AUTHOR=Wang Tao , Hua Yingpeng , Chen Moxian , Zhang Jianhua , Guan Chunyun , Zhang Zhenhua 

TITLE=Mechanism Enhancing Arabidopsis Resistance to Cadmium: The Role of NRT1.5 and Proton Pump

JOURNAL=Frontiers in Plant Science

VOLUME=9

YEAR=2018

URL=https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2018.01892

DOI=10.3389/fpls.2018.01892

ISSN=1664-462X

ABSTRACT=<p><bold>Aim:</bold> Heavy metal pollution is serious in China, and abscisic acid (ABA) is an important stress hormone. How it regulates plant tolerance to cadmium remains unclear, so we aimed to explore the molecular mechanism responsible for enhanced cadmium resistance in Arabidopsis wild-type and mutant plants and <italic>Brassica napus</italic> seedlings.</p><p><bold>Methods:</bold> Arabidopsis/<italic>B. napus</italic> were cultured hydroponically for 28/15 days and then treated with 20/10 μM Cd/Cd+ABA (5 μM) for 3/4 days. Chlorophyll degradation rate, SPAD values, proline, MDA, ABA, <inline-formula><mml:math id="M1" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msubsup><mml:mrow><mml:mi mathvariant="normal">NO</mml:mi></mml:mrow><mml:mn>3</mml:mn><mml:mo>−</mml:mo></mml:msubsup></mml:mrow></mml:math></inline-formula>, and Cd concentrations were measured in root vacuoles and protoplasts; root to shoot <inline-formula><mml:math id="M2" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msubsup><mml:mrow><mml:mi mathvariant="normal">NO</mml:mi></mml:mrow><mml:mn>3</mml:mn><mml:mo>−</mml:mo></mml:msubsup></mml:mrow></mml:math></inline-formula> and Cd concentration ratios were determined and <italic>NRT1.5-, NRT1.8-, BnNRT1.5-</italic>, and <italic>BnNRT1.8-</italic>related gene expression was studied.</p><p><bold>Results:</bold> Cytoplasmic ABA levels in root cells of <italic>bglu10</italic> and <italic>bglu18</italic> Arabidopsis mutants were significantly lower than those in the wild-type, apparently making the latter more resistant to Cd. <inline-formula><mml:math id="M3" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msubsup><mml:mrow><mml:mi mathvariant="normal">NO</mml:mi></mml:mrow><mml:mn>3</mml:mn><mml:mo>−</mml:mo></mml:msubsup></mml:mrow></mml:math></inline-formula> long-distance transporter <italic>NRT1.5</italic> responded to ABA signaling by downregulating its own expression, while <italic>NRT1.8</italic> did not respond. Concomitantly, proton pump activity in wild-type plants was higher than in the <italic>bglu10</italic> and <italic>bglu18</italic> mutants; thus, more <inline-formula><mml:math id="M4" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msubsup><mml:mrow><mml:mi mathvariant="normal">NO</mml:mi></mml:mrow><mml:mn>3</mml:mn><mml:mo>−</mml:mo></mml:msubsup></mml:mrow></mml:math></inline-formula> and Cd accumulated in the vacuoles of wild-type root cells. ABA application inhibited Cd absorption by <italic>B. napus</italic>. <italic>BnNRT1.5</italic> responded to exogenous ABA signal by downregulating its own expression, while the lack of response by <italic>BnNRT1.8</italic> resulted in increased amount of <inline-formula><mml:math id="M5" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msubsup><mml:mrow><mml:mi mathvariant="normal">NO</mml:mi></mml:mrow><mml:mn>3</mml:mn><mml:mo>−</mml:mo></mml:msubsup></mml:mrow></mml:math></inline-formula> accumulating in the roots to participate in the anti-cadmium reaction.</p><p><bold>Conclusion:</bold><italic>NRT1.5</italic> responds to the ABA signal to inhibit its own expression, whereas unresponsiveness of <italic>NRT1.8</italic> causes accumulation of <inline-formula><mml:math id="M6" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msubsup><mml:mrow><mml:mi mathvariant="normal">NO</mml:mi></mml:mrow><mml:mn>3</mml:mn><mml:mo>−</mml:mo></mml:msubsup></mml:mrow></mml:math></inline-formula> in the roots; thus, enhancing Cd resistance. In Arabidopsis, because of proton pump action, more <inline-formula><mml:math id="M7" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msubsup><mml:mrow><mml:mi mathvariant="normal">NO</mml:mi></mml:mrow><mml:mn>3</mml:mn><mml:mo>−</mml:mo></mml:msubsup></mml:mrow></mml:math></inline-formula> and Cd accumulate in the vacuoles of Arabidopsis root cells, thereby reducing damage by Cd toxicity. However, in <italic>B. napus</italic>, the addition of exogenous ABA inhibited Cd absorption. Our data provide a sound basis to the theoretical molecular mechanism involved in hormone signaling during response of plants to heavy metal stress.</p>