Page 10 - Molecular Plant Breeding

Molecular Plant Breeding 2011, Vol.2, No.16, 109-118
http://mpb.sophiapublisher.com
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was closer than that between Asian lily and Oriental
lilies, which might have certain correlations existing
in the hybridizing between Easter lily and Asian lily
easier than that between Oriental lily and Asia lily.
Lily clustering into the group with the grass family
belonging to monocotyledonous plant showed that
this genetic relationship should be in line with
classification in plants. Cluster analysis found that
dicotyledonous plants of Rosaceae, Solanaceae, and
cruciferous plants were clustered into a group to form
branch, indicating that LTP genes during evolution
should be relatively conservative. However, Arabidopsis
LTPs including LTP6, LTP10, LTP8 and LTP11 were
not clustered into the group with cruciferous plant in
this dendrogram, while LTP9 had much closer genetic
distance with the other LTPs, it may be due to
Arabidopsis LTP encoding by a small multi-gene
family (Arondel et al., 2000), resulting in differences
in amino acid sequence of LTP proteins. Barley LTP2,
wheat LTP2- 1 and wheat LTP2- 2 did not come
together in the grass group, but went to form a single
large branch with the cowpea nsLTP, which would be
the reason that LTP can be divided into LTP Ⅰ and
LTP Ⅱ the size of the protein molecular weight. LTP
Ⅰ located in the cell wall consisted by a 90- 95
amino acid residues with the molecular weight of
about 10 kDa, mainly existing in the aerial parts of
plant tissues and organs, while LTP Ⅱ consisted
about 70 amino acid residues molecular weight of
about 7 kDa, mainly existing in the roots (Li et al.,
2009). Cowpea nsLTP, barley LTP2, wheat LTP2-1
and wheat LTP2-2 belongs to type Ⅱ LTP, other
sequences belong to the type Ⅰ LTP.
Prediction of protein secondary structure was carried
out (Figure 5) in this study, the results showed that
there were also quite differences between the
secondary structure of type Ⅰ and type Ⅱ LTP.
Type Ⅰ LTP has more α-helix than that of the type
Ⅱ LTP, while Type Ⅰ LTP has less β-turn than that
of the type Ⅱ LTP. The overall difference of the
secondary structure among the sequences of the type I
LTP was not significant, α-helix and irregular coil are
the main structural elements and four-stranded α-helix
mainly distributed in the middle of the amino acid
sequence, whereas irregular coiled form and extension
mainly distributed in the end. In monocots, there was
no β-turn appearing in the first α-helix region of SCA,
and LTP protein, while in dicots, some β-turn structure
existed in the region of LTP proteins. Based on the
phylogenetic tree and the secondary structure
prediction we might predict that the differentiation of
plant LTP genes should occur before differentiation of
monocots and dicots.
2 Discussions
SCA protein was first isolated from the stigma and
style of Lilium longiflorum 'Nellie White' in Lord’s
laboratory of the University of California in vivo
observations by immunogold electron microscopy as
well as in vitro adhesion assay have proved that SCA
protein has adhesion functions to pollen tube (Park et
al., 2000). We found that lily SCA gene had an open
reading frame containing two exons and one intron.
Regarding exons, the amino acid sequences deduced
by the exons of SCAs of three lily species had high
similarity, more than 80%, of which contained the
conserved structural features of the LTP genes.
However, there were significant amino acid differences
occurring in the sites of the 22 nd, 64th and 71st among
species (Figure 3). Do the amino acid differences of
these sites lead to change in the functions of SCA
protein? Or do changes of these sites affect the
specific recognition between the SCA protein and
SCA receptors on the top plasma membrane pollen
tube, and then affecting pollen tube adhesion and
directional growth between different species, thus
influence on the affinity of hybridization among the
species? At present we are carrying out in vitro protein
expression and purification of lily SCA genes of the
different species as well as in vitro adhesion assay to
determine the induction ability of the pollen tube
adhesion and directional growth of the protein between
interspecies and intra-species, accordingly, to determine
the changes of these amino acid sites whether the SCA
proteins are subject to cause the influences on pollen
tube adhesion among lily species.
In introns, the locations of the intron of three species
were quite conservative, but the lengths of the introns
were differences. The homology of the introns was
lower than that of the exons, there was little or a little