Regulation mechanism of SLs signaling-mediated tillering in S. alterniflora under different salinities
According to the above results, a schematic diagram was presented to describe the regulation mechanism of SLs signaling-mediated tillering process in S. alterniflora under different salinity treatments (Figure 7). Under the control treatment (0‰ salinity), the highly expressed SaD10 and SaD17 , which are the main genes to regulate the biosynthesis of SLs (Alder et al. , 2012), would result in more SLs production (Figure 7a). Nevertheless, the addition of salt reduced the expression of SaD10 and SaD17 and lead to lower SLs production (Figure 7b, c). SLs was subsequently perceived by hormone receptor D14 (Yao et al. , 2016). Thereafter, D14 interact with D3 and D53 proteins to form a Skp-Cullin-F-box (SCF) E3 ubiquitin ligase complex and trigger the ubiquitination and degradation of D53 (Jianget al. , 2013). As the repressor of strigolactone signaling pathway, the degradation of D53 would lead to less tiller (Jianget al. , 2013). On the contrary, the accumulation of D53 would propagate the signal to downstream and result in more tiller. The high expression level of SaD14 in 0‰ salinity treatment would activate the SLs signaling to degrade D53 protein and resulted in less tillers inS. alterniflora (Figure 7a). However, salt addition reduced the expression of SaD14 to block the SLs signaling pathway that inhibited the degradation of D53 protein. Moreover, the S. alterniflora grown in moderate salinity condition (15‰) facilitated the expression of SaD53 to further promote the accumulation of D53 protein and result in the activation of tillering process (Figure 7b). Whereas, high salinity (30‰) reduced SaD53 expression in S. alterniflora basal nodes and led to less D53 protein accumulation, which would cause a reduction of tiller outgrowth (Figure 7c). Taken together, our data suggested that moderate salinity promote the tillering process by inhibiting the SLs biosynthesis and perception but enhancing the accumulation of D53 in S. alterniflora seedlings.
As mentioned above, the asexual propagation by vegetative tillering and the high tolerance to salt stress in S. alterniflora were the main reasons for its fast invasion (Daehler et al ., 1994; Proffitt et al ., 2003; Taylor et al ., 2004; Tang et al ., 2014). And SLs was the key phytohormone to regulate the outgrowth of tiller (Gomez-Roldan et al. , 2008; Mikihisa et al. , 2008; Umehara et al ., 2008). Based on these findings and our analysis, we suggested that the character of more tillers regulated by SLs signaling pathway under moderate salinity condition should be one of the reasons for the rapid spread of S. alterniflora . In addition, the application of artificial SLs, such as GR24, could be a considerable way to control the invasion of S. alterniflora along the coast of China.