The components of the SAGA

The components of the SAGA complex including Gcn5 have recently been found to be involved in regulating pathways that are required for development and virulence in different species under the environmental stresses, around 10% of S. cerevisiae genes are activated by the transcriptional coactivator SAGA complex [38], [39], [40]. Neurospora light signaling is regulated by PP242 modifications. The transcription of early light-inducible genes al-3 and vvd is required for acetylation of histone H3K14 in the promoters of these genes. The acetylation and transient gene activation in Neurospora crassa was shown to be under the control of histone acetyltransferase ngf-1, a homolog of Gcn5 in S. cerevisiae[41]. In C. neoformans, the histone acetyltransferase Gcn5 plays the key important role for chromatin remodeling in regulating specific genes expression leading to the C. neoformans appropriate response to the human host environment, and it is therefore essential for virulence [14]. In Candida albicans, Ada2 protein, one of the components of the SAGA complex, is essential for protein folding, and the defects showed the hypersensitivity to oxidative stress and antifungals, and attenuated virulence [42]. In addition, the Aspergillus nidulans histone acetyltransferase complexes gcnE and adaB, the homologous of the S. cerevisiae GCN5 and ADA2 genes respectively, are not required for chromatin modification and transcriptional regulation, but are required for the suppression of proline metabolic genes prnD and prnB, and for the repositioning of nucleosomes in the divergent promoter region [43]. The S. cerevisiae Spt3 protein, one of the components of the SAGA complex, is required for diploid pseudohyphal and haploid invasive growth. In addition, C. albicans spt3Δ/spt3Δ mutants are avirulent in mice. Furthermore, C. albicans spt3Δ/spt3Δ mutants are hyperfilamentous, the opposite phenotype of S. cerevisiae spt3Δ/spt3Δ mutants. These prove that Spt3 plays important but opposite roles in filamentous growth in S. cerevisiae and C. albicans[44]. In this research, we found no evidence that PsGcn5-silenced mutants are different from P6497 under the stress conditions, elevated temperature and high salt concentration. However, PsGcn5 was responsible for the regulation of oxidative stress tolerance and was required for the P. sojae virulence, which was similar with the phenotype of the C. neoformans Gcn5 deletion mutant. These results show that divergent microbial pathogens using special histone modification control the expression of distinct gene sets required for survival within their particular microenvironments and extracellular stimuli. Given that only partly silenced gene mutants were obtained in this study, the non-silenced part of PsGcn5 may have been sufficient to be effective and to modify the related target genes. It would be desirable for future studies to obtained more effectively silenced mutants. Plant-derived ROS accumulation at the penetration site is considered to be one of the earliest responses for PAMP-triggered immunity mechanisms in plants [21], [45], [46]. With respect to the toxicity of ROS molecules and their importance in plant defense responses, plant pathogens have to develop strategies to scavenge ROS for survival in harsh environments and successfully invade host cells [47]. In fungal plant pathogens, several genes have proven to be responsible for ROS detoxification at the infection site, and the defects of these corresponding genes have shown attenuated virulence on host plants [25], [27]. In this study, we found that the PsGcn5-silenced mutant had no apparent defects in hyphae growth and cyst germination, but had limited infectious hyphae growth and reduced virulence in the susceptible soybean cv. Williams. Due to the hypersensitivity of the silenced mutant strain to H2O2, we suggest that this result may be the loss or severe weakening of the ability to degrade host-derived H2O2 in the silenced mutants. The DAB staining assay was used to determine whether this is associated with reduced colonization by the mutant. The assay indicated that greater amounts of H2O2 accumulated in soybean leaf tissues around the initial infection site when inoculated with the PsGcn5-silenced mutant compared with the wild-type strain. The flavoenzyme inhibitor, DPI, inhibits the activity of the plant NADPH oxidases that is necessary for ROS generation [35]. When DPI was applied to reduce the host-derived ROS, development of infectious hyphae of PsGcn5-silenced mutant was restored to that of the wild-type strain. This result suggests that host-derived ROS is important for the virulence of the PsGcn5-silenced mutant. Inhibited ROS accumulation in soybean may weaken the plant's defense response enabling the silenced mutants to be infective. Linked with hypersensitivity to H2O2 in vitro, we postulate that the histone acetyltransferase PsGcn5 in P. sojae is responsible for the scavenging of host-derived ROS and that this function is required for the inhibition of plant defense and the spread of infection hyphae in plant tissue. With reduced PsGcn5, it is likely that the silenced strain is unable to appropriately acetylate histones, and thus, the transcriptional complexes cannot be recruited by the promoter of related genes needed to induce expression for responding to oxidative stresses. In P. sojae, it is only known that the mitogen-activated protein kinase PsMPK7 is responsible for ROS detoxification at the infection site and virulence on host plants [36]. However, the transcription of PsMPK7 was not regulated by PsGcn5. Whether the ROS detoxification and the loss of pathogenicity was a direct or indirect effect of PsGcn5 silencing remains unclear. In the future, we will perform comparative transcriptional profiling between the wild type and the PsGcn5-silenced mutant strain to determine which processes are transcriptionally regulated by PsGcn5, and further examine how these genes have been regulated through chromatin remodeling by acting as a histone acetyltransferase.