Chromatin remodeling by HIRA and FAS2 restricts geminivirus pathogenesis in plants

Geminiviruses represent a significant threat to global agriculture due to the substantial crop loss they inflict worldwide. Histone chaperones, a highly conserved family of proteins primarily recognized for their roles in nucleosome assembly and disassembly, have demonstrated crucial involvement in animal virus biology. However, their contribution to the process of plant virus infection remains largely uncharacterized. To address this knowledge gap, we aimed to elucidate the role of two major histone chaperones, the replication-coupled Fasciata2 (FAS2) and the replication-independent Histone regulator A (HIRA), in the pathogenesis of geminiviruses in Nicotiana benthamiana.

We investigated the influence of histone chaperones NbHIRA and NbFAS2 during Pepper leaf curl Bangladesh virus (PepLCBV) infection within a permissive host, N. benthamiana. We employed quantitative real-time PCR (qRT-PCR) to quantify gene expression levels, discontinuous sucrose gradient assay to analyse viral minichromosome assembly, and Chromatin immunoprecipitation (ChIP) to examine histone-DNA interactions. We also evaluated both active and repressive histone methylation marks on viral minichromosomes, providing crucial insight into the regulatory influence of NbHIRA and NbFAS2 on viral gene activity.

Our findings indicate that downregulation of NbHIRA and NbFAS2 leads to an increase in virus transcript accumulation. The virus may utilize the elevated NbRAD51 levels in the knockdown plants, contributing to the increased viral load. Conversely, overexpression of both NbHIRA and NbFAS2 exerts a significant antiviral effect, resulting in the reduction of virus transcripts in both local as well as distal plant tissues. This observed decrease in viral titre linked to a high degree of compaction of virus minichromosomes. Concurrently, these compacted virus minichromosomes exhibit an increase in accumulation of repressive histone mark H3K9me2, rendering them less virulent compared to wild-type virus minichromosomes.

Overexpression of NbHIRA and NbFAS2 significantly boosts antiviral activity against geminivirus infections by altering the structure of virus minichromosomes. Consequently, these findings propose that targeting histone chaperones such as NbHIRA and NbFAS2 could offer a promising strategy for managing geminivirus-related infections in crop plants and agricultural applications.