Glandular-haired Alfalfa: Trichome Morphologies and Their Roles in Resistance to the Potato Leafhopper.

Christopher M. Ranger and Arthur A. Hower

Alfalfa, Medicago sativa, possessing resistance to the potato leafhopper, Empoasca fabae, was released by eight seed companies in 1997. The major resistant morphological feature of all of these leafhopper resistant germplasms is the glandular trichomes on selected vegetative and floral structures. All three mechanisms of host plant resistance (antibiosis, antixenosis, and tolerance) have been suggested as being expressed by this plant material. Because our research eliminated lignification, vascular bundles, stem diameter, and phloem toxins as possible mechanical mechanisms for resistance, our focus became the glandular trichomes. The objectives of this study were to: 1. determine the location of the glandular trichomes on the alfalfa plant, 2. characterize the external and internal morphologies of the glandular trichomes, and 3. determine the mode of action of these glandular trichomes against the potato leafhopper. Both light and electron microscopy were used to examine the glandular trichomes on the clonal resistant alfalfa FGplh13. Two distinct types of glandular trichome morphologies were detected: an erect and a procumbent form. The erect glandular trichome was characterized by a multicellular stalk and gland head while the procumbent trichome was characterized by a one or two cell stalk bent parallel to the epidermal surface from which it originated. The procumbent glandular head consisted of two tiers of four cells each. The length of the erect glandular trichome was a mean 340.6 + 34.1 um and a mean gland head width of 31.5 + 1.4 um. The procumbent trichome was significantly smaller with a mean length of 79.9 + 5.6 um and a gland head width of 24.8 + 1.9 um. Examination of a 2mm section of alfalfa stem, petiole, leaf midvein, and leaf perimeter revealed that the erect glandular trichome was the most significantly abundant morphology at each location. Nonglandular trichomes were present but in comparatively lower numbers. An exudate was found associated with both the erect and procumbent glandular trichomes. In the erect glandular trichome, a cuticular boil or secretion blister was formed by the localization of an exudate between the cuticle and cell wall prior to secretion. In the procumbent glandular trichome the cuticular boil formed a single large space between the cuticle and cell wall with the exudate being released by the most distal tier of cells. The exudate from the procumbent glandular trichome was observed attached to the cuticle of a leafhopper nymph. The sticky exudate produced by both trichome types entrapped some first and possibly second instar potato leafhoppers. This entrapment of potato leafhoppers by an exudate has never been documented with perennial alfalfa varieties. Until now researchers have mostly focused on the erect glanduar trichome as providing the resistance factor. The procumbent glandular trichome has not been considered in the resistance mechanism. In fact, the exudate from the procumbent glandular trichomes on annual and perennial Medicagos has been described as dry and hard. Conversely, this research showed that the exudate was somewhat sticky and might play a role in the resistance of these new alfalfa varieties to the potato leafhopper. This research has elucidated the morphology, location, abundance, and one potential mode of action for the glandular trichomes associated with the recently released potato leafhopper resistant alfalfas and thus, provides a better understanding of the relevance of these glandular-haired trichomes for managing this important alfalfa pest.

The objectives of this study were to: 1. determine the location of the glandular trichomes on the alfalfa plant, 2. characterize the external and internal morphologies of the glandular trichomes, and 3. determine the mode of action of these glandular trichomes against the potato leafhopper.

Elden and McCaslin (1997) reported that entrapment did not appear to be associated with these new leafhopper resistant varieties.

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