Soluble signals in Arabidopsis exudates strongly enhance the expression of T3SS-associated genes in DC3000 and are genetically regulated by MKP1 and MPK6. (A) Arabidopsis exudate was mixed with DC3000 in minimal medium with or without 50 mM fructose. qRT-PCR analysis of hrpL and avrPto transcripts was performed using samples isolated at the indicated times. Graphed data are means ± SE; n = 4. ◆, −fructose/−exudate; □, +fructose/−exudate; ▲, +fructose/+exudate. **P < 0.01, ***P < 0.001 based on a t test comparison of +fructose/−exudate and +fructose/+exudate treatments. (B) Immunoblot of AvrPto from DC3000 treated as described in A. CBB, Coomassie Brilliant Blue staining. (C) DC3000 was incubated with WT or mkp1 exudate, and hrpL (Left) and avrPto (Right) transcript levels were measured by qRT-PCR 2 and 24 h posttreatment, respectively. Graphed data are means ± SE; n = 4. *P < 0.05. (D) Immunoblot of AvrPto in DC3000 24 h posttreatment with WT, mkp1, or mkp1 mpk6 exudate. A cross-reacting (x-react) band shows equal loading.