Effect of Pectin Methylesterase Gene Expression on Pea Root Development

Abstract

Expression of an inducible gene with sequences common to genes encoding pectin methylesterase (PME) was found to be tightly correlated, both spatially and temporally, with border cell separation in pea root caps. Partial inhibition of the gene's expression by antisense mRNA in transgenic pea hairy roots prevented the normal separation of root border cells from the root tip into the external environment. This phenotype was correlated with an increase in extracellular pH, reduced root elongation, and altered cellular morphology. The translation product of the gene exhibited PME activity in vitro. These results are consistent with the long-standing hypothesis that the demethylation of pectin by PME plays a key role in cell wall metabolism. INTRODUCTIONBetween the plant cytoplasm and its external environment lies a complex carbohydrate-based cell wall, which is a dynamic interface that participates directly in cellular responses to exogenous stimuli (reviewed in Albersheim et al., 1994; de Lorenzo et al., 1994). In addition to a direct role in perceiving and responding to incoming signals, the cell wall is a repository of oligosaccharides whose activity can alter the metabolism of the plant cell it encloses as well as that of other organisms that find their way into proximity with the cell. These sugar-based signal molecules are released from cell wall polymers by the action of enzymes that can come from fungi, bacteria, or other organisms in the environment, or from the plant itself. The role of specific plant cell walldegrading enzymes in cell wall metabolism during growth and development remains unclear (reviewed in Carpita et al., 1996).Plant enzymes that degrade pectin, or methylated polygalacturonic acid, are of special interest because this polymer is a major constituent of cell walls and because such pectolytic enzymes can solubilize cell walls (Collmer and Keen, 1986;Koutojansky, 1987). For example, genes encoding certain polygalacturonases (PGs) or pectate lyases (PLs) individually allow soft rot pathogens to macerate potato tuber tissue and to infect plants systemically (Collmer and Keen, 1986). Pectin methylesterase (PME), although it does not by itself solubilize cell walls, has been postulated to regulate cell wall degradation by several mechanisms (e.g., Goldberg et al., 1992). The action of PME reduces pH by the release of a proton when methoxyl groups of pectin are converted to carboxyl groups. This change in pH has been proposed to control the activity of other cell wall-degrading enzymes that are optimally active at low pH and thereby to facilitate cell expansion and growth ) and/or cell separation (Koutojansky, 1987).Demethylation by PME can alter sensitivity of polymers to the action of hydrolases (e.g., Fischer and Bennett, 1991;Liu and Berry, 1991) and expansins (Carpita et al., 1996). Small pectic fragments released by the action of such hydrolases act as signals to induce expression of other pectolytic enzymes, and the degree of methylation of such fragments, dictated by PME activity, ...

The Organization Pattern of Root Border-Like Cells of Arabidopsis Is Dependent on Cell Wall Homogalacturonan

“…Therefore, we suggest that, unlike in pea, the hydrolysis of homogalacturonan present in the cell wall of Arabidopsis border-like cells does not occur, possibly because the enzymes, if present in the cell wall of border-like cells, may not be functional. This is supported by the observation that antisense inhibition of pectin-methylesterase expression in transgenic pea resulted in the inhibition of border cell separation (Wen et al, 1999). In the transformed plants, border cells remained attached together as clusters of cells, a phenotype that is somehow similar to that of border-like cells in Arabidopsis.…”

Targeted Modification of Homogalacturonan by Transgenic Expression of a Fungal Polygalacturonase Alters Plant Growth

“…HGA is synthesized and incorporated into the cell wall in a highly methyl-esterified form, and during cell development methyl groups are removed by pectin methylesterases, thereby creating a substrate for endogenous PGs . The relative amounts of methyl-esterified and unesterified HGA are likely to be critical for determining many processes occurring during plant development (Kim and Carpita, 1992;Steele et al, 1997;Femenia et al, 1998;Wen et al, 1999). The observations reported here indicate that the deesterified form of HGA is a critical factor in plant growth.…”

PECTIN METHYLESTERASE48 Is Involved in Arabidopsis Pollen Grain Germination

“…The regulation of the DM of HG has been implicated in many aspects of plant development (Wolf et al, 2009a), including cell adhesion (Tieman and Handa, 1994;Wen et al, 1999;Mollet et al, 2000;Bouton et al, 2002;Mouille et al, 2007;Durand et al, 2009), adventitious rooting (Guénin et al, 2011), primordia emergence at the shoot apical meristem (Peaucelle et al, 2008(Peaucelle et al, , 2011, fruit ripening (Brummell and Harpster, 2001;Phan et al, 2007), and plant defense (Bethke et al, 2014). Although data have shown an important role for Figure 5.…”