Proceedings of the XLVII Italian
Society of Agricultural Genetics - SIGA Annual Congress
Verona,
Italy - 24/27 September, 2003
ISBN 88-900622-4-X
Poster
Abstract - 2.57
NEW CLADES OF R-GENES CANDIDATE SEQUENCES
HOMOLOGOUS TO NUCLEOTIDE BINDING SITE DOMAIN IN PHASEOLUS VULGARIS L.
F. DEL BIANCO,
B. PARISI, P. RANALLI, A. CARBONI
Istituto
Sperimentale per le Colture Industriali, Bologna (Italy)
common bean, root knot nematodes, plant-pathogen
interaction, evolution, NBS-LRR resistance genes
The aim of this
work, carried out together an extensive breeding program to introduce nematode
resistance in common bean, was to exploit the potentiality of Resistance Gene
Analogs studies to understand the evolution of plant-pathogen interaction and
to elucidate the distribution of these RGAs in Phaseolus vulgaris L.
germplasm.
A large number of
sequences (more than 1000) homologous to the Nucleotide Binding Site (NBS)
domain of NBS-leucine-rich repeat (LRR) resistance genes have been isolated
through PCR amplification with degenerate primers, cloned and subsequently
sequenced in 7 different genotypes of Phaseolus vulgaris L.:
two different cultivars (BAT 93 and Jalo EEP558), respectively representative
of the two geographically distinct gene pools (Mesoamerica and the southern
Andes), and five accessions from post-domestication centres of diversity
(Messico, Peru, Colombia, etc.), showing different rates of resistance to Meloydogine spp.
(root knot nematodes).
Phylogenetic
analysis classified these new sequences into TIR (toll and interleukin-1
receptor) and non-TIR subfamilies, by the presence or absence of a region
homologous to the TI Receptor domain at the N-terminus and by specific amino
acid motifs within the NBS domain itself. Both the TIR and non-TIR groups show
long branch lengths and clearly clustered nodes, reflecting a high level of
sequence divergence.
The comparison of
these new sequences with those retrieved in different genebanks of Phaseoleae
and related tribes, such as Trifoleae, Vicieae, etc., confirmed that major
clades within the NBS-LRR family pre-date evolution of these legumes but
introduced also the hypothesis of a secondary development and differentiation
of specific Resistance Gene Analogs (RGAs) in Phaseolus vulgaris L.;
two previously never identified cluster of RGAs within each subgroup (TIR and
non-TIR) were retrieved and here described: a huge cluster near the M non-TIR
region and another cluster with “intermediate” characteristics
within the TIR subfamily. We proposed to call these cluster respectively M6 and
N, according to Zhu (Zhu et al., 2002, MPMI 15(6):529-539).
The presence of
new clusters of highly diverged NBS sequences suggests that multiple ways of
sequence dispersion are occurring in common bean, but further studies are necessary
to confirm the divergence of the RGA family due to plant-nematode evolution.