In npr1-5 mutant exposed to iturin A, PR1 gene expression was not
upregulated in the untreated leaves. Iturin A also upregulated
PDF1.2 gene expression in iturin A untreated leaves of wild plants
12 h after exposure to iturin A (Fig. 2B). The upregulation was
maintained up to 72 h after exposure to iturin A. A similar upregulation
of PDF1.2 gene expression was confirmed in the untreated
leaves of npr1-5 mutant after exposure to iturin A, but not in those
of atmyc2 mutant, although PDF1.2 transcripts were transiently
increased in the untreated leaves of atmyc2 mutant 6 h after
exposure to iturin A.
Taken together, the results suggest that iturin A induces PR1 and
PDF1.2 gene expression in both local and systemic tissues through
the SA and JA signaling pathways, respectively.
3.2. Structure-dependent activity of iturin A in the induction of
plant defense response
To determine whether plant defense response induction by
iturin A is structure-dependent, we synthesized three iturin A derivative
peptides (Fig. 3A). Iturin A inhibited the mycelial growth of
fungal plant pathogen C. gloeosporioides, whereas all the derivative
peptides lost their antagonistic effect on the fungus (Fig. 3B).
As described above, iturin A induced PR1 and PDF1.2 gene
expression in the untreated leaves of wild plants (Fig. 3C and D).
When wild plants were exposed to the three iturin A derivative
peptides, PR1 and PDF1.2 gene expression was not systemically
upregulated in the untreated leaves. The ATAGPA peptide, in
particular, drastically decreased the expression of both genes in the
untreated leaves compared with control plants (Fig. 3D).