FIG. 3.

Systemic induction of plant defense responses by synthetic peptides P1, P2, and P3 in cucumber plants. (A) Induction of defense-related gene expression. A 3.1-nmol sample of each peptide was applied at the root site of cucumber seedlings for 48 h. Total RNA was extracted 24 h after a bacterial challenge from a pool of three leaves and used for quantitative RT-PCR analysis of hpl, pal1, and prx gene expression. n-fold mRNA induction was compared to that in untreated seedlings (control [C]), to that in seedlings infected with the bacteria without previous peptide treatment (P. syringae pv. lachrimans [Psl]), and to that in seedlings treated with peptides (P1, P2, and P3) but not infected with bacteria. The results shown are averages of six independent treatment replicates ± the standard error tested by one-way factorial ANOVA, followed by TukeyHSD with P < 0.05 in comparison to the controls. (B) Growth-inhibitory activity toward P. syringae pv. lachrimans bacteria of phenol aglyconic extracts from plants pretreated with T. asperellum T203 or ultrashort synthetic peptides. A bioassay compared the antimicrobial activity of the aglycone fraction obtained by acid hydrolysis of crude phenolic extract of cucumber cotyledons from seedlings treated for 48 h with 3.1 nmol of P1, P2, or P3 (bars 2, 3, and 4, respectively) or Trichoderma spores (bar 1) before a challenge with P. syringae pv. lachrimans. The bioassay was performed with P. syringae pv. lachrymans as the test microorganism. Untreated plants (bar 9) or plants treated with peptides (bars 6, 7, and 8) or bacteria only (bar 5) served as controls. The diameter of the lytic zone was measured. Each column represents the mean inhibition diameter of three independent experiments ± the standard deviation. *, significantly different from the other treatments (one-way factorial ANOVA; P < 0.01).