Soil salinity is one of the primary causes of yield decline in rice. Pokkali (Pok) is a highly salt-tolerant landrace, whereas IR29 is a salt-sensitive but widely cultivated genotype. Comparative analysis of these genotypes may offer a better understanding of the salinity tolerance mechanisms in rice. Although most stress-responsive genes are regulated at the transcriptional level, in many cases, changes at the transcriptional level are not always accompanied with the changes in protein abundance, which suggests that the transcriptome needs to be studied in conjunction with the proteome to link the phenotype of stress tolerance or sensitivity. Published reports have largely underscored the importance of transcriptional regulation during salt stress in these genotypes, but the regulation at the translational level has been rarely studied. Using RNA-Seq, we simultaneously analyzed the transcriptome and translatome from control and salt-exposed Pok and IR29 seedlings to unravel molecular insights into gene regulatory mechanisms that differ between these genotypes. Clear differences were evident at both transcriptional and translational levels between the two genotypes even under the control condition. In response to salt stress, 57 differentially expressed genes (DEGs) were commonly upregulated at both transcriptional and translational levels in both genotypes; the overall number of up/downregulated DEGs in IR29 was comparable at both transcriptional and translational levels, whereas in Pok, the number of upregulated DEGs was considerably higher at the translational level (544 DEGs) than at the transcriptional level (219 DEGs); in contrast, the number of downregulated DEGs (58) was significantly less at the translational level than at the transcriptional level (397 DEGs). These results imply that Pok stabilizes mRNAs and also efficiently loads mRNAs onto polysomes for translation during salt stress. Under salt stress, Pok is more efficient in maintaining cell wall integrity, detoxifying reactive oxygen species (ROS), translocating molecules and maintaining photosynthesis. The present study confirmed the known salt stress-associated genes and also identified a number of putative new salt-responsive genes. Most importantly, the study revealed that the translational regulation under salinity plays an important role in salt-tolerant Pok, but such regulation was less evident in the salt-sensitive IR29.

Recently, post-transcriptional regulation of mRNA mediated by N⁶-methyladenosine (m⁶A) has been found to have profound effects on transcriptome regulation during plant responses to various abiotic stresses. However, whether this RNA modification can affect an oxidative stress response in plants has not been studied. To assess the role of m⁶A modifications during copper-induced oxidative stress responses, m⁶A-IP-seq was performed in Arabidopsis seedlings exposed to high levels of copper sulfate. This analysis revealed large-scale shifts in this modification on the transcripts most relevant for oxidative stress. This altered epitranscriptomic mark is known to influence transcript abundance and translation; therefore we scrutinized these possibilities. We found an increased abundance of copper-enriched m⁶A-containing transcripts. Similarly, we also found increased ribosome occupancy of copper-enriched m⁶A-containing transcripts, specifically those encoding proteins involved with stress responses relevant to oxidative stressors. Furthermore, the significance of the m⁶A epitranscriptome on plant oxidative stress tolerance was uncovered by assessing germination and seedling development of the mta (N⁶-methyladenosine RNA methyltransferase A mutant complemented with ABI3:MTA) mutant exposed to high copper treatment. These analyses suggested hypersensitivity of the mta mutant compared to the wild-type plants in response to copper-induced oxidative stress. Overall, our findings suggest an important role for m⁶A in the oxidative stress response of Arabidopsis.

High temperature impairs starch biosynthesis in developing rice grains and thereby increases chalkiness, affecting the grain quality. Genome encoded microRNAs (miRNAs) fine-tune target transcript abundances in a spatio-temporal specific manner, and this mode of gene regulation is critical for a myriad of developmental processes as well as stress responses. However, the role of miRNAs in maintaining rice grain quality/chalkiness during high daytime temperature (HDT) stress is relatively unknown. To uncover the role of miRNAs in this process, we used five contrasting rice genotypes (low chalky lines Cyp, Ben, and KB and high chalky lines LaGrue and NB) and compared the miRNA profiles in the R6 stage caryopsis samples from plants subjected to prolonged HDT (from the onset of fertilization through R6 stage of caryopsis development). Our small RNA analysis has identified approximately 744 miRNAs that can be grouped into 291 families. Of these, 186 miRNAs belonging to 103 families are differentially regulated under HDT. Only two miRNAs, Osa-miR444f and Osa-miR1866-5p, were upregulated in all genotypes, implying that the regulations greatly varied between the genotypes. Furthermore, not even a single miRNA was commonly up/down regulated specifically in the three tolerant genotypes. However, three miRNAs (Osa-miR1866-3p, Osa-miR5150-3p and canH-miR9774a,b-3p) were commonly upregulated and onemiRNA (Osa-miR393b-5p) was commonly downregulated specifically in the sensitive genotypes (LaGrue and NB). These observations suggest that few similarities exist within the low chalky or high chalky genotypes, possibly due to high genetic variation. Among the five genotypes used, Cypress and LaGrue are genetically closely related, but exhibit contrasting chalkiness under HDT, and thus, a comparison between them is most relevant. This comparison revealed a general tendency for Cypress to display miRNA regulations that could decrease chalkiness under HDT compared with LaGrue. This study suggests that miRNAs could play an important role in maintaining grain quality in HDT-stressed rice.