Recognition of a wide-range of S-RNases by S locus F-box
like 2, a general-inhibitor candidate in the Prunus-speciﬁc
S-RNase-based self-incompatibility system
Received: 24 November 2015 / Accepted: 4 April 2016 / Published online: 12 April 2016
Ó Springer Science+Business Media Dordrecht 2016
Abstract Many species in the Rosaceae, the Solanaceae,
and the Plantaginaceae exhibit S-RNase-based gameto-
phytic self-incompatibility (GSI). This system comprises
S-ribonucleases (S-RNases) as the pistil S determinant and
a single or multiple F-box proteins as the pollen S deter-
minants. In Prunus, pollen speciﬁcity is determined by a
single S haplotype-speciﬁc F-box protein (SFB). The
results of several studies suggested that SFB exerts cognate
S-RNase cytotoxicity, and a hypothetical general inhibitor
(GI) is assumed to detoxify S-RNases in non-speciﬁc
manner unless it is affected by SFB. Although the identity
of the GI is unknown, phylogenetic and evolutionary
analyses have indicated that S locus F-box like 1–3 (or S
locus F-box with low allelic sequence polymorphism 1–3;
SLFL1–3), which are encoded by a region of the Prunus
genome linked to the S locus, are good GI candidates.
Here, we examined the biochemical characteristics of
SLFL1–3 to determine whether they have appropriate GI
characteristics. Pull-down assays and quantitative expres-
sion analyses indicated that Prunus avium SLFL1–3 mainly
formed a canonical SCF complex with PavSSK1 and
PavCul1A. Binding assays with PavS
that PavSLFL1, PavSLFL2, and PavSLFL3 bound to
-RNase, all PavS-RNases tested, and none of the
PavS-RNases tested, respectively. Together, these results
suggested that SLFL2 has the appropriate characteristics to
be the GI in sweet cherry pollen, while SLFL1 may
redundantly work with SLFL2 to detoxify all S-RNases.
We discuss the possible roles of SLFL1–3 as the GI in the
Prunus-speciﬁc S-RNase-based GSI mechanism.
Keywords General inhibitor Á Prunus Á SCF complex Á
Self-incompatibility Á SLFL Á S-RNase
Self-incompatibility (SI) is one of the most common
reproductive systems in ﬂowering plants to prevent
inbreeding and promote outcrossing by enabling the pistil
to recognize and reject pollen from genetically related
individuals (de Nettancourt 2001; Franklin-Tong 2008).
The SI system is genetically controlled by the polymorphic
S locus containing pistil S and pollen S determinant genes.
The most widespread SI system is the S-RNase-based
gametophytic SI (GSI) that exists in members of the
Rosaceae, the Solanaceae, and the Plantaginaceae. The GSI
system comprises an extracellular cytotoxic S-RNase as the
pistil S determinant and F-box protein(s) as pollen S de-
terminants (Franklin-Tong 2008; McClure et al. 2011).
This GSI system is suggested to have a single origin dating
back to 120 million years ago (Igic and Kohn 2001;
Steinbachs and Holsinger 2002; Vieira et al. 2008a; Nowak
et al. 2011; Asquini et al. 2011). The S-RNase is secreted
into stylar transmitting tissues, and is non-selectively taken
up into the cytoplasm of the pollen tube elongating in those
tissues (Luu et al. 2000; Goldraij et al. 2006; McClure et al.
2011; Boivin et al. 2014). The S-RNase exerts its cyto-
toxicity to stop pollen tube growth unless it is inactivated.
Electronic supplementary material The online version of this
article (doi:10.1007/s11103-016-0479-2) contains supplementary
material, which is available to authorized users.
& Ryutaro Tao
Faculty of Agriculture, Yamagata University,
Tsuruoka 997-8555, Japan
Graduate School of Agriculture, Kyoto University,
Kyoto 606-8502, Japan
Plant Mol Biol (2016) 91:459–469