Significance: Recently N ⁶ -methyladenosine (m ⁶ A) methylation has emerged as a biological process with significant impact on cellular functions. However, almost all the research regarding m ⁶ A methylation has been based on mRNAs. In our research, we focus on how m ⁶ A methylation affects microRNA (miRNA) biogenesis in Arabidopsis . In brief, we show that m ⁶ A methylation is necessary to maintain proper levels of mature miRNAs as well as their precursors. m ⁶ A mark affects pri-miRNA secondary structures and affects the recruitment of the Microprocessor to pri-miRNAs. We also demonstrate the interactions of MTA (m ⁶ A writer) with other proteins involved in miRNA biogenesis, namely RNA Polymerase II and TOUGH. Our study provides evidence of the role played by m ⁶ A in plant miRNA biogenesis.

SERRATE/ARS2 is a conserved RNA effector protein involved in transcription, processing and export of different types of RNAs. In Arabidopsis, the best-studied function of SERRATE (SE) is to promote miRNA processing. Here, we report that SE interacts with the nuclear exosome targeting (NEXT) complex, comprising the RNA helicase HEN2, the RNA binding protein RBM7 and one of the two zinc-knuckle proteins ZCCHC8A/ZCCHC8B. The identification of common targets of SE and HEN2 by RNA-seq supports the idea that SE cooperates with NEXT for RNA surveillance by the nuclear exosome. Among the RNA targets accumulating in absence of SE or NEXT are miRNA precursors. Loss of NEXT components results in the accumulation of pri-miRNAs without affecting levels of miRNAs, indicating that NEXT is, unlike SE, not required for miRNA processing. As compared to se-2, se-2 hen2-2 double mutants showed increased accumulation of pri-miRNAs, but partially restored levels of mature miRNAs and attenuated developmental defects. We propose that the slow degradation of pri-miRNAs caused by loss of HEN2 compensates for the poor miRNA processing efficiency in se-2 mutants, and that SE regulates miRNA biogenesis through its double contribution in promoting miRNA processing but also pri-miRNA degradation through the recruitment of the NEXT complex.

N⁶-methyladenosine (m⁶A) is known to occur in plant and animal messenger RNAs (mRNAs) since the 1970s. However, the scope and function of this modification remained un-explored till very recently. Since the beginning of this decade, owing to major technological breakthroughs, the interest in m⁶A has peaked again. Similar to animal mRNAs, plant mRNAs are also m⁶A methylated, within a specific sequence motif which is conserved across these kingdoms. m⁶A has been found to be pivotal for plant development and necessary for processes ranging from seed germination to floral development. A wide range of proteins involved in methylation of adenosine have been identified alongside proteins that remove or identify m⁶A. This review aims to put together the current knowledge regarding m⁶A in Arabidopsis thaliana.

Argonaute1 activity is not limited to the cytoplasm and has been found to be associated with the regulation of gene expression in the nucleus and to be tightly associated with chromatin and transcription.

Summary: Among many mRNA modifications, adenine methylation at the N⁶ position (N⁶‐methyladenosine, m⁶A) is known to affect mRNA biology extensively. The influence of m⁶A has yet to be assessed under drought, one of the most impactful abiotic stresses. We show that Arabidopsis thaliana (L.) Heynh. (Arabidopsis) plants lacking mRNA ADENOSINE METHYLASE (MTA) are drought‐sensitive. Subsequently, we comprehensively assess the impacts of MTA‐dependent m⁶A changes during drought on mRNA abundance, stability, and translation in Arabidopsis. During drought, there is a global trend toward hypermethylation of many protein‐coding transcripts that does not occur in mta. We also observe complex regulation of m⁶A at a transcript‐specific level, possibly reflecting compensation by other m⁶A components. Importantly, a subset of transcripts that are hypermethylated in an MTA‐dependent manner exhibited reduced turnover and translation in mta, compared with wild‐type (WT) plants, during drought. Additionally, MTA impacts transcript stability and translation independently of m⁶A. We also correlate drought‐associated deposition of m⁶A with increased translation of modulators of drought response, such as RD29A, COR47, COR413, ALDH2B, ERD7, and ABF4 in WT, which is impaired in mta. m⁶A is dynamic during drought and, alongside MTA, promotes tolerance by regulating drought‐responsive changes in transcript turnover and translation.

Since their discovery, microRNAs have led to a huge shift in our understanding of the regulation of key biological processes. The discovery of epigenetic modifications that affect microRNA expression has added another layer of complexity to the already tightly controlled regulatory machinery. The presence of N6-methyl-adenosine (m6A) mark and its critical importance in miRNA biogenesis in animals adds to our understanding of the regulatory mechanisms.