As lateral root primordia development occurs deep within primary roots, remodeling of overlying tissue cell walls is required to accommodate growing primordia during LRE. Previous research involving cell wall remodeling (CWR) induced by the auxin signaling pathway, and IDA-peptide signaling, identified several enzymes responsible for alterations in plant cell walls. For example, xylosyltransferases such as XTR6 (Péret et al., 2013) and XXT1 (Cavalier et al., 2008) both synthesize xyloglucan, which forms a major component of hemicellulose in plant cell walls. Down-regulation of these two genes was observed during lateral root emergence, suggesting that diminished hemicellulose content is required to weaken cell wall structures that may inhibit lateral root primordia elongation. In addition, down-regulation of the genes PRC1, which produces a cellulose synthase required for cellulose microfibril assembly (Fagard et al., 2000), and XEG113, a xyloglucan transferase required to increase cross-linking of extensins in the cell wall matrix (Roycewicz and Malamy, 2014), were similarly observed during lateral root emergence and may also imply that a decrease in specific cell wall component is required for successful LRP emergence.
Other cell wall remodeling enzymes have been identified that destabilize or degrade specific cell wall components. For example, up-regulation of CEL3/GH9B3 (Lewis et al., 2013) and EXP17 (Sato and Miura, 2011) both weaken cell wall structures by either degrading cellulose directly (CEL3/GH9B3), or reducing the cross-linking between cellulose microfibrils to destabilize their position in the cell wall matrix (EXP17). In addition, several enzymes appear to target pectin (e.g. homogalacturonan), another significant structural component of the cell wall, to destabilize the cell wall matrix to permit deformation of cells contacting emerging lateral root primordia. AIR3 (Vilches-Barro and Maizel, 2015), PGLR and PGAZAT (Kumpf et al., 2013) genes yield enzyme products that either act as subtilisin-like serine proteases that degrade homogalacturonan by enabling de-methyl esterification of the pectin backbone and subsequent cleavage by polygalacturonases (AIR3), or directly serve as polygalacturonases which hydrolyze the bonds between residues in homogalacturonan (PGLR and PGAZAT).
The cell wall itself is a distinguishing feature of plant cells and is critical to the successful development and reproduction of plants. Plant cell walls are commonly divided into two types: primary and secondary. The primary cell wall consists of a rigid layer of complex polysaccharides on the outer surface of the plasma membrane that encases the entire plant cell and changes little after initial synthesis. The secondary cell wall forms underneath the primary cell wall after it has ceased developing and provides additional structural support to plant cells. Unlike primary walls, secondary walls continue to form after the plant cell has ceased expanding and typically exhibit different component ratios than primary cell walls. For instance, secondary cell walls found in xylem and sclerenchyma tissue typically contain significantly more lignin than primary cell walls and serve as strong support structures to ensure proper water transport and maintain overall plant form, respectively.
The primary cell wall is further described according to two distinct classes of cell wall matrix contents: Type-I and Type-II. Dicotyledonous and non-graminaceous monocotyledonous plants possess Type-I cell walls which are characterize as interlinking matrices of xyloglucan and cellulose microfibrils in a hydrated pectin polymer network (Carpita, 1996). Graminaceous monocotyledonous plants, such as rice, possess Type-II cell walls that possess hemicellulose, feruloyated arabinoxylans, and mixed-linkage glucans (1,3; 1,4)-β-D-glucan) as major components with minor quantities of xyloglucans, pectic polysaccharides, and arabinogalactan proteins (Vega-Sánchez et al., 2013)
Remodeling of primary cell wall components is known to occur in overlaying Zea mays and Arabidopsis root tissues during LRE, but is poorly described in many cereal grains such as Oryza sativa. Rice primary wall, similar to other grasses, are characterized by the significant abundance of hemicellulosic polysaccharides, glucuronoarabinoxylans and mixed-linkage glucans, with relatively minor proportions of xyloglucans, pectic polysaccharides, and structural proteins such as arabinogalactan (Pattathil et al., 2015). By contrast, other components such as lignin are located in secondary cell walls that form after primary cell wall development. Modification of these cell wall matrix constituents may influence the properties of the wall matrix such as tensile strength, recalcitrance to enzymatic digestion and proper root elongation (Tenhaken, 2014).
Pectin, a relatively minor primary wall component, is subject to modification in the form of de-methyl esterification of homogalacturonan (Ochoa-Villarreal et al., 2012). Homogalacturonan (HG) itself is the most abundant pectic polysaccharide, constituting ~65% of total pectin in both Type I (dicots) and Type II (commelinoid monocots) plant cell walls. Homogalacturonan consists of a linear α-1,4-linked galacturonic acid (GalA) homopolymer with a typical degree of polymerization of ∼100 (Mohnen, 2008). During cell wall synthesis, the homogalacturonan backbone structure is produced in a highly methyl-esterified form from cellular Golgi complex, with methyl-esterification occurring on C2-C3 and C6 GalA residue carbons (Figure 4) (Ridley et al., 2001). Activity from pectinesterases cleave methyl ester bonds later in cell wall development, yielding epitopes of de-methyl esterified pectin homopolymers that play significant roles in overall primary wall integrity (Arancibia and Motsenbocker, 2006).
HG methyl-esterification directly influences structural interactions in plant tissues such as cell wall matrix stability. A “Loosening Model” of primary cell wall alteration suggests that de-methyl esterification enhances the vulnerability of the HG backbone structure to degradation by polygalacturonase enzyme activity – resulting in pectin degradation and a subsequent loss of cell wall matrix structural integrity.
Understanding the developmental changes experienced by cell walls overlaying lateral root primordia can have significant impacts on agricultural research and development, including: expanded knowledge of cereal grain root system development, potential for increases in root system volume, and identification of molecular targets to modify rice cell wall recalcitrance, leading to improvement of rice feedstock digestibility for biofuel production.
To better understand developmental changes during rice LRE, immunohistochemical studies were performed on sectioned root tissue from rice root seedlings. Antibody binding patterns from these results identified epitopes of primary cell wall components that appear to be modified in cells near the primordium during LRE. Further testing with enzymatic assays illuminated the potential chemical interactions between select cell wall components during lateral root emergence. In addition, groundwork for future relative gene expression and transcriptome analysis of tissues subject to cell wall matrix alterations was initiated with the development of an optimized laser capture microdissection protocol for rice root tissue.
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