A molecular rheostat adjusts auxin flux to promote root protophloem differentiation

First author: P. Marhava; Affiliations: University of Lausanne (瑞士洛桑大学): Lausanne, Switzerland

Corresponding author: C. S. Hardtke

Auxin influences plant development through several distinct concentration-dependent effects. In the Arabidopsis root tip, polar auxin transport by PIN-FORMED (PIN) proteins creates a local auxin accumulation that is required for the maintenance of the stem-cell niche (龛). Proximally, stem-cell daughter cells divide repeatedly before they eventually differentiate. This developmental gradient is accompanied by a gradual decrease in auxin levels as cells divide, and subsequently by a gradual increase as the cells differentiate. However, the timing of differentiation is not uniform across cell files. For instance, developing protophloem sieve elements (PPSEs; 原生韧皮部筛管元件) differentiate as neighbouring cells still divide. Here we show that PPSE differentiation involves local steepening of the post-meristematic auxin gradient. BREVIS RADIX (BRX) and PROTEIN KINASE ASSOCIATED WITH BRX (PAX) are interacting plasma-membrane-associated, polarly localized proteins that co-localize with PIN proteins at the rootward end of developing PPSEs. Both brx and pax mutants display impaired PPSE differentiation. Similar to other AGC-family kinases, PAX activates PIN-mediated auxin efflux (外排), whereas BRX strongly dampens (抑制) this stimulation. Efficient BRX plasma-membrane localization depends on PAX, but auxin negatively regulates BRX plasma-membrane association and promotes PAX activity. Thus, our data support a model in which BRX and PAX are elements of a molecular rheostat (变阻器) that modulates auxin flux through developing PPSEs, thereby timing PPSE differentiation.

生长素通过一些完全不同的浓度依赖型效应影响植物的发育。在拟南芥的根尖，生长素由PIN蛋白极性运输，会造成局部性的生长素积累，这对于干细胞龛的维持是必要的。干细胞的子细胞会不断的向外重复分裂，直至最终分化。这个细胞不断分裂的连续发育过程伴随着生长素水平的连续降低，随后当细胞进入分化后，生长素水平又会连续增高。然而，在不同细胞类型中，分化的时间总是不一样的。比如，发育中的原生韧皮部筛管元件PPSE在进入分化后，其邻近的细胞还处于分裂的状态。本文的研究揭示了PPSE的分化涉及到了局部生长素梯度分生后陡变的过程。BRX和PAX是质膜互作的极性定位蛋白，与PIN蛋白共定位于发育中的PPSE朝向根的一端尾部。brx和pax突变体都表现出了PPSE分化缺陷。与其它AGC家族蛋白一样，PAX会激活PIN介导的生长素外流，而生长素负调控BRX质膜相关，并促进PAX的活性。因此，本文的研究表明BRX和PAX作为一个分子变阻器调节生长素在发育中的PPSE中的流动，因此决定了PPSE的分化时间。

通讯：Christian Hardtke (https://wp.unil.ch/hardtkelab/lab-members/)

个人简介：1997年，慕尼黑大学，博士；1998年，多伦多大学，博后。

研究方向： 植物根发育的分子遗传研究。

doi: 10.1038/s41586-018-0186-z

Journal: Nature 

Published date: 06 June, 2018

（P.S. 原文下载：链接:https://pan.baidu.com/s/1qFiSsqwlF_7JjmGMbedfhQ  密码:dmu6）