Auxin movement between meristem and leaf primordia controls leaf polarity development
Jiyan Qi^{1, §}, Ying Wang^{1,2, §}, Bihai Shi¹, Elliot Meyerowitz², Yuling Jiao^1,*

¹State Key Laboratory of Plant Genomics, Institute of Genetics and Developmental Biology, Chinese Academy of Sciences, Beijing 100101, China.
²Division of Biology, California Institute of Technology, Pasadena, California 91125, USA.
^§Equal contribution.
*Corresponding Author：Tel: (+8610) 64807656; Fax: (+8610)64806595; E-mail: yljiao@genetics.ac.cn

Abstract
One of the central questions in developmental biology is that of pattern formation in organs. Leaves of most plant species have distinct dorsoventral (adaxial-abaxial) polarity, which is formed early in leaf primordium growth at the shoot apex. A self-organized auxin flow from emerging leaf primordia to the shoot apical meristem was visualized by live imaging and predicted by computational models of PIN-facilitated auxin transport. Here we show that this self-organized auxin flow controls leaf dorsoventral polarity. Local application of polar auxin transport inhibitors abolished this auxin flow and dorsoventral polarity of the related leaf, and this is confirmed by mutant analysis. Furthermore, auxin application allowed correlation of leaf dorsoventral pattern formation with a gradient of auxin concentration in emerging leaf primordia. Such a self-organized auxin flow coincides with the long-proposed Sussex signal for formation of leaf dorsoventral polarity. The data indicate that the auxin gradient is an early signal for dorsoventral pattern formation. We also find that mechanical signals serve as cues for dorsoventral patterning.

Key Words: leaf; polarity; auxin