Introduction
Potassium (K+) is an essential element with various functions in many physiological processes, such as osmoregulation, photosynthesis, enzyme activation, membrane potential maintenance, and ion homeostasis (Clarkson & Hanson, 1980). These functions rely on high and relatively stable concentrations of K+ in cellular compartments and K+ movement between different compartments, cells, and tissues. Accordingly, K+ must by readily transported or regulated. K+ exists in plants as an ion and is highly mobile. In the soil, K+is taken up by plants through root absorption. The K+concentration differs among subcellular regions of plant cells; for example, the concentration of K+ in the cytoplasm is generally maintained at approximately 100 mM, whereas that in the vacuoles varies to facilitate the storage of K+ in the cell (Besford & Maw, 1975). Compared with the high concentration of K+ in cells, the concentration of K+in the soil is very low. Moreover, because the roots of the plant are in direct contact with the soil, the K+ deficiency signal is first perceived by root cells, particularly root epidermal cells and root hair cells (Song et al. 2017). Plants respond to low K+ concentrations by altering root growth and root configuration, such as inhibiting primary roots and stimulating root hair elongation (Cao et al. 1993). In plants, low K+signal transduction causes a downstream response, ultimately promoting the adaptation of the response to K+ deficiency in plants (Han et al. 2018). Notably, K+ deficiency can activate K+ uptake through plant roots by regulating the activity of K+ transporters (Gierth & Maser, 2007).
Some signalling molecules, including reactive oxygen species (ROS), Ca2+, plant hormones, and microRNAs (miRNAs) are involved in plant responses to low K+ stress (Wang & Wu, 2017). In Arabidopsis , K+ deficiency induces the production of ROS and the expression of the NADPH oxidase gene RHD2 and the peroxidation enzyme gene RCI3 (Shin & Schachtman, 2004). In addition to ROS, Ca2+ also acts as a low K+ response signal. Recently, the Ca2+ reporter YC3.6 was observed to be induced by low K+ (Behera et al. 2017). Additionally, previous studies have revealed that some phytohormones (ethylene, auxin, cytokinin [CTK], and jasmonic acid) are involved in signal transduction of plant responses to K+-deficiency stress. In Arabidopsis thaliana , low K+-induced ethylene signalling regulates K+ transporter 5 (AtHAK5 ) transcription and root growth through modulation of ROS signalling (Schachtman, 2015). Moreover, by regulating the localisation of auxin transporter AtPIN1, the K+ transporter AtTRH1/AtKUP4 plays important roles in the regulation of K+-dependent root architecture in Arabidopsis thaliana (Rigas et al. 2013; Dolan, 2013). Low-K+stress downregulated CTK levels, which stimulates ROS accumulation, root hair growth, and AtHAK5 expression (Nam et al. 2012). The expression of the K+ transporter gene OsCHX14, which is involved in K+ homeostasis in rice flower, is regulated by jasmonic acid signalling (Chen et al. 2016). Taken together, these phytohormone signalling pathways may constitute a regulatory network and synergistically control root architecture and K+transporter expression under low-K+ conditions.
Compared with miRNA studies of other nutrient (e.g., N, P, S, and Cu) deficiencies, few reports have evaluated K+ deficiency (Hu et al. 2015; Kulcheski et al. 2015). A recent study showed thatmiR-399 is involved in responses to multiple nutrient deficiencies in rice (Oryza sativa ) by regulating the absorption of multiple nutrient (Hu et al. 2015). In O. sativa ,miR-399 is also induced by low-K+ stress and represses its downstream target listerin E3 ubiquitin protein ligase 1/PHO2. In plants, after RNase III Dicer-like 1 cutting, the miRNA strand of the miRNA:miRNA* duplex is loaded into an Argonaute (AGO) protein, which has a single-stranded RNA-binding PAZ domain and an RNaseH-like PIWI domain to catalyses mRNA cleavage or translational repression (Reinhart et al. 2002; Tomari & Zamore, 2005). miRNAs are loaded into AGO1, which acts as an RNA slicer (Qi et al. 2005). Among the 10 plant AGO mRNA homologs, only AGO1 has extensive complementary to miR-168 or any other known miRNAs, suggesting that AGO1 may be the only member of the AGO family to be regulated by miRNAs (Vaucheret et al. 2004). In Arabidopsis , fine-tuned post-transcriptional regulation of miR-168 andAGO1 levels maintains the expression balance of other miRNAs, which, together with AGO1, control the mRNA expression levels of miRNA targets (Vaucheret et al. 2006).
In this study, we aimed to evaluate the balance between miR-168aand AGO1 expression in response to K+deficiency stress in Solanum lycopersicum . We also verified influence of SlmiR-168a -dependent SlAGO1 on plant growth under K+-deficiency stress and used miRNA-Seq and mRNA-Seq results to assess the regulatory mechanism ofSlmiR-168a -mediated SlAGO1 in the response to K+ deficiency.