In nature, plants often encounter chronic or recurring stressful conditions. An increasing number of observations suggest that plants can remember a past exposure to stress in order to be better prepared for a future stress incident. My lab studies heat stress memory in plants as a model case for environmental stress memory. Seedlings acquire thermotolerance through a heat treatment at sublethal temperatures (priming heat stress (HS)) that enables them to survive an otherwise lethal HS. This thermotolerance is actively maintained for several days as indicated by the existence of mutants which are able to establish thermotolerance, but fail to maintain it.
We have been exploring the molecular basis of HS memory in the model plant Arabidopsis thaliana. We have found that HS induces sustained histone methylation at HS memory-related loci that outlasts the transcriptional activity of these loci and hence marks them as recently transcriptionally active. At certain loci, this is correlated with hyper-induction of gene expression upon a recurring HS, - a signature readout of transcriptional memory. In a forward genetics approach, we have found that regulated nucleosome occupancy is also required for sustained activation of memory gene expression. Sustained low nucleosome occupancy is mediated by the highly conserved FORGETTER1 protein through direct interaction with chromatin remodeling proteins. FORGETTER1 is the orthologue of metazoan strawberry notch. I will present latest our finding on the role of chromatin structure in stress memory.
In summary, the physiologically defined phenomenon of HS memory has a molecular equivalent in the transcriptional memory and changes in chromatin structure at the level of histone modifications and nucleosome occupancy are associated with this. In the long term, our research will provide new approaches to improve stress tolerance in crops.