Xiao-yu Zheng, Mian Zhou, Heejin Yoo, Jose L. Pruneda-Paz, Natalie Weaver Spivey, Steve A. Kay, and Xinnian Dong

Significance

Biosynthesis of the plant immune signal salicylic acid (SA) is normally induced upon pathogen challenge through transcriptional activation of the key SA synthetic enzyme gene, ICS1. However, how different pathogenic signals trigger SA synthesis in both local and systemic tissues and during different immune responses is poorly understood. Our study filled this knowledge gap by the identification of two transcription factors (TFs): one is required for SA biosynthesis in stomata to prevent pathogen entry through these epidermal openings, and the other is essential for both the circadian oscillation in SA levels and the accumulation of SA in distal tissue during systemic acquired resistance. Our study shows that SA biosynthesis is regulated by multiple TFs in a spatial and temporal manner.

Abstract

The plant hormone salicylic acid (SA) is essential for local defense and systemic acquired resistance (SAR). When plants, such as Arabidopsis, are challenged by different pathogens, an increase in SA biosynthesis generally occurs through transcriptional induction of the key synthetic enzyme isochorismate synthase 1 (ICS1). However, the regulatory mechanism for this induction is poorly understood. Using a yeast one-hybrid screen, we identified two transcription factors (TFs), NTM1-LIKE 9 (NTL9) and CCA1 HIKING EXPEDITION (CHE), as activators of ICS1 during specific immune responses. NTL9 is essential for inducing ICS1 and two other SA synthesis-related genes, PHYTOALEXIN-DEFICIENT 4 (PAD4) and ENHANCED DISEASE SUSCEPTIBILITY 1 (EDS1), in guard cells that form stomata. Stomata can quickly close upon challenge to block pathogen entry. This stomatal immunity requires ICS1 and the SA signaling pathway. In the ntl9 mutant, this response is defective and can be rescued by exogenous application of SA, indicating that NTL9-mediated SA synthesis is essential for stomatal immunity. CHE, the second identified TF, is a central circadian clock oscillator and is required not only for the daily oscillation in SA levels but also for the pathogen-induced SA synthesis in systemic tissues during SAR. CHE may also regulate ICS1 through the known transcription activators CALMODULIN BINDING PROTEIN 60g (CBP60g) and SYSTEMIC ACQUIRED RESISTANCE DEFICIENT 1 (SARD1) because induction of these TF genes is compromised in the che-2 mutant. Our study shows that SA biosynthesis is regulated by multiple TFs in a spatial and temporal manner and therefore fills a gap in the signal transduction pathway between pathogen recognition and SA production.

See: http://www.pnas.org/content/112/30/9166.abstract.html?etoc

PNAS July 28, 2015 vol. 112 no. 30 9166-9173

Fig. 6.

CHE is required for pathogen-mediated systemic induction of SARD1 and CBP60g. Plants were infiltrated with 10 mM MgSO₄ (mock) or Psm ES4326/ avrRpt2 (avr, OD_600nm = 0.02), and systemic tissues were collected at 0, 1, and 2 d postinoculation (dpi) to analyze the transcripts of SARD1 (A) and CBP60g (B) using qPCR. Error bars represent SEs from three biological replications. One-way ANOVA, followed by Tukey’s multiple comparisons test, was used for statistical analysis (P < 0.05). Means with the same letter are not significantly different from each other.