Defense-Related Signaling by Interaction of Arabinogalactan Proteins and β-Glucosyl Yariv Reagent Inhibits Gibberellin Signaling in Barley Aleurone Cells

Arabinogalactan proteins (AGPs) are hydroxyproline-rich glycoproteins present at the plasma membrane and in extracellular spaces. A synthetic chemical, β-glucosyl Yariv reagent (β-GlcY), binds specifically to AGPs. We previously reported that gibberellin signaling is specifically inhibited by β-GlcY treatment in barley aleurone protoplasts. In the present study, we found that β-GlcY also inhibited gibberellin-induced programmed cell death (PCD) in aleurone cells. We examined the universality and specificity of the inhibitory effect of β-GlcY on gibberellin signaling using microarray analysis and found that β-GlcY was largely effective in repressing gibberellin-induced gene expression. In addition, >100 genes were up-regulated by β-GlcY in a gibberellin-independent manner, and many of these were categorized as defense-related genes. Defense signaling triggered by several defense system inducers such as jasmonic acid and a chitin elicitor could inhibit gibberellin-inducible events such as α-amylase secretion, PCD and expression of some gibberellin-inducible genes in aleurone cells. Furthermore, β-GlcY repressed the gibberellin-inducible Ca 2+ -ATPase gene which is important for gibberellin-dependent gene expression, and induced known repressors of gibberellin signaling, two WRKY genes and a NAK kinase gene. These effects of β-GlcY were also phenocopied by the chitin elicitor and/or jasmonic acid. These results indicate that gibberellin signaling is under the regulation of defense-related signaling in aleurone cells. It is also probable that AGPs are involved in the perception of stimuli causing defense responses.