IMMUNOCYTOLOGICAL ANALYSIS OF POTATO TUBER PERIDERM AND CHANGES IN PECTIN AND EXTENSIN EPITOPES ASSOCIATED WITH PERIDERM MATURATION

Authors: Sabba, Robert; Lulai, Edward

Submitted to: Journal of the American Society for Horticultural Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/13/2005
Publication Date: 11/1/2005
Citation: Sabba, R.P., Lulai, E.C. 2005. Immunocytological analysis of potato tuber periderm and changes in pectin and extensin epitopes associated with periderm maturation. Journal of the American Society for Horticultural Science. 130(6):936-942.

Interpretive Summary: The potato skin forms a barrier at the surface of the tuber that protects it from infection and dehydration. An immature potato is susceptible to skinning injury, which leads to potato loses in storage. As the potato matures, it becomes resistant to skinning. The potato skin is created by the phellogen (cork cambium) layer. Changes in the architecture of the phellogen cell walls upon potato maturation are directly associated with development of resistance to skinning injury. The walls of phellogen cells are thin and labile to fracture in immature potato, but thicken and become resistant to fracture upon maturation of the potato. Here we show that staining with toluidine blue O can be used to distinguish between the skin and phellogen cell layers. Immunolabeling with antibodies which react with the cell wall ploymers pectin and extensin indicates that both types increase in phellogen walls during maturation of the potato. These results identify pectin and extensin deposition in phellogen cell walls as key biochemical processes responsible for potato maturation and resistance to tuber skinning.

Technical Abstract: The potato (Solanum tuberosum L.) periderm forms a barrier at the surface of the tuber that protects it from infection and dehydration. Potato periderm consists of three different cell types: phellem, phellogen, and phelloderm. The phellogen serves as a lateral meristem that undergoes periclinal divisions to produce the phellem and the phelloderm. An immature periderm is characterized by a meristematically active phellogen whose cell walls are thin and labile to fracture, which renders the tuber susceptible to periderm excoriation. Upon inactivation of the phellogen as a lateral meristem, these cell walls undergo poorly understood biochemical changes which strengthen them and reduce the tuber's susceptibility to periderm excoriation. Here we show that staining with toluidine blue O can be used to distinguish between the phellem and phellogen cell layers. We utilized immunological probes for both homogalacturonan (HG)and rhamnogalacturonan I (RG-I) epitopes to determine changes in the expression pattern of these pectin polymers in tuber periderm cell walls during periderm maturation. Immunolabeling with the monoclonal antibodies JIM 5 (recognizing unesterified HG) and JIM 7 (recognizing esterified HG) indicates that while phelloderm cell walls are rich in both unesterified and esterified HG, phellem walls are rich in highly-esterified HG, but are lacking in un-esterified pectin. In immature periderm, labeling of phellogen walls by Jim5 and JIM7 was weak, but increased greatly upon maturation of the periderm. Expression of a (1,4)-beta-galactan epitope found in RG-I and recognized by the monoclonal antibody LM5 was strong in phelloderm cell walls, but weak in most phellem cell walls. LM5 labeling was very weak in the walls of meristematically active phellogen cells of immature periderm, but labeling of phellogen walls intensified dramatically upon meristematic inactivation and periderm maturation. Expression of a (1,5)-alpha-L-arabinan epitope found in RG-I and recognized by LM6 was strong in phelloderm and phellogen cell walls, but weak in phellem cell walls. LM6 labeling of phellogen walls did not change after meristematic inactivation and periderm maturation. In addition, labeling with the monoclonal antibody LM1 for extensin indicates that extensin is lacking in phellem cell walls, but is strong in phelloderm cell walls. Phellogen cell walls did not label with LM1 in immature periderm, but labeled strongly with LM1 in mature periderm. These results identify pectin and extensin deposition in phellogen cell walls as key biochemical processes involved in the inactivation of the phellogen layer as a lateral meristem and maturation of the periderm in potato tuber.