Are epicuticular waxes a surface defense comparable to trichomes? A test using two Solanum species and a specialist herbivore

Abstract

Although plants possess a suite of structural defenses, most studies have focused on trichomes. Trichomes can have both pre- and post-ingestive effects and have been consistently found to reduce herbivory. Along with trichomes, a few studies have focused on epicuticular waxes as an important defense; however, manipulated comparisons examining herbivore growth and development is limited. In this study, using two Solanum species (Solanum glaucescens and Solanum macrocarpon) that vary in both defenses, we tested the hypothesis that variation in defenses will affect herbivore feeding, primarily by restricting feeding commencement. We used electron microscopy together with a series of plant- and diet-based manipulative experiments, using tobacco hornworm (Manduca sexta; Lepidoptera: Sphingidae) as the herbivore. We found that S. glaucescens leaves had significantly fewer trichomes and significantly higher wax content when compared to S. macrocarpon. We also found that S. glaucescens waxes acted as a strong physical barrier resulting in lower mass gain and higher mortality of caterpillars compared to S. macrocarpon. Artificial diet manipulation experiments also suggested the possible toxicity of waxes. Collectively, we show that epicuticular waxes can play a significant role as a strong surface barrier and should be examined further.

Revisiting plant defense-fitness trade-off hypotheses using Solanum as a model genus

“…Structural defenses in Solanum mainly consist of trichomes, spines and waxes. An enormous variation in morphology, types, density and dimensions have been reported for trichomes of Solanum (Anjana et al, 2008;Howe and Jander, 2008;Kim et al, 2012;Cho et al, 2017;Kariyat et al, 2018;Kariyat, 2021a, 2022), with variation for density and types on abaxial and adaxial surfaces, with functional consequences for herbivores (Malakar and Tingey, 2000;Horgan et al, 2007;Kariyat et al, 2013;Kaur I. and Kariyat R. R., 2020;Watts and Kariyat, 2021b), a similar trend of interspecific variation and antiherbivore defenses functions have also been found for spines (Kariyat et al, 2017), and epicuticular wax (Watts and Kariyat, 2022). Moving past the structural defense barrier in Solanum, the herbivores are then exposed to a gamut of chemical defenses starting from the leaf surface messengers, Ca 2+ and Reactive Oxygen Species (ROS).…”

Cascading Effects of Cover Crops on the Subsequent Cash Crop Defense against the Polyphagous Herbivore Fall Armyworm (Spodoptera frugiperda)

“…A suite of previous studies has shown that plants employ both chemical and physical defenses against herbivores [ 25 , 28 , 35 ]. Although most studies have focused on how plants elicit chemical defense compounds to deter herbivores, some studies have also shown that physical traits such as trichomes and epicuticular wax impact herbivore feeding [ 28 , 36 ]. We quantified the wax amounts on sorghum leaves planted either in control or cover crop plots to determine if epicuticular wax impacts the feeding of FAW.…”

“…Although not significantly different, there were higher amounts of epicuticular wax in the sorghum leaves planted in the control plots. This, however, might be a probable reason why FAW had higher mass gains when fed on sorghum leaves planted in the cover crop plots [ 28 , 36 , 37 ].…”

“…To test whether the cash crop defenses (from both cover grown and control fields) were affected, we estimated the activity of polyphenol oxidase (PPO). PPO is a widely distributed enzyme that plays a vital role in plant defense against plant diseases and herbivores [ 28 , 29 ]. Quantification of PPO was performed using the equation in the Polyphenol Oxidase Assay Kit manual (Catalog# MBS822343; MyBioSource, San Diego, CA, USA).…”

“…These tubes were vortexed for 1.5 min, after which the leaf punches were removed. The chloroform was allowed to evaporate, after which the microcentrifuge tubes were reweighed to quantify epicuticular wax (mg) [ 28 , 31 ].…”