Altered Middle Lamella Homogalacturonan and Disrupted Deposition of (1→5)-α-l-Arabinan in the Pericarp ofCnr, a Ripening Mutant of Tomato

Abstract

Cnr (colorless non-ripening) is a pleiotropic tomato (Lycopersicon esculentum) fruit ripening mutant with altered tissue properties including weaker cell-to-cell contacts in the pericarp (A.J. Thompson, M. Tor, C.S. Barry, J. Vrebalov, C. Orfila, M.C. Jarvis, J.J. Giovannoni, D. Grierson, G.B. Seymour [1999] Plant Physiol 120: 383-390). Whereas the genetic basis of the Cnr mutation is being identified by molecular analyses, here we report the identification of cell biological factors underlying the Cnr texture phenotype. In comparison with wild type, ripe-stage Cnr fruits have stronger, non-swollen cell walls (CW) throughout the pericarp and extensive intercellular space in the inner pericarp. Using electron energy loss spectroscopy imaging of calcium-binding capacity and anti-homogalacturonan (HG) antibody probes (PAM1 and JIM5) we demonstrate that maturation processes involving middle lamella HG are altered in Cnr fruit, resulting in the absence or a low level of HG-/calcium-based cell adhesion. We also demonstrate that the deposition of (135)-␣-l-arabinan is disrupted in Cnr pericarp CW and that this disruption occurs prior to fruit ripening. The relationship between the disruption of (135)-␣-larabinan deposition in pericarp CW and the Cnr phenotype is discussed.The modification of cell walls (CW) is an important aspect of plant cell development. During fruit ripening the regulated swelling and dissolution of primary CW and the modification of middle lamellae (ML) between adherent primary CW are important factors contributing to tissue softening (Brady, 1987; Fischer and Bennett, 1991). The biochemistry and the spatial regulation of the dissolution of primary CW and ML are not fully understood, but in all cases appear to involve modifications to the network of pectic polysaccharides.The multi-functional pectic polysaccharides are the most complex class of polysaccharides in primary plant CW (Jarvis, 1984). Core backbone structures of contiguous 1,4-linked ␣-d-galacturonic acid (homogalacturonan, HG) or repeats of the disaccharide [34)-␣-d-GalA-(132)-␣-l-Rha-(13] (rhamnogalacturonan, RG) are elaborated with a range of modifications and substitutions. These include methylesterification, acetylation, and the addition of neutral polysaccharide side chains. Side chains may be attached to HG and RG to form the branched polysaccharides RG-II and RG-I, respectively, the latter often rich in (135)-␣-l-arabinan and (134)-␤-d-galactan components (O'Neill et al., 1990; Albersheim et al., 1996; Mohnen 1999). Several of these pectic structures appear to be capable of being enzymatically modified in muro. For example, the de-esterification of HG by pectin methyl esterases (PMEs) influences its capacity to form calcium cross-linked gels. The relationships of pectic polysaccharide domains within large polymer structures and their functional properties in relation to factors such as porosity, cell adhesion and expansion, ionic and hydration status, and cell signaling, are far from clear.Ripening-related textural changes in t...

Loss of Highly Branched Arabinans and Debranching of Rhamnogalacturonan I Accompany Loss of Firm Texture and Cell Separation during Prolonged Storage of Apple

“…The Cnr tomato mutant shows a disrupted deposition of (1/5)-a-L-arabinan, a modified HG network with reduced calcium-binding capacity in the middle lamellae, and a reduced cell adhesion concomitant with a failure to swell (Orfila et al, 2001(Orfila et al, , 2002. A similar phenomenon is observed in the arabinan-rich walls of Commelina guard cells, in which the wall shape can be frozen open or closed by digestion of 5-linked arabinans (Jones et al, 2003).…”

“…In the tobacco (Nicotiana plumbaginifolia) mutant nolac-H14, RG Is with low levels of associated arabinans were not retained in the cells walls and middle lamella (Iwai et al, 2001). The altered HG network and reduced cell adhesion observed in the Cnr tomato mutant could be a consequence of disrupted arabinan deposition in this tomato cell wall (Orfila et al, 2001(Orfila et al, , 2002. A role for arabinans in anchoring pectin in the wall was suggested by these authors, but the decreased abundance and altered location of arabinan side chain when RG I was fragmented in muro by a rhamnogalacturonan lyase in transgenic potato plants indicated that cross-linkage between arabinans and other components of the wall is unlikely (Oomen et al, 2002).…”

“…Redgwell et al (1997) found no correlation between the loss of Gal and the swelling and softening of several kinds of fruit. However, the Cnr tomato fruit ripening mutant, which has a disrupted deposition of (1/5)-a-L-arabinans, exhibits altered tissue properties (Orfila et al, 2001).…”

Plant cell adhesion: A bioassay facilitates discovery of the first pectin biosynthetic gene

“…Less attention has been paid to this role for adhesion in plants. By using a variety of approaches, genetic studies have implicated pectins in adhesion (17)(18)(19)(20)(21), but none are as definitive as the Iwai et al 2002 study (5). A very recent report (22) of two dwarf Arabidopsis mutants that have a 25% reduction in galacturonic acid levels is the most intriguing.…”

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

“…In the Arabidopsis parvus mutants, characterized by reduced levels of RG-I branching, growth and fertility are influenced by humidity (Lao et al, 2003). The tomato (Lycopersicon esculentum) Cnr mutant has an altered deposition of both HGA and arabinan in the fruit pericarp, affecting cell wall properties and intercellular adhesion (Thompson et al, 1999;Orfila et al, 2001). …”