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
Abbreviations: JA, jasmonic acid; LAR, local acquired resistance; NPR1, NONEXPRESSOR
OF PATHOGENESIS-RELATED GENES1; PAMP, pathogen-associated
molecular pattern; SA, salicylic acid; SAR, systemic acquired resistance.
* Corresponding author. Fax: þ81 55 220 8768.
E-mail address: suzukis@yamanashi.ac.jp (S. Suzuki).
Contents lists available at ScienceDirect
Biochemical and Biophysical Research Communications
journal homepage: www.elsevier.com/locate/ybbrc
http://dx.doi.org/10.1016/j.bbrc.2015.03.143
0006-291X/© 2015 Elsevier Inc. All rights reserved.
Biochemical and Biophysical Research Communications 460 (2015) 1015e1020
[10]. Overall, the SA and/or JA signaling pathways are presumed to
be involved in plant defense response triggered by iturin A, as iturin
A induced SAR in strawberry plants [4].
Here we report the defense signaling pathways triggered by
iturin A. We demonstrate that both SA and JA signaling pathways
are involved in the defense response triggered by iturin A. Moreover,
we show that the activity of iturin A as an elicitor may be
depend on its structure.