Glandular trichomes of S. berthaultii

Table II. Deterrence of green peach aphid feeding by sucrose esters from type B glandular trichomes of S. berthaultii (PI 473331) applied to diet membranes. Adapted from (24)...

Table IV. Effect of removing type A (wipe) and type B (methanol dip) glandular trichomes of S. berthaultii (PI 310927) on feeding, growth, and mortality of neonate Colorado potato beetle. Adapted from (29)...

The sesquiterpenes of j>. tuberosum foliage are not present in sufficient quantities at the leaf surface interface to discourage aphid settling. By contrast, the exudate of the Type B glandular trichomes on S. berthaultii foliage contains sesquiterpenes, thus enhancing the external level of these insect-repellent compounds. 

The inheritance of glandular trichomes in S, berthaultii and in crosses with 5. tuberosum has been studied by several groups (11. 35-371. Type B trichome density, droplet size and presumably the presence of sucrose esters appear to be controlled by relatively few recessive genes. Heritability estimates ranged from 20-30% for density of Type A and B trichomes and 60% for Type B droplet size. The presence of polyphenoloxidase in type A trichomes is controlled by a single dominant gene (381. The inheritance of sesquiterpenes in type A trichomes has not been studied. 

There are primarily two classes of foliar glandular trichomes that contribute to the insect resistance of S. berthaultii and L. pennellii. Type A trichomes of S. berthaultii (referred to as Type VI in the genus Lycopersicon (H.) ) condition resistance to small-bodied insect pests such as aphids and leafhoppers by an... 

Browning and hardening of exudate on the insect s mouthparts and tarsi is due to the enzymatic activity of an O2- requiring oxidase (12), and has been associated with both peroxidase and polyphenol oxidase activities (2/14-fl2.). We recently showed that these enzymatic activities are due to a 59 kD polyphenol oxidase (PPO) which is present in the type A trichomes of S. berthaultii at approximately 1 ng/trichome head (a concentration of approximately 0.3 mM) (lA). The specialization of the glandular trichome is reflected in the fact that PPO comprises approximately 60% of the total soluble protein of the organ. 

Behavioral and sensory disturbance. Potato glandular trichomes, in addition to acting as a physical barrier to pests, manufacture and/or store a profusion of plant metabolic products, some of which profoundly influence insect behavior and metabolism (19,20). The sesquiterpene components of potato trichome glands (21,22,23) are potentially powerful semiochemicals and one of these, E-6-farnesene, is well known for its ability to initiate evasive behavior in aphids (24). The dramatic alteration of aphid feeding behavior on S. berthaultii reported by Lapointe and Tingey (25) may be due to allomonal sesquiterpenes in trichome exudate and will be discussed later. 

Such subde effects of the trichome barrier might explain our recent finding that feeding is a much smaller portion of the activity budget of Colorado potato beetle larvae on S. berthaultii than on non-glandular susceptible cultivars (Table V) (34) and thus provide an explanation for several of the major impacts of resistance on this pest, i.e. decreased food consumption leads to the reduced growth rates, retarded development, and their cumulative suppressive effects on survival, fecundity, and population dynamics. 

Table V. Activity of neonate Colorado potato beetle larvae on excised leaflets of either S. tuberosum, S. berthaultii PI 310927, or PI 310927 from which most of the glandular trichome exudate had been removed by wiping between tissue papers. Adapted from (24)...

The wild tomato, Lycopersicon pennellii, and the wild potato, Solanum berthaultiif are two species which exhibit insect resistance conferred by glandular trichomes. S, berthaultii and L. pennellii have been the focus of efforts at Cornell University to transfer trichome-based insect resistance traits. This chapter reviews our knowledge of the biochemistry of glandular trichome-based insect resistance in these species. 

---