Juvenile hormone (JH) has an ability to repress the precocious metamorphosis of insects during their larval development. Krüppel homolog 1 (Kr-h1) is an early JH-inducible gene that mediates this action of JH; however, the fine hormonal regulation of Kr-h1 and the molecular mechanism underlying its antimetamorphic effect are little understood. In this study, we attempted to elucidate the hormonal regulation and developmental role of Kr-h1. We found that the expression of Kr-h1 in the epidermis of penultimate-instar larvae of the silkworm Bombyx mori was induced by JH secreted by the corpora allata (CA), whereas the CA were not involved in the transient induction of Kr-h1 at the prepupal stage. Tissue culture experiments suggested that the transient peak of Kr-h1 at the prepupal stage is likely to be induced cooperatively by JH derived from gland(s) other than the CA and the prepupal surge of ecdysteroid, although involvement of unknown factor(s) could not be ruled out. To elucidate the developmental role of Kr-h1, we generated transgenic silkworms overexpressing Kr-h1. The transgenic silkworms grew normally until the spinning stage, but their development was arrested at the prepupal stage. The transgenic silkworms from which the CA were removed in the penultimate instar did not undergo precocious pupation or larval-larval molt but fell into prepupal arrest. This result demonstrated that Kr-h1 is indeed involved in the repression of metamorphosis but that Kr-h1 alone is incapable of implementing normal larval molt. Moreover, the expression profiles and hormonal responses of early ecdysone-inducible genes (E74, E75, and Broad) in transgenic silkworms suggested that Kr-h1 is not involved in the JH-dependent modulation of these genes, which is associated with the control of metamorphosis.
ORCID ID: 0000-0003-4940-288X (S.S.). Jasmonic acid (JA) plays an important role in the induction of herbivore resistance in many plants. However, JA-independent herbivore resistance has been suggested. An herbivore-resistance-inducing substance was isolated from Tobacco mosaic virusinfected tobacco (Nicotiana tabacum) leaves in which a hypersensitive response (HR) was induced and identified as loliolide, which has been identified as a b-carotene metabolite. When applied to tomato (Solanum lycopersicum) leaves, loliolide decreased the survival rate of the two-spotted spider mite, Tetranychus urticae, egg deposition by the same pest, and the survival rate of larvae of the common cutworm Spodoptera litura without exhibiting toxicity against these herbivores. Endogenous loliolide levels increased not only with an infestation by S. litura larvae, but also with the exogenous application of their oral secretions in tomato. A microarray analysis identified cell-wall-associated defense genes as loliolide-responsive tomato genes, and exogenous JA application did not induce the expression of these genes. Suppressor of zeaxanthin-less (szl), an Arabidopsis (Arabidopsis thaliana) mutant with a point mutation in a key gene of the b-carotene metabolic pathway, exhibited the decreased accumulation of endogenous loliolide and increased susceptibility to infestation by the western flower thrip (Frankliniella occidentalis). A pretreatment with loliolide decreased susceptibility to thrips in the JA-insensitive Arabidopsis mutant coronatine-insensitive1. Exogenous loliolide did not restore reduced electrolyte leakage in szl in response to a HR-inducing bacterial strain. These results suggest that loliolide functions as an endogenous signal that mediates defense responses to herbivores, possibly independently of JA, at least in tomato and Arabidopsis plants.
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