1. Introduction
Iturin A is the most well studied broad-spectrum antifungal
cyclic lipopeptide (a heptapeptide with a b-hydroxy fatty acid
chain) produced by Bacillus species [1]. It is thought to be the
principal antifungal substance responsible for the biological control
activity of Bacillus species against fungal pathogens [2,3]. On the
other hand, iturin A triggers systemic acquired resistance (SAR),
which is a whole-plant defense response to an earlier and localized
exposure to pathogens, in strawberry plants, thereby resulting in
the suppression of strawberry anthracnose disease by accumulating
pathogenesis-related (PR) proteins [4]. Accordingly, iturin A
produced by biological control agents plays a role in the suppression
of plant diseases by acting as a bifunctional molecule through
its antifungal activity and the activation of plant defense systems.
How does iturin A trigger plant defense response? No evidence
of the mechanisms is available so far. The simplest mode would be
the direct interaction of iturin A with specific receptors on plant
cells to activate signal transduction pathways. Typically, plant cells
recognize both avirulence factors produced by specific pathogens
and non-specific elicitors constitutively present in pathogens,
thereby inducing local acquired resistance (LAR) in the localized
tissue [5]. Through the recognition, signals are transmitted to nuclei
in plant cells to result in the induction of defense-related proteins,
including PR proteins [6]. Salicylic acid (SA) synthesized in plant
cells functions as a long-distance SAR signal [7]. Plant cells
receiving SA signals accumulate PR proteins. A study of the mechanism
of SAR demonstrated that the positive regulator protein
NON-EXPRESSOR OF PATHOGENESIS-RELATED GENES1 (NPR1) is
transported to the nucleus in response to SA where it activates the
expression of defense genes, including PR proteins PR1 and PR2 [8].
On the other hand, jasmonic acid (JA) also plays a role in the
acquisition of plant disease resistance as a signaling molecule [9].
The JA signaling pathway is positively regulated by the nuclearlocalized
helix-loop-helix-leucine zipper-type transcription factor
MYC2 and induces plant defense related proteins, such as PDF1.2