Potato Glandular Trichomes: A Physicochemical Defense Mechanism Against Insects

A wild potato species, Solanum berthaultii Hawkes, is resistant to insect pests by virtue of two types of glandular trichomes on its foliage. Type A is a short trichome with a four-lobed membrane-bound gland at its apex. Type B is a longer, simple trichome with an ovoid gland at its tip which continuously exudes a clear viscous exudate. Studies of this physicochemical resistance mechanism suggest the following series of events: (a) the insect lands upon the foliage and encounters Type B trichomes, (b) Type B trichomes coat the insect with a sticky exudate and, due to sesquiterpenoid action, agitate the insect, (c) the struggling insect disrupts the heads of Type A trichomes, and an oxidation reaction between a unique polyphenoloxidase enzyme (PPO) and phenolic substrate(s) results in quinone formation, (d) these events result in insect immobilisation, cessation of feeding and death. The relevance of this mechanism to breeding for improved potato resistance to insect pests is discussed.