We provide evidence that alterations in DNA methylation patterns contribute to the regulation of stress-responsive gene expression for an intergenerational resistance of β-aminobutyric acid (BABA)-primed potato to Phytophthora infestans . Plants exposed to BABA rapidly modified their methylation capacity toward genome-wide DNA hypermethylation. De novo induced DNA methylation (5-mC) correlated with the up-regulation of Chromomethylase 3 (CMT3), Domains rearranged methyltransferase 2 (DRM2), and Repressor of silencing 1 (ROS1) genes in potato. BABA transiently activated DNA hypermethylation in the promoter region of the R3a resistance gene triggering its downregulation in the absence of the oomycete pathogen. However, in the successive stages of priming, an excessive DNA methylation state changed into demethylation with the active involvement of potato DNA glycosylases. Interestingly, the 5-mC–mediated changes were transmitted into the next generation in the form of intergenerational stress memory. Descendants of the primed potato, which derived from tubers or seeds carrying the less methylated R3a promoter, showed a higher transcription of R3a that associated with an augmented intergenerational resistance to virulent P. infestans when compared to the inoculated progeny of unprimed plants. Furthermore, our study revealed that enhanced transcription of some SA-dependent genes ( NPR1, StWRKY1 , and PR1 ) was not directly linked with DNA methylation changes in the promoter region of these genes, but was a consequence of methylation-dependent alterations in the transcriptional network.

In this paper we analyzed β-aminobutyric acid (BABA)-primed epigenetic adjustment of potato cv. “Sarpo Mira” to Phytophthora infestans. The first stress-free generation of the potato genotype obtained from BABA-primed parent plants via tubers and seeds showed pronounced resistance to the pathogen, which was tuned with the transcriptional memory of SA-responsive genes. During the early priming phase before the triggering stress, we found robust bistable deposition of histone marks (H3K4me2 and H3K27me3) on the NPR1 (Non-expressor of PR genes) and the SNI1 gene (Suppressor of NPR1, Inducible), in which transcription antagonized silencing. Switchable chromatin states of these adverse systemic acquired resistance (SAR) regulators probably reprogrammed responsiveness of the PR1 and PR2 genes and contributed to stress imprinting. The elevated levels of heritable H3K4me2 tag in the absence of transcription on SA-dependent genes in BABA-primed (F0) and its vegetative and generative progeny (F1) before pathogen challenge provided evidence for the epigenetic mark for intergenerational memory in potato. Moreover, our study revealed that histone acetylation was not critical for maintaining BABA-primed defense information until the plants were triggered with the virulent pathogen when rapid and boosted PRs gene expression probably required histone acetyltransferase (HAT) activity both in F0 and F1 progeny.

Peroxynitrite (ONOO-) exhibits a well-documented nitration activity in relation to proteins and lipids; however, the interaction of ONOO- with nucleic acids remains unknown in plants. The study uncovers RNA and mRNA nitration as an integral event in plant metabolism intensified during immune response. Using potato-avr/vr Phytophthora infestans systems and immunoassays we documented that potato immunity is accompanied by two waves of boosted ONOO- formation affecting guanine nucleotides embedded in RNA/mRNA and protein tyrosine residues. The early ONOO- generation was orchestrated with an elevated level of protein nitration and a huge accumulation of 8-nitroguanine (8-NO2-G) in RNA and mRNA pools confirmed as a biomarker of nucleic acid nitration. Importantly, potato cells lacking ONOO- due to scavenger treatment and attacked by the avr pathogen exhibited a low level of 8-NO2-G in the mRNA pool correlated with reduced symptoms of programmed cell death (PCD). The second burst of ONOO- coincided both with an enhanced level of tyrosine-nitrated proteins identified as subtilisine-like proteases and diminished protease activity in cells surrounding the PCD zone. Nitration of both RNA/mRNA and proteins via NO/ONOO- may constitute a new metabolic switch in redox regulation of PCD, potentially limiting its range in potato immunity to avr P. infestans.

Nitric oxide (NO) is an essential redox-signaling molecule operating in many physiological and pathophysiological processes. However, evidence on putative NO engagement in plant immunity by affecting defense gene expressions, including histone modifications, is poorly recognized. Exploring the effect of biphasic NO generation regulated by S-nitrosoglutathione reductase (GNSOR) activity after avr Phytophthora infestans inoculation, we showed that the phase of NO decline at 6 h post-inoculation (hpi) was correlated with the rise of defense gene expressions enriched in the TrxG-mediated H3K4me3 active mark in their promoter regions. Here, we report that arginine methyltransferase PRMT5 catalyzing histone H4R3 symmetric dimethylation (H4R3sme2) is necessary to ensure potato resistance to avr P. infestans. Both the pathogen and S-nitrosoglutathione (GSNO) altered the methylation status of H4R3sme2 by transient reduction in the repressive mark in the promoter of defense genes, R3a and HSR203J (a resistance marker), thereby elevating their transcription. In turn, the PRMT5-selective inhibitor repressed R3a expression and attenuated the hypersensitive response to the pathogen. In conclusion, we postulate that lowering the NO level (at 6 hpi) might be decisive for facilitating the pathogen-induced upregulation of stress genes via histone lysine methylation and PRMT5 controlling potato immunity to late blight.

Our previous study concerning the pathogen-induced biphasic pattern of nitric oxide (NO) burst revealed that the decline phase and a low level of NO, due to S-nitrosoglutathione reductase (GSNOR) activity, might be decisive in the upregulation of stress-sensitive genes via histone H3/H4 methylation in potato leaves inoculated with avr P. infestans. The present study refers to the NO-related impact on genes regulating DNA (de)methylation, being in dialog with histone methylation. The excessive amounts of NO after the pathogen or GSNO treatment forced the transient upregulation of histone SUVH4 methylation and DNA hypermethylation. Then the diminished NO bioavailability reduced the SUVH4-mediated suppressive H3K9me2 mark on the R3a gene promoter and enhanced its transcription. However, we found that the R3a gene is likely to be controlled by the RdDM methylation pathway. The data revealed the time-dependent downregulation of the DCL3, AGO4, and miR482e genes, exerting upregulation of the targeted R3a gene correlated with ROS1 overexpression. Based on these results, we postulate that the biphasic waves of NO burst in response to the pathogen appear crucial in establishing potato resistance to late blight through the RdDM pathway controlling R gene expression.