To study the functions of the early auxin-responsive genes Cs GH3.1 and Cs GH3.6 in citrus resistance against canker disease, we cloned Cs GH3.1and Cs GH3.6 in ‘Newhall’ Navel Orange(Citrus sinensis Osbeck). They are 1 797 bp and 1 887 bp and encode 598 and 629 amino acids, respectively. In vitro mature leaves from susceptible ‘Newhall’ and resistant Calamondin(C. madurensis) were inoculated by a Xanthomonas axonopodis pv. citri(Xac) bacterial suspension, and expression of Cs GH3.1 and Cs GH3.6 in the two varieties were analyzed using quantitative real-time PCR(q RT-PCR). ‘Newhall’ leaves were treated with different hormones for 3 days, inoculated by Xac bacterial suspension, and then the symptoms in these leaves were investigated. We used q RT-PCR to analyze the effect of different hormones on Cs GH3.1 and Cs GH3.6 expression in ‘Newhall’leaves. The expression levels of both Cs GH3.1 and Cs GH3.6 were significantly induced by Xac in ‘Newhall’ leaves, compared with levels in Calamondin leaves. 1-naphthy acetic acid(NAA) increased the hypertrophy of infection sites in ‘Newhall’ leaves, while naphthyl-phthalamic acid(NPA)had no visible effect on lesion development. NAA hormone greatly improved expression of Cs GH3.1 in ‘Newhall’, but not Cs GH3.6. These results indicate that the auxin primary-response gene Cs GH3.1 plays an important role in citrus susceptibility to Xac. To study the functions of the early auxin-responsive genes Cs GH3.1 and Cs GH3.6 in citrus resistance against canker disease, we cloned Cs GH3.1 and Cs GH 3.6 in ’Newhall’ Navel Orange (Citrus sinensis Osbeck). are 1 797 bp and 1 887 bp and encode 598 and 629 amino acids, respectively. In vitro mature leaves from susceptible ’Newhall’ and resistant Calamondin (C. madurensis) were inoculated by a Xanthomonas axonopodis pv. citri (Xac) bacterial suspension, and expression of Cs GH3.1 and Cs GH3.6 in the two varieties were analyzed using quantitative real-time PCR (q RT-PCR). ’Newhall’ leaves were treated with different hormones for 3 days, inoculated by Xac bacterial suspension, and then the symptoms in these leaves were investigated. We used q RT-PCR to analyze the effect of different hormones on Cs GH3.1 and Cs GH3.6 expression in ’Newhall’leaves. The expression levels of both Cs GH3.1 and Cs GH3.6 were significantly induced by Xac in ’Newhall’ leaves, compared with levels in 1-naphthy acetic acid (NAA) increased the hypertrophy of infection sites in ’Newhall’ leaves, while naphthyl-phthalamic acid (NPA) had no visible effect on lesion development. NAA hormone greatly improved expression of Cs GH3.1 in ’Newhall’, but not Cs GH3.6. These results indicate that the auxin primary-response gene Cs GH3.1 plays an important role in citrus susceptibility to Xac.