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Disruption of the pollen-expressed FERONIA homologs ANXUR1 and ANXUR2 triggers pollen tube discharge
Monica A. Grobei1,
Julian I. Schroeder2, and
1 Institute of Plant Biology and Zürich-Basel Plant Science Center,
University of Zürich, Zollikerstrasse 107, 8008 Zürich,
Switzerland. 2 Division of Biological Sciences, Cell and Developmental Biology Section, and
Center for Molecular Genetics, University of California San Diego, La Jolla,
CA 92093, USA.
Authors for correspondence
Accepted for publication 31 July 2009.
The precise delivery of male to female gametes during reproductionin
eukaryotes requires complex signal exchanges and a flawlesscommunication
between male and female tissues. In angiosperms,molecular mechanisms have
recently been revealed that are crucialfor the dialog between male (pollen
tube) and female gametophytesrequired for successful sperm delivery. When
pollen tubes reachthe female gametophyte, they arrest growth, burst and
dischargetheir sperm cells. These processes are under the control ofthe
female gametophyte via the receptor-like serine-threoninekinase (RLK) FERONIA
(FER). However, the male signaling componentsthat control the sperm delivery
remain elusive. Here, we showthat ANXUR1 and ANXUR2
(ANX1, ANX2), which encode the closesthomologs of the FER-RLK in
Arabidopsis, are preferentially expressedin pollen. Moreover,
ANX1-YFP and ANX2-YFP fusion proteins displaypolar localization to the plasma
membrane at the tip of thepollen tube. Finally, genetic analyses demonstrate
that ANX1and ANX2 function redundantly to control the
timing of pollentube discharge as anx1 anx2 double-mutant pollen
tubes ceasetheir growth and burst in vitro and fail to reach the female
gametophytesin vivo. We propose that ANX-RLKs constitutively inhibit pollen
tuberupture and sperm discharge at the tip of growing pollen tubesto sustain
their growth within maternal tissues until they reachthe female gametophytes.
Upon arrival, the female FER-dependentsignaling cascade is activated
to mediate pollen tube receptionand fertilization, while male
ANX-dependent signaling is deactivated,enabling the pollen tube to
rupture and deliver its sperm cellsto effect fertilization.