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.