Caratterizzazione strutturale e funzionale dei geni che codificano per l’inibitore proteico delle poligalatturonasi in riso e frumento
Author(s)
Di Giovanni, Michela
Date Issued
March 14, 2008
Type
Doctoral Thesis
Abstract
Polygalacturonase-inhibiting proteins (PGIPs) are leucine-rich repeat (LRR) proteins involved in plant
defence. A number of PGIPs have been characterized from dicot species, whereas only a few data are
available from monocots.
Database searches and genome-specific cloning strategies allowed the identification of four rice (Oryza
sativa L.) and three wheat (Triticum aestivum L.) Pgip genes. The rice Pgip genes (Ospgip1, Ospgip2,
Ospgip3 and Ospgip4) are distributed over a 30 Kbp region of the short arm of chromosome 5, whereas the
wheat Pgip genes, Tapgip1, Tapgip2 and Tapgip3, are localized on the short arm of chromosome 7B, 7D and
7A, respectively. By Southern blots and sequence analysis of BAC clones we demonstrated that wheat
contains a single copy Pgip gene per genome and the one from the A genome, Tapgip3, is inactivated by the
insertion of a LTR copia retrotranspon within the fourth LRR. We demonstrated also that this
retrotransposon insertion is present in T. urartu and all the polyploidy wheats assayed, but is absent in T.
monococcum (Tmpgip3), suggesting that this insertion took place after the divergence between T.
monococcum and Triticum urartu, but before the formation of the polyploid wheats.
We identified also two independent insertion events of new Class II transposable elements, Vacuna,
belonging to the Mutator superfamily, that interrupted the Tdipgip1 gene of T. turgidum ssp. dicoccoides.
The occurrence of these transposons within the coding region of Tdipgip1 facilitated the mapping of the Pgip
locus in the pericentric region of the short arm of chromosome group 7.
Deduced amino acid sequences of both OsPGIP and TaPGIP show the typical LRR modular organization
and a conserved distribution of the eight cysteines at the N- and C- regions. Sequence comparison suggests
that monocot and dicot PGIPs form two separate clusters sharing about 40% identity and shows that this
value is close to the extent of variability observed within each cluster.
Gene-specific RT-PCR and biochemical analyses demonstrate that both Ospgips and Tapgips are expressed
in the whole plant or in a tissue-specific manner, and that OsPGIP1, lacking an entire LRR repeat, is an
active inhibitor of fungal polygalacturonases. This last finding can contribute to define the molecular features
of PG-PGIP interactions and highlights that the genetic events that can generate variability at the Pgip locus
are not only limited to substitutions or small insertions/deletions, as so far reported, but can also involve
variation in the number of LRRs.
Finally, on the basis of the presence of PGIP activity in T. dicoccoides genotypes having both inactive
Tapgip genes we speculate that their inactivation is tolerated because of redundancy of PGIP activities in the
wheat genome.
Additional information
Dottorato di ricerca in Biotecnologie vegetali
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