Download Sticking the Landing: Probing the Roles of LORELEI

The Plant Cell, Vol. 28: 995–996, May 2016, www.plantcell.org ã 2016 American Society of Plant Biologists. All rights reserved.
IN BRIEF
Sticking the Landing: Probing the Roles of LORELEI in Pollen
Tube Reception
OPEN
A tiny, dry ball comprising two sperm cells
and a vegetative cell protected by a tough
layer of sporopollenin (otherwise known
as a pollen grain) lands on a stigma. Upon
hydration, the vegetative cell gives rise to
a pollen tube, which travels through the
style to the micropyle, a minute opening
in the ovule. Given the proper signals from
the synergids in the waiting embryo sac, the
pollen tube initiates double fertilization.
During this process, the synergids facilitate
pollen tube burst (to release the two sperm
cells), as well as the fusion of one sperm cell
with the egg cell and one with the central
cell to form the embryo and endosperm,
respectively. The filiform apparatus (FA) is
an impressive group of finger-like projections at the micropylar ends of synergids
that extend back into the synergid cytoplasm. The FA increases the area of contact
between the synergids and pollen tube to
facilitate the release of a suite of signaling
peptides, which we are only beginning to
identify (Qu et al., 2015). One such peptide,
LORELEI (LRE), appears to function in pollen tube reception at the FA together with
FERONIA (FER), a receptor-like kinase
that it chaperones from the endoplasmic
reticulum (Li et al., 2015), although the precise mechanisms are unclear. LRE might
also play a separate role at the synergid
surface.
Through clever structure-function analysis of LRE in Arabidopsis thaliana, Liu et al.
(2016) obtained experimental evidence for
the roles of LRE in the synergid cell. Previous
predictions indicated that LRE is a glycosylphosphatidylinositol (GPI)-linked protein.
These ubiquitous eukaryotic proteins contain a signal sequence directing them to the
endoplasmic reticulum lumen. After this sequence and a hydrophobic tail are removed,
a GPI anchor is linked to a specific site on
the protein, allowing it to stick to the cell
surface. Through sequence analysis, the authors confirmed that LRE is a GPI-anchored
OPEN
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Confocal image of an unfertilized ovule showing LRE-cYFP expression in two adjacent synergid cells.
Green signals are from the nuclear marker ACT11:H2B:GFP. (Figure courtesy of R. Palanivelu.)
surface protein with a conserved eightcysteine motif (8CM) required for structural stability, although this motif is modified
(M8CM), a feature found in many plants.
LRE localizes to both the FA and synergid
cytoplasm, as shown using LRE fused to
citrine yellow fluorescent protein (LRE-cYFP;
see figure). Transformation with an LREcYFP construct restored seed set in lre
mutants to wild-type levels and rescued
transmission of the lre mutation through
the female gametophyte. Deletion of putative GPI anchor addition domains affected
the localization but not the functionality of
LRE-cYFP. Analysis of gpi8, a mutant defective in GPI attachment, indicated that
attachment of GPI to LRE helped localize
LRE-cYFP to the FA. Surprisingly, this mutant showed wild-type pollen tube reception and seed set, suggesting that the GPI
anchor addition domains in LRE are not
required for its function. However, a modified LRE containing a typical 8CM instead
of the M8CM failed to rescue lre mutant
phenotypes and exhibited altered localization, indicating that the modification in
M8CM is critical for LRE’s role in pollen tube
reception.
The authors then supplied LRE-cYFP directly to the synergid cell surface of lre by
ectopically expressing it in pollen tubes
where endogenous LRE was not expressed.
Even though LRE-cYFP was not delivered to
the FA, it rescued the impaired female gametophyte phenotype of the mutant, indicating that LRE plays a role at the interface of
the pollen tube and synergid cell independent of its intracellular role in the synergid.
By contrast, such pollen harboring LREcYFP failed to rescue the pollen tube reception and reduced seed set defects in lre
fer double mutants, indicating that both LRE
and FER are required at the pollen tube/
synergid cell interface to induce pollen tube
reception.
Similar structure-function studies of other
signaling peptides should provide further
insights into the fascinating processes that
996
The Plant Cell
occur when a pollen tube approaches its
target cells in the ovule.
Jennifer Lockhart
Science Editor
[email protected]
ORCID ID: 0000-0002-1394-8947
REFERENCES
Li, C., et al. (2015). Glycosylphosphatidylinositolanchored proteins as chaperones and coreceptors for FERONIA receptor kinase signaling
in Arabidopsis. eLife 4: e06587.
Liu, X., Castro, C.A., Wang, Y., Noble, J.A.,
Ponvert, N.D., Bundy, M.G., Hoel, C.R.,
Shpak, E.D., and Palanivelu, R. (2016). The
role of LORELEI in pollen tube reception at the
interface of the synergid cell and pollen tube
requires the modified eight-cysteine motif and
the receptor-like kinase FERONIA. Plant Cell
28: 1035–1052.
Qu, L.J., Li, L., Lan, Z., and Dresselhaus, T.
(2015). Peptide signalling during the pollen tube
journey and double fertilization. J. Exp. Bot. 66:
5139–5150.
Sticking the Landing: Probing the Roles of LORELEI in Pollen Tube Reception
Jennifer Lockhart
Plant Cell 2016;28;995-996; originally published online April 15, 2016;
DOI 10.1105/tpc.16.00308
This information is current as of June 17, 2017
Supplemental Data
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References
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