Immunocytological Comparison of Native and Wound Periderm Maturation in Potato Tuber

American Journal of Potato Research, Mar/Apr 2004 by Sabba, Robert P, Lulai, Edward C

An immature periderm has a phellogen layer made up of cells with thin radial walls that fracture easily, allowing the phellem to excoriate or scuff-off in both native (Lulai and Freeman 2001) and wound periderm (Sabba and Lulai 2002). These walls strengthen and thicken during maturation, resulting in resistance to excoriation in both types of periderm (Lulai 2002).

Pectins are one of the primary constituents of the plant cell wall, functioning as an agent in hydration and cell-to-cell adhesion. One of the most common types of pectin is HG, which is a polymer of D-galacturonic acid with varying degrees of methyl esterification (Thakur et al. 1997). We have previously shown that increases in phellogen cell wall pectins accompany the thickening of phellogen radial walls upon maturation in native periderm (Sabba et al. 2002). While there are similarities between native and wound periderm, we have shown previously that there are histological differences in the staining patterns of cell wall polymers involved in the maturation of native and wound periderm (Sabba and Lulai 2002). In this study, we utilized immunological probes to analyze the deposition of cell wall pectin polymers during the maturation of wound periderm and compared these results with our recent findings for native periderm. This is the first report that we know of that directly compares the immunological properties of tuber pectin polymers during maturation of native periderm with wound periderm.

MATERIALS AND METHODS

Preparation of Periderm Sample Material

Tubers (cv Russet Burbank) harvested early, after the growing season, were susceptible to skinning and were therefore considered to be immature. This was confirmed by mechanically testing skinning susceptibility as described previously (Lulai and Orr 1993). Tubers from this harvest that were stored for at least 4 wk under 96% relative humidity at 21 C in the dark were resistant to skinning and were therefore considered mature. Tubers harvested late, after the growing season, were also resistant to skinning and were therefore considered mature.

Tubers were reproducibly wounded using a surgically sharp industrial steel blade to cleanly shave away a 0.75-mm-thick slice of tissue from either the natural tuber surface or the rough cut surface of a tuber that was first cut in half (stem to bud end) with a conventional blade. This technique produced surfaces that were more sharply cut than those obtained with a conventional knife and consequently wound-healed with greater uniformity. A wound periderm was allowed to develop under optimal conditions (Morris et al. 1989), at 21 C and 96% relative humidity in the dark for the desired time period (either 7 d for immature periderm or 18 to 29 d for mature periderm). Wound periderm that was 1 wk old was susceptible to excoriation and was therefore considered to be immature. Wound periderm that was more than 2 wk old was resistant to excoriation and was therefore considered to be mature.

Tissue blocks, including periderm (either native or wound), were cut from the tubers and fixed in 2.5% glutaraldehyde in phosphate buffer (pH 7.3). Fixed periderm blocks were washed in phosphate buffer (pH 7.3), hand sectioned with a razor blade and dehydrated through a graded ethanol series (25%-100%). Dehydrated samples were embedded in L.R. White resin (Polysciences Inc., Warrington, PA, USA) over a period of one week and polymerized in BEEM capsules (Ted Pella Inc., Redding, CA, USA) according to manufacturer's instructions.