Introduction
Spartina alterniflora , a gramineous plant native to the salt marsh of North America, is rapidly spreading along the southeast coast of China since introduced in 1979 for the purpose of intertidal mudflat control (Anet al. , 2007). Nowadays, S. alterniflora had caused a serious threat to the native mangroves and salt marshes especially at the intertidal area with moderate salinity (13-26‰) (Zhang et al. , 2012). Previous studies indicated that the asexual propagation by vegetative tillering was proposed as the main reason for the fast expansion of S. alterniflora (Daehler et al ., 1994; Proffitt et al ., 2003; Taylor et al ., 2004). Since the seeds were germinated in a suitable habitat, a tussock would be quickly formed by the vegetative tillering and rhizoming and these tussocks finally merged into dense meadows, which lay a firm foundation for further invasion (Davis et al ., 2004). In addition, salinity in soil was an important environmental factor to affect the tillering inS. alterniflora (Wang et al ., 2006; Liu et al ., 2016). Under a proper salinity conditions (10‰-20‰), S. alterniflora grew much better and produced more tillers (Wang et al ., 2006, Xiao et al ., 2011; Tang et al ., 2014). Moreover, the high tolerance to salt was proposed to be another reason of fast invasion of S. alterniflora (Tang et al ., 2014). The ability of salt tolerance of S. alterniflora was investigated by transcriptome sequencing, suggesting that some genes participated in various biological processes, such as ion homeostasis, carbohydrate metabolism, protein modification, antioxidant system, stress protein biosynthesis and transcriptional regulation play an important role inS. alterniflora against high salinity condition (Bedre et al ., 2016; Ye et al ., 2020). However, those results cannot explain why S. alterniflora produced more tiller under a proper salinity conditions (10‰-20‰) which may help to understand the fast invasion of S. alterniflora into new intertidal habitats.
Tillering is one of the most important agronomic characteristics that not only determine the yield of crops but also affect the capacity of interspecific competition (Moral & Moral, 1995; Ruan et al. , 2008; Mennan et al. , 2012; Huang et al. , 2020). The outgrowth of tiller/branch from the basal node of shoot is largely contributed to the fast expansion of population into a new habitat, which is more likely controlled by the phytohormone, such as auxin, zeatin, abscisic acid and so on (Liu et al ., 2011; Cai et al. , 2018). Moreover, various environmental factors, such as salt, drought, and high/low temperature, could also affect the development and survival of tillers (Xu & Huang, 2001; Li et al. , 2010; Xu et al. , 2020). Soil salinization as a common environmental factor usually cause a negative effect on plant branching/tillering, especially in glycophyte (Ruan et al. , 2008; Zhao et al. , 2009). Even in some salt-tolerance crop species, the salinity (below 200 mM NaCl/11.69‰ salinity) treatment also inhibit the tillering (Ruan et al. , 2008). However, such salinity is coincidently the optimal salinity for the growth of many halophyte, such as S. alterniflora (Wang et al. , 2006; Xiao et al. , 2011). Therefore, a better understanding of tillers outgrowth and development under salt treatment in halophyte is required.
As a new plant hormone, strigolactones (SLs) was initially identified as germination stimulants in Striga and Orobanche (Cooket al ., 1996; Nomura et al ., 2013). Recently, SLs have been proved to negatively control the tillering/branching formation in plants (Gomez-Roldan et al. , 2008; Mikihisa et al. , 2008; Umehara et al ., 2008). In plant kingdom, many kinds of natural SLs were found, and 5-deoxystrigl (5-DS) was proposed to be the common precursor of other SLs (Xie et al ., 2010). Most of the SLs are synthesized by the carotenoid-derived pathway in plant roots and transported to the shoot for regulating the secondary growth (Dunet al. , 2009; Kohlen et al . 2011; Seto et al . 2012). Genes participated in SLs biosynthesis and signaling have been identified and characterized. The geneRMS5 /D17 /MAX3encoding carotenoid cleavage dioxygenase 7 (CCD7) catalyzes an oxidative cleavage in 9-cis -β-carotene to produce 9-cis -β-apo-10’-carotenal. Subsequently, 9-cis -β-apo-10’-carotenal is cleaved by carotenoid cleavage dioxygenase 8 (CCD8), which encoded byRMS1 /D10 /MAX4 , to produce the SLs precursor carlactone (CL) (Alder et al. , 2012). The defective mutants of CCD7 and CCD8 exhibited an excessive tiller or branch production in plants (Auldridge et al. , 2006; Zou et al. , 2006). Several studies demonstrated that SLs production and genes related to SLs biosynthesis were significantly reduced under abiotic stresses, such as drought and osmotic stress (Liu et al. , 2015; Zhuang et al. , 2017; Xu et al. , 2020). After biosynthesis in cell, the phytohormone would be transported to various tissues and recognized by the specific receptor for different biological functions.D14 /MAX2 , which encode a α/β hydrolase, was recently identified as the SLs receptor to function in SLs perception and signaling in Arabidopsis (Yao et al. , 2016). The mutant lack of D14 exhibited a low sensitive to SLs and more tillers (Arite et al. , 2009). Under osmotic stress, the tillers number ofFestuca arundinacea seedling was dramatic decreased with a high expression of FaD14 (Zhuang et al. , 2017). In addition, D14 protein interacts with F-box protein D3 to form a ubiquitination complex and catalyze the degradation of D53 protein, which belong to P-loop nucleoside triphosphate hydrolase superfamily (Jiang et al. , 2013). As a key repressor in SLs signaling pathway, D53 directly binds to a transcriptional factor named Ideal Plant Architecture 1 (IPA1) and suppresses the activity of IPA1 to regulate the tiller development in rice (Songet al. , 2017). Despite the knowledge of SLs-mediated tillering in glycophyte, little is known about how salt affect the SLs signaling-mediated tillering in halophyte.
In the present study, the regulation mechanism of SLs signaling-mediated tillering process in S. alterniflora under different salinity conditions were investigated. The seedlings of S. alterniflorawere firstly transplanted to the field to evaluate the effect of salinity on S. alterniflora tillering. Moreover, the seedlings ofS. alterniflora grew in greenhouse were subjected to different salinities to further explore the regulation mechanism of salinity mediated tillering process. We used the HPLC-MS/MS to assess the variation of SLs content under different salinity conditions. Afterwards, the genes involved in SLs biosynthesis (D10 ,D17 ) and signaling (D14 , D53 ) were cloned and quantified. Finally, the regulation mechanism of SLs signaling-mediated tillering process in S. alterniflora under different salinity treatments was discussed. The conclusions will help us to understand the mechanism of fast invasion of S. alterniflora into new intertidal salt habitats.