Knock-Down of CsNRT2.1, a Cucumber Nitrate Transporter, Reduces Nitrate Uptake, Root length, and Lateral Root Number at Low External Nitrate Concentration

Abstract

Nitrogen (N) is a macronutrient that plays a crucial role in plant growth and development. Nitrate ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mi>NO</mml:mi><mml:mi>3</mml:mi><mml:mn>-</mml:mn></mml:msubsup></mml:math> ) is the most abundant N source in aerobic soils. Plants have evolved two adaptive mechanisms such as up-regulation of the high-affinity transport system (HATS) and alteration of the root system architecture (RSA), allowing them to cope with the temporal and spatial variation of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mi>NO</mml:mi><mml:mi>3</mml:mi><mml:mn>-</mml:mn></mml:msubsup></mml:math> . However, little information is available regarding the nitrate transporter in cucumber, one of the most important fruit vegetables in the world. In this study we isolated a nitrate transporter named <i>CsNRT2.1</i> from cucumber. Analysis of the expression profile of the <i>CsNRT2.1</i> showed that <i>CsNRT2.1</i> is a high affinity nitrate transporter which mainly located in mature roots. Subcellular localization analysis revealed that CsNRT2.1 is a plasma membrane transporter. In N-starved <i>CsNRT2.1</i> knock-down plants, both of the constitutive HATS (cHATS) and inducible HATS (iHATS) were impaired under low external <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mi>NO</mml:mi><mml:mi>3</mml:mi><mml:mn>-</mml:mn></mml:msubsup></mml:math> concentration. Furthermore, the <i>CsNRT2.1</i> knock-down plants showed reduced root length and lateral root numbers. Together, our results demonstrated that CsNRT2.1 played a dual role in regulating the HATS and RSA to acquire <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mi>NO</mml:mi><mml:mi>3</mml:mi><mml:mn>-</mml:mn></mml:msubsup></mml:math> effectively under N limitation.