1. Division of Molecular Life Sciences,
2. School of Interdisciplinary Bioscience and Bioengineering, Pohang University of Science and Technology, Pohang 790-784, South Korea
3. Department of Biology, University of Leicester, University Road, Leicester LE1 7RH, UK
4. Present address: School of Life Sciences, Kyungpook National University, Daegu 702-701, South Korea.

Correspondence to: David Twell³Hong Gil Nam^1,2 Correspondence and requests for materials should be addressed to D.T. (Email: twe@le.ac.uk) or H.G.N. (Email: nam@postech.ac.kr).

Abstract

Flowering plants possess a unique reproductive strategy, involving double fertilization by twin sperm cells¹. Unlike animal germ lines, the male germ cell lineage in plants only forms after meiosis and involves asymmetric division of haploid microspores, to produce a large, non-germline vegetative cell and a germ cell that undergoes one further division to produce the twin sperm cells². Although this switch in cell cycle control is critical for sperm cell production and delivery, the underlying molecular mechanisms are unknown. Here we identify a novel F-box protein of Arabidopsis thaliana, designated FBL17 (F-box-like 17), that enables this switch by targeting the degradation of cyclin-dependent kinase A;1 inhibitors specifically in male germ cells. We show that FBL17 is transiently expressed in the male germ line after asymmetric division and forms an SKP1–Cullin1–F-box protein (SCF) E3 ubiquitin ligase complex (SCF^FBL17) that targets the cyclin-dependent kinase inhibitors KRP6 and KRP7 for proteasome-dependent degradation. Accordingly, the loss of FBL17 function leads to the stabilization of KRP6 and inhibition of germ cell cycle progression. Our results identify SCF^FBL17 as an essential male germ cell proliferation complex that promotes twin sperm cell production and double fertilization in flowering plants.

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