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.