Plant Sciences Department, University of Cambridge, Downing Street, Cambridge CB2 3EA, UK; School of Biological Sciences, University of Bristol, 24 Tyndall Avenue, Bristol BS8 1TQ, UK; Laboratoire Reproduction et Développement des Plantes, Université de Lyon, ENS de Lyon, UCB Lyon 1, CNRS, INRA, 69342 Lyon, France.
Laboratory of Growth Regulators, Centre of the Region Haná for Biotechnological and Agricultural Research, Faculty of Science of Palacký University and Institute of Experimental Botany CAS, Šlechtitelů 27, 78371 Olomouc, Czech Republic.
Curr Biol. 2019 Aug 19;29(16):2743-2750.e5. doi: 10.1016/j.cub.2019.06.083. Epub 2019 Aug 1.
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 [1]. 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 [1]. Genetic mechanisms regulating indeterminacy are well characterized in flowering plants, involving a feedback loop between class I KNOX genes and cytokinin [2, 3], and class I KNOX expression is a conserved feature of vascular plant meristems [4]. 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 [5]. 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.
今天占主导地位的维管植物区系的多种形式是由孢子体分生组织在植物的芽和根尖的持续增殖活动产生的，这种特性被称为不定性[1]。与维管植物亲缘关系密切的苔藓类植物缺乏这种不定性分生组织，其整体孢子体形式包括一个单一的小轴，在形成生殖孢子囊时停止生长[1]。调控不定性的遗传机制在开花植物中得到了很好的描述，涉及到 I 类 KNOX 基因和细胞分裂素之间的反馈环[2,3]，并且 I 类 KNOX 表达是维管植物分生组织的一个保守特征[4]。在进化过程中，从有性生长到不定性的转变是维管植物多样化的先决条件，但使不定性创新的机制尚不清楚[5]。在这里，我们表明 I 类 KNOX 基因的活性是从有性苔藓芽的居间区延伸轴所必需和充分的。与拟南芥一样，I 类 KNOX 活性可以通过 ISOPENTENYL TRANSFERASE 基因 PpIPT3 促进细胞分裂素的生物合成。PpIPT3 促进轴的延伸，PpIPT3 和外源细胞分裂素可以部分补偿 I 类 KNOX 功能的丧失。通过与外群的比较，结果表明，一个预先存在的 KNOX-细胞分裂素调节模块在进化过程中被招募到维管植物的芽分生组织中，以促进不定性，从而使维管植物芽形式的辐射成为可能。
