Regulation of SlAGO1 by SlmiR-168a mediated root development in response to K+ deficiency stress
K+ deficiency in soil is of great agricultural importance (Laegreid et al. 1999). One important aspect of plant adaptation to K+ deficiency stress is cellular and tissue homeostasis of K+, which involves transport of K+ across various membranes in several tissues (Amtmann et al. 2005). The two tomato genotypes (low K+ tolerant JZ34 and low K+sensitive JZ18) exhibit marked differences in sensitivity to K+ deficiency and root morphology (Zhao et al. 2018). Moreover, JZ34 has more root hairs under K+ deficiency treatment than JZ18 and exhibits stronger nutritional uptake capability of K+ than JZ18 (Zhao et al. 2018). Thus, JZ34 maintains higher K+ contents under K+deficiency stress than JZ18. In our current study, the expression ofSlmiR-168a was increased in response to low K+, whereas the expression of its target SlAGO1 was decreased following low K+ treatment (Fig. 1). BothSlmiR-168a and SlAGO1 were expressed at higher levels in roots than in other tissues (Fig. 2). Additionally,35S:SlmiR-168a had more root hairs than 35S:rSlAGO1 and JZ18 (Fig .4a). Notably, the potassium deficiency signal is first perceived by root cells, particularly root epidermal cells and root hair cells (Song et al. 2017).
Integrated analysis of mRNA-Seq and miRNA-Seq results in35S:rSlAGO1 showed that a member of the miR-171 family was significantly induced and that its target Solyc08g069180.3.1 was downregulated (Table 1). Further analysis showed that this target gene was involved in root epidermal cell differentiation and stress responses. Previous studies have shown that miR-171 expression is higher in the vascular bundle and cuticle layer of roots inArabidopsis (Mahale et al. 2014) and that this miRNA is upregulated in response to Cd stress, drought, and salt stress (Liu et al. 2008; Zhou et al. 2008). miR-171 has also been shown to be differentially expressed in maize roots in response to salt stress (Ding et al. 2009). Therefore, we concluded that regulation ofSolyc08g069180.3.1 by miR-171a may explain differences in root development between 35S:SlmiR-168a and 35S:rSlAGO1under K+ deficiency stress (Fig. 9).