A method for field assessment of plant injury elicited by the salivary proteins of Apolygus lucorum

Abstract

Mirid bugs (Hemiptera: Miridae) are an important group of insect pests on many agricultural crops, including cotton, legumes, cereals, vegetables, and fruits (Wheeler, 2001). The adults and nymphs feed via piercing and sucking mouthparts on the terminal meristems, young flower buds, fruits (including cotton bolls), and other tissues, often causing the formation of bushy plants, the abscission of flower buds and fruits, and the malformation of mature fruits (Wheeler, 2001). These injuries greatly reduce crop yield and quality, causing serious economic losses in years of high infestation levels. Unlike many other common piercing/sucking insects (e.g., aphids, planthoppers, whiteflies) that are vascular feeders, mirid bugs are mostly considered ‘lacerate-and-flush’ feeders (Miles, 1972; Backus, 1988). More specifically, mirid bugs use their stylets to first lacerate the plant cells, simultaneously secreting a watery saliva into the ruptured cellular matter, and then ingest the resulting lacerated/macerated ‘soup’ (Backus et al., 2007). However, certain species – e.g., Helopeltis clavifer (Walker) – move their stylets very little while probing and use the watery saliva itself to macerate plant tissue, a strategy that is termed ‘macerate-and-flush’ feeding (Miles, 1987; Miles & Taylor, 1994). As both feeding strategies of mirid bugs are apparently related to the presence of digestive enzymes in their saliva, knowledge of the extra-oral digestion of mirid bugs is important for fully understanding the interactions between mirid bugs and their host plants (CelorioMancera et al., 2008). Among the various digestive enzymes in the salivary glands of mirid bugs, polygalacturonase (PG) is the most important in the induction of visible plant injury. This enzyme catalyzes the hydrolysis of the a-1,4-glycosidic linkages in polygalacturonic (pectic) acid in the cell walls. Strong & Kruitwagen (1968) determined the presence of PG in the salivary glands of Lygus hesperus Knight, and Strong (1970) further found that the plant injury caused by L. hesperus was mainly due to the activity of PG, rather than tomechanical damage caused by stylet probing. Shackel et al. (2005) injected a crude and partially purified PG solution into alfalfa (Medicago sativa L.) florets and cotton (Gossypium hirsutum L.) flowers and observed damage similar to that caused by Lygus feeding, whereas the control plant injected with a solution lacking PG showed no damage. Meanwhile, Celorio-Mancera et al. (2008) compared the damage of an enzymatically active PG and a mutant inactive PG in alfalfa florets and found that the former elicited Lygus damage symptoms, whereas the latter did not. These two recent studies support the results of previous studies (Strong & Kruitwagen, 1968; Strong, 1970) and highlight the significance of PG proteins in the formation of plant symptoms caused bymirid bug feeding. In China, the mirid bug Apolygus lucorum (Meyer-D€ ur) is a common polyphagous pest (Cao & Wan, 1983; Lu & Wu, 2008). Since the late 1990s, with the wide-scale adoption of transgenic Bt (Bacillus thuringiensis Berliner) cotton and the associated reduction of insecticide use in this crop, A. lucorum population levels have drastically increased, resulting in high infestations in such crops as cotton, local Chinese dates, cherries, grapes, apples, and pears (Lu et al., 2010; Lu &Wu, 2011). Although A. lucorum causes the same damage symptoms as other common mirid bug species (e.g., Lygus spp.) (Lu & Wu, 2008), the role of its salivary gland digestive enzymes in the formation of plant damage symptoms has not been determined. In addition, although the micropressure probe system is suitable for simulating mirid bug feeding on transplanted plants in the laboratory (Shackel et al., 2005), it requires a specialized micropressure probe system which is not suitable for assay*Correspondence: E-mail: yhlu@ippcaas.cn