Effect of Btl19-13 on reporter gene activity in a transient expression assay in N. benthamiana leaves. (A) Schematic of the two reporter constructs used. GUS is driven by a pepper Bs3 minimal promoter harboring a binding element for the Xanthomonas TAL effector AvrBs3 and either a binding element for Btl19-13 (BE) or a scrambled binding element (sBE). (B and C) Fluorometric assays of GUS activity in planta. N. benthamiana leaves were coinfiltrated with Agrobacterium tumefaciens strains delivering one of the two reporter constructs, as indicated, and one (B) or more (C) expression constructs for the following effector proteins: AvrBs3; dT19-13, a designer TAL effector with similar RVD sequence to Btl19-13; Btl19-13; Tal1c, a Xanthomonas TAL effector which has no predicted binding element in the Bs3 promoter. GUS activity was assayed 48 hpi and is shown relative to the corresponding reporter construct with only Tal1c. Each value is an average of three replicates, and the experiment was repeated twice with similar results. An asterisk (*) over two values indicates a significant difference (paired Student’s t test, P < 0.05). Error bars denote SD.