SlmiR-168a regulated SlAGO1-mediated CTK/ABA signalling in response to K+ deficiency stress
In addition to root architecture, phytohormones also are involved in signal transduction of plant responses to K+deficiency stress. Low K+ stress results in decreased CTK levels, which may stimulate ROS accumulation, root hair growth, andAtHAK5 expression (Nam et al. 2012). The KAT1 potassium channel is a target for ABA signal transduction through SRK1/OST1/SnRK2.6 (Sato et al. 2009).
Additionally, expression of the K+ release channel gene GORK is induced by ABA in the presence of extracellular Ca2+ (Becker et al. 2003). In this study, we found that the low K+ tolerant tomato JZ34 had higher CTK/ABA contents under K+ deficiency stress than the low K+ sensitive tomato JZ18.
Integrated analysis of mRNA-Seq and miRNA-Seq results for the comparison of 35S:rSlAGO1 versus JZ18 showed that miR-384 , miR-530 , and miR-858 were upregulated and that their downregulated targets were enriched in the CTK signalling pathway and CTK responses. Moreover, we found that targets of miR-384 , miR-530 , andmiR-858 were also involved in plant hormone signal transduction. Additionally, target genes of the novel miRNAPC-3p-276756_24 were found to be involved in CTK responses. Interestingly, in 35S:SlmiR-168a , only miR-8006 and miR-8007b were downregulated, and their upregulated targets were enriched in response to salt stress and ABA. Accordingly, our results showed that SlAGO1 induced the expression of various miRNAs, including miR-384 , miR-530 ,miR-858 , miR-8007 , and PC-3p-276756_24 , through regulation of SlmiR-168a . These miRNA/mRNA pairs may influence tolerance to K+ deficiency stress in plants via the CTK/ABA signalling pathway (Fig. 9). CTK- and ABA-related genes that were downregulated in 35S:rSlAGO1 and upregulated in35S:SlmiR-168a are listed in Table S12.
K+ transport via ABA signalling requires extracellular Ca2+ (Becker et al. 2003), and P68 protein combines with AGO1 to interact with CaM and enhance accumulation of K+ in rice (Banu et al. 2015). Thus, P68 expression was investigated in 35S:SlmiR-168a and 35S:rSlAGO1 (Fig. S4). P68 expression levels were decreased in 35S:rSlAGO1 but increased in 35S:SlmiR-168a compared with those in JZ18. Based on these findings, the pathway through which SlAGO1 was regulated bySlmiR-168a in response to K+ deficiency stress via ABA signalling may require Ca2+.