></mml:math></inline-formula> and nitrate (<inline-formula><mml:math display="inline" id="M3" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>N</mml:mi><mml:msubsup><mml:mi>O</mml:mi><mml:mn>3</mml:mn><mml:mo>−</mml:mo></mml:msubsup></mml:mrow></mml:math></inline-formula>) on rice (<italic>Oryza sativa</italic> L.) growth, accumulation and distribution of Mn, accumulation of iron (Fe), zinc (Zn) and copper (Cu), root cell wall components, and expression of Mn and Fe transporter genes. After rice seedlings were grown in non-pH-buffered nutrient solution for 2 days, the pH of growth medium changed from an initial value of 4.5 to 3.5 and to 5.5 in the presence of <inline-formula><mml:math display="inline" id="M4" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>N</mml:mi><mml:msubsup><mml:mi>H</mml:mi><mml:mn>4</mml:mn><mml:mo>+</mml:mo></mml:msubsup></mml:mrow></mml:math></inline-formula> and in the presence of <inline-formula><mml:math display="inline" id="M5" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>N</mml:mi><mml:msubsup><mml:mi>O</mml:mi><mml:mn>3</mml:mn><mml:mo>−</mml:mo></mml:msubsup></mml:mrow></mml:math></inline-formula>, respectively. Compared with <inline-formula><mml:math display="inline" id="M6" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>N</mml:mi><mml:msubsup><mml:mi>O</mml:mi><mml:mn>3</mml:mn><mml:mo>−</mml:mo></mml:msubsup></mml:mrow></mml:math></inline-formula>, ammonium decreased nutrient-solution pH and alleviated Mn toxicity and accumulation in rice under non-pH-buffered conditions. This alleviation disappeared when 5 mM Homo-PIPES pH buffer was added. Regardless of N form, roots, shoots, root cell sap, and xylem sap accumulated much lower Mn at pH 3.5 than at pH 5.5, whereas Mn distribution in different leaves and Mn accumulation in root cell walls was affected by neither N form nor pH. Ammonium decreased the expression of the Mn influx transporter gene <italic>OsNramp5</italic> in roots under non-pH-buffered conditions, but not under pH-buffered ones. <italic>OsNramp5</italic> expression was down-regulated at pH 3.5 compared with pH 5.5. Another efflux Mn transporter gene, Os<italic>MTP9</italic>, was not regulated by either N form or pH. High pH (5.5) enhanced the expression of the Fe transporter gene <italic>OsIRT1</italic> and increased the accumulation of Zn but not Fe or Cu in shoots compared with pH 3.5. Taken together, our results indicate that <inline-formula><mml:math display="inline" id="M7" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>N</mml:mi><mml:msubsup><mml:mi>H</mml:mi><mml:mn>4</mml:mn><mml:mo>+</mml:mo></mml:msubsup></mml:mrow></mml:math></inline-formula> alleviates Mn toxicity and accumulation in rice through the down-regulatory effects of rhizosphere acidification on the Mn influx transporter gene <italic>OsNramp5</italic>. In addition, the up-regulation of <italic>OsIRT1</italic> expression may contribute to the increased Zn uptake by rice at high pH of nutrient solution.</p>