Thrips Resistance Screening Is Coming of Age: Leaf Position and Ontogeny Are Important Determinants of Leaf-Based Resistance in Pepper

Abstract

Capsicum is a genus containing important crop species, many of which severely suffer from thrips infestation. Thrips feeding damages leaves and fruits, and often results in virus infections. Only a few insecticides are still effective against thrips, underlining the importance of finding natural resistance in crops. Capsicum is a perennial plant which is usually cultivated for several months, during which time the fruits are harvested. From the young vegetative stage to the mature fruit bearing stage, the plants are at risk to thrips infestation. Constitutive resistance to thrips over the entire ontogenetic development is therefore a key trait for a more sustainable and successful cultivation of the hot and sweet pepper. In addition to ontogeny, leaf position can affect the level of thrips resistance. Pest resistance levels are known to differ between young and old leaves. To our knowledge, no studies have explicitly considered ontogeny and leaf position when screening for constitutive resistance to thrips in Capsicum . In this study we analyze whether ontogeny and leaf position affect leaf-based resistance to Frankliniella occidentalis and Thrips tabaci , in 40 Capsicum accessions, comprising five different species. Our results show that resistance to both thrips species in Capsicum varies with ontogenetic stage. This variation in resistance among ontogenetic stages was not consistent among the accessions. However, accessions with constitutive resistance in both the flowering and fruit ripening stage could be identified. In addition, we found that thrips resistance is overall similar at different leaf positions within the ontogenetic stage. This implies that resistance mechanisms, such as defense compounds, are constitutively present at sufficient levels on all leaf positions. Finally, we found that resistance to F. occidentalis and resistance to T. tabaci were not correlated. This indicates that leaf-based resistance in Capsicum is thrips species-specific. Because of the variation in resistance over ontogeny, identifying Capsicum accessions with resistance over their entire lifespan is challenging. For resistance screening, accounting for leaf position may be less of a concern. To identify the defense mechanisms responsible for thrips resistance, it is important to further analyze and compare resistant and susceptible accessions.

The effect of a thrips resistance QTL in different Capsicum backgrounds

“…T. tabaci and Frankliniella schultzei (Trybom), was found in cowpea, although it was not determined whether each QTL contributed to resistance to both thrips species, or to only one of the tested thrips species (Muchero et al 2010). Contrary to our observations, no correlation was found between resistance to F. occidentalis and T. tabaci in another pepper study (Visschers et al 2019a). However, in that study overall thrips resistance was compared in different Capsicum accessions, while our study focused on the effect of a QTL on thrips larval development within and between different backgrounds.…”

High Concentrations of Very Long Chain Leaf Wax Alkanes of Thrips Susceptible Pepper Accessions (Capsicum spp)

“…The classification of relative thrips resistance or susceptibility of the accessions was based on the leaf disc choice assays (Table 1 , Macel et al 2019 ). This resistance classification was compared with resistance in whole plant thrips performance assays and in no-choice leaf discs feeding damage assays (Visschers et al 2019a ) (Table 1 ). All three thrips tests showed that accession 43 is consistently susceptible, whereas accession 63 and 23 were the most resistant C. annuum accessions in all assays (Table 1 ).…”

“…These experiments were previously published as part of Visschers et al 2019a . The purpose of no-choice assays was to determine thrips resistance of the pepper accessions per se and not thrips preference.…”

Overview of Biotic Stresses in Pepper (Capsicum spp.): Sources of Genetic Resistance, Molecular Breeding and Genomics

“…The latter showed a reducing of thrips reproduction. Moreover, the leaf-based resistance to F. occidentalis and T. tabaci have been demonstrated species-specific, being not correlated [239].…”