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.

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

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

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

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