Transcriptional memory and response to adverse temperatures in plants

Temperature is one of the major environmental signals controlling plant development,geographical distribution,and seasonal behavior.Plants perceive adverse temperatures,such as high,low,and freezing temperatures,as stressful signals that can cause physiological defects and even death.As sessile organisms,plants have evolved sophisticated mechanisms to adapt to recurring stressful environments through changing gene expression or transcriptional reprogramming.Transcriptional memory refers to the ability of primed plants to remember previously experienced stress and acquire enhanced tolerance to similar or different stresses.Epigenetic modifications mediate transcriptional memory and play a key role in adapting to adverse temperatures.Understanding the mechanisms of the formation,maintenance,and resetting of stress-induced transcriptional memory will not only enable us to understand why there is a trade-off between plant defense and growth,but also provide a theoretical basis for generating stress-tolerant crops optimized for future climate change.In this review,we summarize recent advances in dissecting the mechanisms of plant transcriptional memory in response to adverse temperatures,based mainly on studies of the model plant Arabidopsis thaliana.We also discuss remaining questions that are important for further understanding the mechanisms of transcriptional memory during the adverse temperature response.

H3K27me3 and H3K4me3 Chromatin Environment at Super-Induced Dehydration Stress Memory Genes of Arabidopsis thaliana

Pre-exposure to a stress may alter the plant＇s cellular, biochemical, and/or transcriptional responses during future encounters as a ＂memory＇ from the previous stress. Genes increasing transcription in response to a first dehydra- tion stress, but producing much higher transcript levels in a subsequent stress, represent the super-induced ＇transcription memory＇ genes in Arabidopsis thaliana. The chromatin environment （histone H3 tri-methylations of Lys 4 and Lys 27, H3K4me3, and H3K27me3） studied at five dehydration stress memory genes revealed existence of distinct memory- response subclasses that responded differently to CLF deficiency and displayed different transcriptional activities dur- ing the watered recovery periods. Among the most important findings is the novel aspect of the H3K27me3 function observed at specific dehydration stress memory genes. In contrast to its well-known role as a chromatin repressive mechanism at developmentally regulated genes, H3K27me3 did not prevent transcription from the dehydration stress- responding genes. The high H3K27me3 levels present during transcriptionally inactive states did not interfere with the transition to active transcription and with H3K4me3 accumulation. H3K4me3 and H3K27me3 marks function indepen- dently and are not mutually exclusive at the dehydration stress-responding memory genes.