DR5 is a Suitable System for Studying the Auxin Response in the Poncirus trifoliata-Xanthomonas axonopodis pv. citri Interaction

The local auxin distribution characteristics in the roots, stems, and leaves of stably transformed plantlets of trifoliate orange (Poncirus trifoliata) with auxin reporter system DR5::GUS-YFP were elucidated in this research. The auxin response maxima could be observed in the apex of the root tip, primary phloem of the tender stem, and the margin of the young leaves according to the activity of the β-glucuronidase (GUS) reporter gene triggered by the auxin responsive DR5 promoter. Auxin responses in the apex of the root tips increased when treated with synthetic auxin 1-naphthylacetic acid (NAA), but decreased when treated with the auxin polar transportation inhibitor 2,3,5-triiodobenzoic acid (TIBA). These results indicated that the DR5 reporter system worked in P. trifoliata for auxin distribution and response observation. Trifoliate orange is highly susceptible to citrus canker disease. Auxin accumulation was observed visually in the invasion sites of the detached leaves inoculated with Xanthomonas axonopodis pv. citri (Xac) by GUS staining; the upregulated expression of the YFP, GH3.1, GH3.9, and SAUR genes assessed by quantitative real-time PCR (qRT-PCR) also identified auxin accumulation in the inoculated tissues following Xac infection. Overall, these findings indicated that the plantlets of P. trifoliata engineered with the auxin reporter gene provided a promising system for studying auxin responses during Xac infection.