A KNOX-Cytokinin Regulatory Module Predates the Origin of Indeterminate Vascular Plants

First author: Yoan Coudert; Affiliations: University of Cambridge (剑桥大学): Cambridge, UK

Corresponding author: C. Jill Harrison

The diverse forms of today’s dominant vascular plant flora are generated by the sustained proliferative activity of sporophyte meristems at plants’ shoot and root tips, a trait known as indeterminacy. Bryophyte sister lineages to the vascular plants lack such indeterminate meristems and have an overall sporophyte form comprising a single small axis that ceases growth in the formation of a reproductive sporangium. Genetic mechanisms regulating indeterminacy are well characterized in flowering plants, involving a feedback loop between class I KNOX genes and cytokinin, and class I KNOX expression is a conserved feature of vascular plant meristems. The transition from determinate growth to indeterminacy during evolution was a pre-requisite to vascular plant diversification, but mechanisms enabling the innovation of indeterminacy are unknown. Here, we show that class I KNOX gene activity is necessary and sufficient for axis extension from an intercalary region of determinate moss shoots. As in Arabidopsis, class I KNOX activity can promote cytokinin biosynthesis by an ISOPENTENYL TRANSFERASE gene, PpIPT3. PpIPT3 promotes axis extension, and PpIPT3 and exogenously applied cytokinin can partially compensate for loss of class I KNOX function. By outgroup comparison, the results suggest that a pre-existing KNOX-cytokinin regulatory module was recruited into vascular plant shoot meristems during evolution to promote indeterminacy, thereby enabling the radiation of vascular plant shoot forms.

现如今，主要维管植物群落的多样化形态由处于植物茎尖和根尖的孢子体分生组织持续增殖活性所形成，这种特性叫做无限生长。维管植物的姊妹支系苔藓植物则缺少这种无限生长的分生组织，具有一个单一的孢子体形式，只有一个体轴，并且在生殖孢子囊形成的时候就会停止生长。在有花植物中有关调控无限生长的遗传机制研究得已经很透彻了，该过程涉及第一类KNOX基因与细胞分裂素之间的反馈回路，并且第一类KNOX基因的表达在所有维管植物的分生组织中十分保守。植物在演化过程中从有限生长向无限生长的转变是维管植物分化形成的前提基础，但促进这种无限生长特性创新形成的机制还不清楚。本文中，作者的研究显示第一类KNOX基因的活性对于苔藓地上部位居间区域的体轴延伸是必要且充分的。在拟南芥中，第一类KNOX基因能够通过一个异戊烯基转移酶PpIPT3基因来促进细胞分裂素的生物合成。PpIPT3基因促进体轴的延伸，并且PpIPT3基因和外施细胞分裂素能够部分互补第一类KNOX基因功能缺失的表型。通过与外群的比较，作者的研究显示一个预先存在的KNOX-细胞分裂素调控模块在演化过程中被招募到维管植物的茎尖分生组织区域，促进该区域的无限生长，因此使得维管植物地上部位的形式辐射开来。

通讯：Jill Harrison (http://www.bristol.ac.uk/biology/people/jill-j-harrison/overview.html)

个人简介：1992-1996年，圣安德鲁斯大学，学士；1997-1998年，爱丁堡大学，硕士；1998-2002年，爱丁堡大学，博士；2002-2008年，牛津大学，博士后。

研究方向：陆地植物演化。

doi: https://doi.org/10.1016/j.cub.2019.06.083

Journal: Current Biology

Published date: August 01, 2019