The Molecular Evolution Of The Wheat Leaf Rust Resistance Gene Lr1

Wheat,as one of the important crops,suffers from the infection of various diseases, wheat leaf rust is one of the major diseases in wheat production.It spreads wildly and can lead to severe yield loss and poor seed quality.The use of resistance genes is the most effective,economical and ecological method to control epidemics of leaf rust disease. However,natural environment and artificial domestication accelerated the co-evolution of host-pathogen interaction,which moreover resulted in the rapid evolution and disappearance of resistance genes.Therefore,it is necessary to study the evolutionary mechanism of plant resistance genes.In our research,we screened 128 Aegilops.Tauschii(goat grass) accessions and 283 hexaploid wheat varieties(including 262 wheat core germplasms) with marker WR003 co-segregated with Lr1,and obtained the full-length sequences of 42 Lr1 alleles and 6 Lr1 homologous genes.Afterwards,we aligned DNA and protein sequences of Lr1 allelic or homologous genes by DNAMAN and Clustal W,and analyzed the molecular evolution mechanism for the wheat leaf rust resistance genes according to the phylogenetic tree established by neighbor-joining(NJ) method by MEGA 4.0.1.The results showed that:1.Wheat leaf rust resistance gene Lr1 was originated from Ae.Tauschii,and differentiated into resistant and susceptible genotypes prior to its evolution into hexaploid wheat. Therefore,wheat leaf rust resistance gene Lr1 has two ancestral genes at least.2.The allelic sequences alignment of Lr1 from resistant and susceptible lines revealed a divergent DNA sequence block of 570 bp,where Lr1 nucleotide sequences were mostly similar between hexaploid wheat and Ae.Tauschii with the same resistance except for two point mutations.So it is concluded that no genetic recombination and gene conversion happened during the evolution from Ae.Tauschii.into hexaploid wheat.3.The rest of the sequences were mostly similar,where only 21 point mutations existed(9 synonymous mutations,and 12 nonsynonymous mutations).Interestingly,these 21 point mutations occur between lines with identical resistance rather than different resistance.4.There was only one point mutation between resistant hexaploid wheat and Ae.Tauschii (at 2400bp down stream of the start codon ATG,a synonymous mutation);9 point mutations took place between susceptible hexaploid wheat and Ae.Tauschii(2 synonymous mutations and 7 nonsynonymous mutations),1 in the 570bp block and 8 in the rest of the sequence.5.Fourteen synonymous mutations and 51 nonsynonymous mutations lined in tandem on the LRR coding region of the allele sequences,Ka(rate of nonsynonymous mutation)/Ks (rate of synonymous mutation)＞1.These results indicated that LRR domain suffered from the pressure of strong selection,consistent with the former reports that LRR domains were involved in the interaction with effector proteins and were the major determinant of resistance specificity.