Introduction
Hypocotyl growth including elongation and thickening is important for the emergence of tomato seedlings from soil and resistance to lodging. In the dark, seedlings undergo skotomorphogenic development including closed cotyledons, formation of an apical hook and elongated hypocotyls. In the light, photomorphogenic development results in cotyledon expansion, leaf development, photosynthesis initiation and reduced hypocotyl elongation. Many mutants of light signaling components exhibit defective hypocotyl growth. Loss-of-function mutant of phyB(Phytochrome B ), the red-light receptor, has elongated hypocotyls (de Lucas et al., 2008; Kim et al., 2016). Phytochrome interacting factors (PIFs ) are transcription factors downstream of light signaling to positively regulate hypocotyl elongation, and monogenicPIF mutants have shorter hypocotyls than wild-type plants under light condition (Huq & Quail, 2002; Kim et al., 2003; Zhong et al., 2012). Ambient temperature also regulates hypocotyl elongation. High temperature promotes hypocotyl elongation through the increase of free IAA in Arabidopsis seedlings (Gray, Ostin, Sandberg, Romano, & Estelle, 1998). PIF4 promotes IAA biosynthesis, and is also required for high temperature induced hypocotyl elongation (Franklin et al., 2011; Koini et al., 2009; Stavang et al., 2009). PIF4 specifically binds to the biologically active Pfr form of phyB, which results in the degradation of PIF4 and prevents its transcription activity in the hypocotyl elongation (Huq & Quail, 2002; Park et al., 2012). PhyB-PIF signaling module also regulates red light-induced hypocotyl elongation under the warm temperature (Johansson et al., 2014). Blue light inhibits Arabidopsis hypocotyl elongation requiring the function ofcryptochrome 1 (CRY1 ) and cryptochrome 2(CRY2 ) (Ahmad & Cashmore, 1993). Under blue light, CRY1 directly interacts with PIF4 to inhibit its transcription activity and represses the hypocotyl elongation induced by high temperature (Ma et al., 2016).
Plant hormones have diverse and strong impacts on hypocotyl growth. Overproduction of auxin in plants promotes hypocotyl elongation (Boerjan et al., 1995; Romano, Robson, Smith, Estelle, & Klee, 1995; Zhao et al., 2001). Chemical screens identify a new auxin analog named pro-2,4-D which significantly promotes hypocotyl elongation (Savaldi-Goldstein et al., 2008). Exogenous application of gibberellins promotes Arabidopsis hypocotyl elongation under light via regulation of cellular elongation (Cowling & Harberd, 1999). Ethylene prevents the hypocotyl elongation in the dark, while promotes hypocotyl elongation in the light (Bleecker, Estelle, Somerville, & Kende, 1988; Smalle, Haegman, Kurepa, Van Montagu, & Straeten, 1997). Brassinosteroid (BR) stimulates hypocotyl elongation of pakchoi (Brassica chinensis cv Lei-Choi) (T. W. Wang, Cosgrove, & Arteca, 1993) and BR signaling is required for GA-induced hypocotyl elongation (Bai et al., 2012). Appropriate endogenous level of Abscisic acid (ABA) is essential for tomato hypocotyl elongation in the dark, which promotes the endoreduplication in hypocotyl cells and inhibits cytokinin biosynthesis (Humplik et al., 2015). Cytokinins inhibit hypocotyl elongation of Arabidopsis seedlings in the dark, while induce hypocotyl elongation in light-grown Arabidopsis plants via suppression of ethylene action or auxin transport (Smets, Le, Prinsen, Verbelen, & Van Onckelen, 2005).
Hypocotyl growth is species- and growth condition-dependent, resulting from cell division and expansion (Gendreau et al., 1997; Raz & Koornneef, 2001; H. Wang & Shang, 2020). In Arabidopsis, hypocotyl growth is exclusively attributed to cell expansion (Boron & Vissenberg, 2014), while both cell division and expansion exist in hypocotyls ofHelianthus annuus (Kutschera & Niklas, 2013). Accurate cell cycle progression including mitotic cycle and endoreduplication results in proper cell division and expansion during plant organ growth and development (De Veylder, Beeckman, & Inze, 2007; Sugimoto-Shirasu & Roberts, 2003). Cell cycle consists of four major phases, G1, S, G2 and M, and the progression is governed by the activity of cyclin-dependent kinases (CDKs)/cyclin protein complex, in which CDK is the catalytic subunit and cyclin is the regulatory subunit (Inze & De Veylder, 2006). CDK inhibitors (CKIs) or Kip-related proteins (KRPs) directly bind to the CDK/cyclin complexes to inhibit CDK activity (De Veylder et al., 2001; Lui et al., 2000; Sherr & Roberts, 1999; H. Wang, Fowke, & Crosby, 1997). SIAMESE(SIM)/SIAMESE RELATED (SMR) family proteins are another type of inhibitors of CDK activity controlling endocycle onset in Arabidopsis (Churchman et al., 2006; Kasili et al., 2010; Walker, Oppenheimer, Concienne, & Larkin, 2000). Cell cycle from G2 to M phase requires E3 ubiquitin ligase complexes Anaphase-promoting complex/cyclosome (APC/C) to degrade cyclins and inhibit CDK activity (Fulop et al., 2005; Marrocco, Bergdoll, Achard, Criqui, & Genschik, 2010; Peters, 2006; Sullivan & Morgan, 2007). APC/C is a large protein complex containing at least 11 different subunits referred as APC1 to APC11, in which APC2 and APC11 are core components with catalytic activities. APC/C has two types of activators, cell division cycle 20 (CDC20 ) and CELL CYCLE SWITCH52 (CCS52 ) (Fulop et al., 2005). Five CDC20 homologs (CDC20.1 -CDC20.5 ) and three CCS52 homologs (CCS52A1 , CCS52A2 and CCS52B ) were identified in Arabidopsis genome. ULTRAVIOLET-B-INSENSITIVE4 (UVI4 ) andOMISSION OF SECOND DIVISION1 (OSD1 ) are two negative regulators of APC/C, which have functional redundancy in the regulation of female gametophyte development (Bao & Hua, 2014; Heyman et al., 2011; Iwata et al., 2011). Either UVI4 or OSD1overexpression leads to reduced plant size, lateral shoots and enhanced disease resistance (Bao, Yang, & Hua, 2013; Iwata et al., 2011).
Studies on APC/C activities in tomato hypocotyl growth were seldom reported. Here we accessed roles of the negative APC/C regulatorSlUVI4 and the positive regulator SlCCS52B in hypocotyl elongation via the modulation of cell cycle progression. High expression levels of both genes correlated with the robust hypocotyl elongation when seedlings grow under different lights or on the growth medium supplemented with MS salts and sugar. Salt, heat and plant hormones auxin and ethylene treatments affect their transcription distinctly to modulate hypocotyl elongation via the perturbation of cell cycle progression. Genetic studies revealed that SlCCS52Boverexpression Arabidopsis plants and SlUVI4 deletion mutants had defective hypocotyl elongation with enhanced endoreduplication. Together, our study establishes cellular and genetic connections between APC/C regulators, cell cycle progression and tomato hypocotyl elongation.