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Small RNAs (sRNAs) are a class of non-coding RNAs that consist of 21–24 nucleotides. They have been extensively investigated as critical regulators in a variety of biological processes in plants. sRNAs include two major classes: microRNAs (miRNAs) and small interfering RNAs (siRNAs), which differ in their biogenesis and functional pathways. Due to global warming, high-temperature stress has become one of the primary causes for crop loss worldwide. Recent studies have shown that sRNAs are involved in heat stress responses in plants and play essential roles in high-temperature acclimation. Genome-wide studies for heat-responsive sRNAs have been conducted in many plant species using high-throughput sequencing. The roles for these sRNAs in heat stress response were also unraveled subsequently in model plants and crops. Exploring how sRNAs regulate gene expression and their regulatory mechanisms will broaden our understanding of sRNAs in thermal stress responses of plant. Here, we highlight the roles of currently known miRNAs and siRNAs in heat stress responses and acclimation of plants. We also discuss the regulatory mechanisms of sRNAs and their targets that are responsive to heat stress, which will provide powerful molecular biological resources for engineering crops with improved thermotolerance.

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MicroRNAs (miRNAs) play crucial roles in plant development and stress responses, and a growing number of studies suggest that miRNAs are promising targets for crop improvement because they participate in the regulation of diverse, important agronomic traits. MicroRNA398 (miR398) is a conserved miRNA in plants and has been shown to control multiple stress responses and plant growth in a variety of species. There are many studies on the stress response and developmental regulation of miR398. To systematically understand its function, it is necessary to summarize the evolution and functional roles of miR398 and its target genes. In this review, we analyze the evolution of miR398 in plants and outline its involvement in abiotic and biotic stress responses, in growth and development and in model and non-model plants. We summarize recent functional analyses, highlighting the role of miR398 as a master regulator that coordinates growth and diverse responses to environmental factors. We also discuss the potential for fine-tuning miR398 to achieve the goal of simultaneously improving plant growth and stress tolerance.