Cloning and Expression Analysis of Citrus Genes CsGH3.1 and CsGH3.6 Responding to Xanthomonas Axonopodis Pv. Citri Infection

To study the functions of the early auxin-responsive genes CsGH3.1 and CsGH3.6 in citrus resistance against canker disease, we cloned CsGH3.1 and CsGH3.6 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 CsGH3.1 and CsGH3.6 in the two varieties were analyzed using quantitative real-time PCR (qRT-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 qRT-PCR to analyze the effect of different hormones on CsGH3.1 and CsGH3.6 expression in ‘Newhall’ leaves. The expression levels of both CsGH3.1 and CsGH3.6 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 CsGH3.1 in ‘Newhall’, but not CsGH3.6. These results indicate that the auxin primary-response gene CsGH3.1 plays an important role in citrus susceptibility to Xac.