Immunocytological Analysis of Potato Tuber Periderm and Changes in Pectin and Extensin Epitopes Associated with Periderm Maturation

Abstract

Potato (Solanum tuberosum L.) periderm forms a barrier at the surface of the tuber that protects it from infection and dehydration. Immature periderm is susceptible to excoriation (skinning injury), which results in costly storage loses and market quality defects. The periderm consists of three different cell types: phellem (skin), phellogen (cork cambium), and phelloderm (parenchyma-like cells). The phellogen serves as a lateral meristem for the periderm and is characterized by thin radial walls that are labile to fracture while the periderm is immature and the phellogen is actively dividing, thus rendering the tuber susceptible to excoriation. As the periderm matures the phellogen becomes inactive, its cell walls thicken and become resistant to fracture, and thus the tuber becomes resistant to excoriation. Little is known about the changes in cell wall polymers that are associated with tuber periderm maturation and the concurrent development of resistance to excoriation. Various changes in pectins (galacturonans and rhamnogalacturonans) and extensin may be involved in this maturational process. The objectives of this research were to compare immunolabeling of homogalacturonan (HG) epitopes to labeling of rhamnogalacturonan I (RG-I) and extensin epitopes to better understand the depositional patterns of these polymers in periderm cell walls and their involvement in tuber periderm maturation. Immunolabeling with the monoclonal antibodies JIM5 and JIM7 (recognizing a broad range of esterified HG) confirmed that HG epitopes are lacking in phellogen walls of immature periderm, but increased greatly upon maturation of the periderm. Labeling of a (1,4)-β-galactan epitope found in RG-I and recognized by the monoclonal antibody LM5 was abundant in phelloderm cell walls, but sparse in most phellem cell walls. LM5 labeling was very sparse in the walls of meristematically active phellogen cells of immature periderm, but increased dramatically upon periderm maturation. Deposition of a (1,5)-α-l-arabinan epitope found in RG-I and recognized by LM6 was abundant in phelloderm and phellogen cell walls, but was sparse in phellem cell walls. LM6 labeling of phellogen walls did not change upon periderm maturation, indicating that different RG-1 epitopes are regulated independently during maturation of the periderm. Labeling with the monoclonal antibody LM1 for an extensin epitope implied that extensin is lacking in phellem cell walls, but is abundant in phelloderm cell walls. Phellogen cell walls did not label with LM1 in immature periderm, but were abundantly labeled with LM1 in mature periderm. These immunolabeling studies identify pectin and extensin depositions as likely biochemical processes involved in the thickening and related strengthening of phellogen walls upon inactivation of the phellogen layer as a lateral meristem and maturation of the periderm in potato tuber. These results provide unique and new insight into the identities of some of the biological processes that may be targeted in the development of new technologies to enhance resistance to tuber skinning injury for improved harvest, handling and storage properties.