Epifluorescence and stereomicroscopy of trichomes associated with resistant and susceptible host plant genotypes of the asian citrus psyllid (Hemiptera: Liviidae), Vector of citrus greening disease bacterium

Abstract

Background and Significance:Foliar trichomes (tiny hair-like structures) are part of the plant defense mechanisms that may confer resistance to some herbivore pests. Trifoliate orange, Poncirus trifoliata, is a genotype resistant to infestations by the Asian citrus psyllid, Diaphorina citri (Hemiptera: Liviidae), vector of the economically important citrus greening (huanglongbing) disease bacterium. We discovered that dense trichomes are associated with young leaves of trifoliate orange plants and hypothesized that these might be responsible for reduced infestation by this psyllid.Materials and Methods:Epifluorescence and stereomicroscopy were used to study the density and structure of trichomes associated with young flush leaves and stems of trifoliate orange and of five other plant genotypes that are highly susceptible to colonization by the psyllid: lemon, grapefruit, sweet orange, curry leaf, and orange jasmine.Results:Simple unicellular trichomes were observed at moderate-to-large densities on young leaves and stems of each genotype except lemon and sweet orange, which had considerably fewer trichomes. Trichomes were generally abundant on young leaves of curry leaf and orange jasmine, two genotypes that are often heavily colonized by the psyllid. Although we did not quantify oviposition rates on these genotypes, we observed that psyllid females deposited eggs on young leaves, buds, and stems regardless of the density of trichomes present, sometimes directly within or close to a dense bed of trichomes.Conclusions:While trichomes were moderately abundant on young leaves of trifoliate orange, our results strongly suggest that these trichomes may play little or no role in reduced colonization by the psyllid on this genotype.

Resistance of True Citrus species to Diaphorina citri

“…Trichome density was a morphological trait assessed in the genotypes used in the antibiosis assay. Although E. glauca had a significantly higher trichome density than Citrus × sinensis , which in turn was considered glabrous (without trichomes), they were both oviposited similarly, suggesting that trichome density did not influence D. citri oviposition, which corroborates the results in the literature comparing this trait in Citrus types versus P. trifoliata , M. paniculata , and B. koenigii 39 . Other morphological mechanisms, such as shoot architecture and tissue hardness, may be involved in D. citri oviposition preference 61,62 .…”

“…51 No studies have associated the resistance of Citrinae genotypes to D. citri with trichome density. In a previous study, trichome density and sizes of six cultivars of the Rutaceae species had little to no role in reducing D. citri oviposition, 39 although the authors did not assess the behavior and development of nymphs. Hence, to the best of our knowledge, this is the first study to suggest that high trichome densities in flush shoots may affect D. citri nymphal viability in Citrinae genotypes.…”

“…Although E. glauca had a significantly higher trichome density than Citrus × sinensis, which in turn was considered glabrous (without trichomes), they were both oviposited similarly, suggesting that trichome density did not influence D. citri oviposition, which corroborates the results in the literature comparing this trait in Citrus types versus P. trifoliata, M. paniculata, and B. koenigii. 39 Other morphological mechanisms, such as shoot architecture and tissue hardness, may be involved in D. citri oviposition preference. 61,62 Chemical compounds produced by the secondary metabolism of citrus plants and the nutritional quality of shoots are other possible mechanisms involved in the deterrence to D. citri oviposition in some of the True Citrus species.…”

“…37,38 In Citrus and Poncirus, trichomes have little to no role in deterring oviposition by D. citri. 39 However, to the best of our knowledge, the influence of trichome density on the shoots of Citrinae species on D. citri development has not been studied.…”

“…Trichome density is a morphological trait that may serve as a defense against insect herbivory 33–36 thereby interfering with insects landing, walking, and feeding on plant surfaces 37,38 . In Citrus and Poncirus , trichomes have little to no role in deterring oviposition by D. citri 39 . However, to the best of our knowledge, the influence of trichome density on the shoots of Citrinae species on D. citri development has not been studied.…”