Review
Small RNAs as big players in plant abiotic stress responses and nutrient deprivation

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Abiotic stress is one of the primary causes of crop losses worldwide. Much progress has been made in unraveling the complex stress response mechanisms, particularly in the identification of stress responsive protein-coding genes. In addition to protein coding genes, recently discovered microRNAs (miRNAs) and endogenous small interfering RNAs (siRNAs) have emerged as important players in plant stress responses. Initial clues suggesting that small RNAs are involved in plant stress responses stem from studies showing stress regulation of miRNAs and endogenous siRNAs, as well as from target predictions for some miRNAs. Subsequent studies have demonstrated an important functional role for these small RNAs in abiotic stress responses. This review focuses on recent advances, with emphasis on integration of small RNAs in stress regulatory networks.

Section snippets

Small RNAs are ubiquitous regulators of gene expression

Plant development, metabolism and stress responses, as well as a myriad of other functions, depend on the correct regulation of gene expression. This is achieved by multiple mechanisms, with perhaps the most important control being exerted at the level of transcription. However, post-transcriptional events also play a crucial role in regulating gene expression. The stability of mRNAs is regulated by a variety of signals acting on specific sequences within the RNAs. This regulation is often

Overview of the role of miRNAs in plant stress responses

Plants are sessile organisms that must endure stressful environments. A large proportion of plant genes are regulated by stresses such as drought, soil salinity and extreme temperatures 19, 20, 21, 22, 23. Of the many gene regulatory mechanisms such as transcriptional, post-transcriptional and post-translational regulation, transcriptional regulation is the most widely studied mechanism. The action of specific transcription factors that bind to conserved cis-acting promoter elements is well

miRNA and oxidative stress

Under normal conditions, plants maintain a delicate balance between reactive oxygen species (ROS) production and scavenging. Plants have developed a highly sophisticated and efficient antioxidant system 20, 22, 35, 36. However, exposure to stress conditions such as drought, cold, salinity, high light and heavy metals results in the accumulation of excess ROS in plant cells. Superoxide radicals (O2) are the primary products of photo-reduction of dioxygen in Photosystem I (PSI) of chloroplasts.

miRNAs and nutrient homeostasis

Inorganic phosphate (Pi) is frequently a limiting factor for plant growth [39]. Plants adapt to low Pi soils in several ways: root growth and architecture are altered to access a larger soil volume; organic acids, phosphatases and nucleases are exuded to solubilize Pi or release Pi from organic sources; the capacity for Pi uptake is increased; and internal Pi is recycled 40, 41, 42. Although some components of Pi starvation signaling in plants have been identified, the overall pathway is still

Other abiotic stress-regulated small RNAs and their target genes

High-through-put gene expression analysis of plants under abiotic stress indicated that several hundred genes have modulated expression 19, 20, 21, 22, 23. Some of these genes are up-regulated or induced, and others are down-regulated under stress conditions. The up- or down-regulation of genes appears to be dependent on their roles. MicroRNAs that are up-regulated by stress might down-regulate their target genes, which might be negative regulators of stress tolerance (e.g. repressors of

miRNAs and growth and development under abiotic stresses

Crop yield under abiotic stresses depends not only on the mere survival of plants under stress conditions but also on the phenological and developmental plasticity of plants. Under abiotic stress conditions, plants adjust the durations of phenological phases, and the rate of developmental processes, which modify biomass and harvest index. Changes in the duration of various phenological phases (e.g. vegetative phase, days to flowering and grain development duration) help plants to avoid critical

siRNAs and stress responses

Endogenous siRNAs are synthesized from long double-stranded RNAs (dsRNAs). The endogenous sources of dsRNAs are: (i) miRNA-directed cleavage products of non-coding transcripts, which are then converted into dsRNAs by RNA-dependent RNA polymerases (RDRs); (ii) dsRNAs formed from the mRNAs encoded by natural cis-antisense gene pairs [74]; and (iii) dsRNAs generated from heterochromatin and DNA repeats [8]. The siRNAs produced by miRNA-directed cleavage of mRNAs are referred to as trans-acting

Conclusions and outlook

Extensive efforts over the past two decades have identified thousands of stress-regulated genes. With the recent identification of miRNAs and siRNAs as components of stress response, another level of gene regulation has been revealed. The evidence thus far suggests important roles for these small RNAs in stress response. The extent of small RNA involvement in abiotic stress response should become clear in the next several years if sufficient effort can be directed to these studies in Arabidopsis

Acknowledgements

The work in R.S.'s laboratory is supported by the Oklahoma Agricultural Experiment Station and OCAST, and the work in J-K.Z.'s laboratory is supported by the National Institutes of Health grants R01GM0707501 and R01GM59138.

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