Ocatin. A Novel Tuber Storage Protein from the Andean Tuber Crop Oca with Antibacterial and Antifungal Activities^{<a href="#FN1" rid="FN1" class=" fn">1</a>}

Abstract

The Andean root and tuber crops constitute a unique reservoir of germplasm biodiversity in the world. They grow at altitudes of 2,400 to 4,000 m in the Andes; therefore, they have great potential for introduction into other highland areas where crops from the Old World are not well adapted (King and Gershoff, 1987). However, the Andean root and tuber crops have been largely overlooked and poorly studied at the biological and biochemical level (Tapia, 1984; National Research Council, 1989; Flores and Flores, 1997).

One of the Andean tuber crops, oca (Oxalis tuberosa Mol.), grows in greatest diversity in the highlands of Ecuador, Peru, and Bolivia, although it is found as far north as Venezuela and as far south as Chiloe Island in Chile. Oca and potato (Solanum tuberosum) are cultivated in the same areas and the same range of altitude, but oca is more resistant to cold temperatures than potato (Pulgar, 1981). Oca is tolerant to temperatures as low as 5°C and grows in moderately cool climates and in very poor soils with pH of 5.3 to 7.8, where other crops cannot survive (Leon, 1964). Oca tubers have a nutritional value as good as, or better than, potato (National Research Council, 1989), and they vary in nutritional value depending on the variety (Cortes, 1977). In a study by Janick and Simon (1988), the authors identified oca as an excellent source of carbohydrates that should be highly digestible by monogastric animals because of the low content of alpha-galactosidase and fiber and the high content of digestible sugars. Compared with other Andean tuber crops, including potato, some oca morphotypes have considerably higher levels of essential amino acids (Hodge, 1957). For example, high concentrations of Val and Lys (4.0 and 4.5 mg g ^{fresh weight, respectively) were found in some of the oca morphotypes. Amounts of riboflavin and thiamine present in oca tubers (0.07 and 0.05 mg g fresh weight, respectively) were lower than those obtained in some potato varieties (1.0 and 0.065 mg g fresh weight, respectively); however, the amounts of iron and phosphorus in oca (0.8 and 34.0 mg g fresh weight, respectively) were considerably higher than those in potato (0.6 and 25 mg g fresh weight, respectively;} Hodge, 1957; Seminario, 1988). Studies by Cortes (1977) on different oca tuber morphotypes showed that oca tubers can be a potential commercial source of flour, starch, and alcohol. According to Ortega (1992), oca is an excellent source of carbohydrates, alcohol, sugar, phosphorus, calcium, iron, and vitamins.

Oca is now cultivated in New Zealand, Australia, and Mexico (National Research Council, 1989), but its basic biology and biochemistry are virtually unknown (Stegemann and Schmiediche, 1981; Stegemann et al., 1988). Little is known about the biochemistry of most vegetative storage organs except for potato, sweet potato (Ipomoea batata), and yam (Dioscorea cayensis; Racussen and Foote, 1980; Maeshima, et al., 1985; Coursey, 1991).

Vegetative storage organs contain a large amount of storage proteins (constituting 30%–80% of total soluble proteins), which serve as a nutritional resource to support sprouting and regrowth (Stegemann and Schmiediche, 1981). Vegetative storage proteins are synthesized with a signal peptide, which targets them to the vacuole (Matsuoka and Nakamura, 1991). In addition to their storage function, they could play additional roles in the plant: sporamin, from sweet potato, has an antiprotease activity and is thought to play a role in defense against insects (Yeh et al., 1997). Patatin, from potato tubers, has phospholipase A₂ activity, inhibits worm larval growth (Strickland et al., 1995), and is suggested to play a role in the hypersensitivity reaction (Senda et al., 1996).

In this study, we found that the protein patterns of the soluble proteins from 36 different oca tuber morphotypes are very similar when separated by SDS-PAGE. The major tuber soluble protein (ocatin) was characterized. The amino acid sequence of ocatin showed homology with proteins that accumulate in response to elicitation, pathogens, or wounding in a wide variety of angiosperms. Ocatin was found to have antibacterial and antifungal activities against several soil-borne microbes. Thus, ocatin serves as a storage protein and might play a role in natural resistance to pathogens.

Inhibition of hyphal growth on different fungal species by ocatin (as percentage of total area covered by fungal hyphae).

ACKNOWLEDGMENTS

Footnotes

LITERATURE CITED