Hypocotyl development is a crucial phase
Hypocotyl development is a crucial phase of early vegetative growth that influences seedling vigor contributing to fitness and productivity of a plant system (Stinchcombe and Schmitt, 2006; Luo et al., 2017). The development of a hypocotyl is characterized by an initial slow growth phase followed by a rapid progressive elongation phase originating from hypocotyl IY-5511 australia (Refrégier et al., 2004; Boron and Vissenberg, 2014). It is now well-established that expansion of hypocotyl cells requires remodeling of the cell wall synthesized during the initial slow growth phase (Derbyshire et al., 2007; Pelletier et al., 2010). Expression studies revealed transcripts of multiple β-galactosidase isoforms in seedlings of flax (Linum usitatissimum L.), rice (Oryza sativa L.) and hemp (Cannabis sativa L.) (Chantarangsee et al., 2007; Roach et al., 2011; Behr et al., 2016), and involvement of β-galactosidase in vascular differentiation, epicotyl elongation and secondary cell wall development is well-documented (Roach et al., 2011; Martín et al., 2013). However, the role of β-galactosidases in hypocotyl development remains poorly explored.
Since plant β-galactosidases have a large number of functionally diverse isoforms, detection of the full spectrum of β-galactosidases expressing in the hypocotyl is a crucial step to understand the enzyme's role in hypocotyl development. Transcriptome profiling by RNA-seq provides a unique opportunity to capture complete array of different isoforms of a gene expressed at a given condition (Wang et al., 2009; Hrdlickova et al., 2017). To date, hypocotyl transcriptome has been characterized to study light and hormonal regulation (Pelletier et al., 2010; Kohnen et al., 2016), vascular differentiation (Behr et al., 2016) and genetic analysis of hypocotyl elongation (Luo et al., 2017). Although these studies did not focus on β-galactosidase, Behr et al. (2016) observed temporal expression of β-galactosidases in hemp hypocotyl. On the contrary, no β-galactosidase was reported to be differentially expressed in developing rapeseed hypocotyl (Luo et al., 2017). Presently, it appears to be unknown how many β-galactosidases are actually expressed during hypocotyl development. Among annual plants, Corchorus capsularis L. (Malvaceae), commonly known as white jute is one of the fastest growing species that exhibits rapid exponential early vegetative growth, with a relative growth rate (RGR) of 140 g kg−1day−1 up to a period of 40–50 days (Johansen et al., 1985). We previously observed that C. capsularis hypocotyl can attain a length of 39.7 mm on 7th day after germination (DAG) (Sharma et al., 2012). In comparison, hypocotyl of the model plant Arabidopsis thaliana L. (Cruciferae) grows only up to 1.8 mm on 9th DAG (Gendreau et al., 1997). Thus, jute hypocotyl seems to be an ideal system for studying growth kinetics in fast-developing hypocotyl and examine the role of β-galactosidase in hypocotyl development.
To address these issues, we first identified the full spectrum of β-galactosidases expressed in jute hypocotyl by high throughput RNA-seq-based transcriptome sequencing of a hypocotyl-defect mutant of C. capsularis and its wild type cv. JRC-212 (WT). Next, we compared hypocotyl-specific expression profiles of β-galactosidases from the mutant with corresponding homologs from the WT. To further investigate the role of this enzyme, we analyzed growth kinetics, anatomy and β-galactosidase activity in the hypocotyls of the mutant and the WT. We also tested inheritance and correlation of hypocotyl-defect traits and enzyme activity in a genetically defined population. Based on these results, we posit a crucial role of β-galactosidase in cell wall modification during hypocotyl development. Our results also bring novel insights into the evolutionary fate of the GH-2 family β-galactosidase in plant lineage.
Results and discussion