Abstract: MdFLS2（MDP0000254122 as registered in GDR）encoding a putative receptor perceiving bacterial flagellin was cloned from the apple callus of‘Zihong 3’. Phylogeny analysis showed that MdFLS2 bears distantly related homologies with FLS2 in Arabidopsis thaliana（AtFLS2，At5g46330.1）， but showed the closest homologies with FLS2 in Pyrus bretshneideri. Temporal and spatial expression analysis indicated that there was no observed difference on the expressional level of this gene in leaves with different senescence degree. But its expression could be significantly up-regulated by exogenously provided salicylic acid（SA）but not IAA and ACC. The gene was expressed predominantly in root with subsequently sequential declining in stem，leaf，and flower and fruit. When heterologously overexpressed in Arabidopsis，MdFLS2 significantly retarded the seedling growth and caused the death and wilt of certain leaves. The expression of pathogenesis related genes AtPR1，AtPR2 and AtPR5 which were regulated by SA are up-regulated similarly as those senescence regulated genes AtORE1 and AtNAP. Overexpression of MdFLS2 in the Arabidopsis fls2 mutant restored the inhibition of root elongation by flg22，while the induction of flg22-induced marker genes 30 and 60 minutes after flg22 treatment and the initiation of MAPK signaling pathway 20 and 40 minutes after flg22 treatment in MdFLS2 overexpression transgenic plants was significantly lower than that of wild type plants. Moreover，the disease resistance of Arabidopsis leaves against the apple ring rot causal fungi Botryosphaeria dothidea was significantly enhanced by MdFLS2 overexpression and the immune responses were also significantly strengthened compared with the fls2 mutant during the infection of Arabidopsis leaves with the bacterial pathogen Pseudomonas syringae pv. tomato DC3000. Our result suggested that MdFLS2 functions as an immune gene but displayed differences in the specific implementation of the function and mechanism of action.

Key words: apple, MdFLS2, Arabidopsis, fls2 mutant, disease resistance, apple ring rot, flg22