High toxin concentration in pollen may deter collection by bees in butterfly-pollinated Rhododendron molle.

BACKGROUNDS AND AIMS: The hypothesis that plants evolve features that protect accessible pollen from consumption by flower visitors remains poorly understood. METHODS: To explore potential chemical defenses against pollen consumption, we examined the pollinator assemblage, foraging behaviour, visitation frequency and pollen transfer efficiency in Rhododendron molle, a highly toxic shrub containing Rhodojaponin III. Nutrient (protein and lipid) and toxic components in pollen and other tissues were measured. KEY RESULTS: Overall in the five populations, floral visits by butterflies and bumblebees were relatively more frequent than visits by honeybees. All foraged for nectar but not pollen. Butterflies did not differ from bumblebees in the amount of pollen removed per visit, but deposited more pollen per visit. Pollination experiments indicated that R. molle was self-compatible, but both fruit and seed production were pollen limited. Our analysis indicated that the pollen was not protein-poor and had a higher concentration of the toxic compound Rhodojaponin III than petals and leaves, which compound was undetectable in nectar. CONCLUSION: Pollen toxicity in Rhododendron flowers may discourage pollen robbers (bees) from taking the freely accessible pollen grains, while the toxin-free nectar rewards effective pollinators, promoting pollen transfer. This preliminary study supports the hypothesis that chemical defense in pollen would be likely to evolve in species without physical protection from pollinivores.

Potential mechanisms involved in ceramide-induced apoptosis in human colon cancer HT29 cells.

OBJECTIVE: To investigate the potential mechanisms of cell death after the treatment with ceramide. METHODS: MTT assay, DNA ladder, reporter assay, FACS and Western blot assay were employed to investigate the potential mechanisms of cell death after the treatment with C2-ceramide. RESULTS: A short-time treatment with C2-ceramide induced cell death, which was associated with p38 MAP kinase activation, but had no links with typical caspase activation or PARP degradation. Rather than caspase inhibitor, Inhibitor of p38 MAP kinase blocked cell death induced by a short-time treatment with ceramide (<12 h). However, inhibition of p38 MAP kinase could not block cell death induced by a prolonged treatment with ceramide (>12 h). Moreover, incubation of cells with ceramide for a long time (>12 h) increased subG1, but reduced S phase accompanied by caspase-dependent and caspase-independent changes including NFkappaB activation. CONCLUSION: Ceramide-induced cell apoptosis involves both caspase-dependent and -independent signaling pathway. Caspase-independent cell death occurring in a relatively early stage, which is mediated via p38 MAP kinase, can progress into a stage involving both caspase-dependent and -independent mechanisms accompanied by cell signaling of MAPKs and NFkappaB.

Mechanisms involved in ceramide-induced cell cycle arrest in human hepatocarcinoma cells.

AIM: To investigate the effect of ceramide on the cell cycle in human hepatocarcinoma Bel7402 cells. Possible molecular mechanisms were explored. METHODS: [3- (4, 5)-dimethylthiazol-2-yl]-2, 5-diphenyltetrazolium bromide (MTT) assay, plasmid transfection, reporter assay, FACS and Western blotting analyses were employed to investigate the effect and the related molecular mechanisms of C2-ceramide on the cell cycle of Bel7402 cells. RESULTS: C2-ceramide was found to inhibit the growth of Bel7402 cells by inducing cell cycle arrest. During the process, the expression of p21 protein increased, while that of cyclinD1, phospho-ERK1/2 and c-myc decreased. Furthermore, the level of CDK7 was downregulated, while the transcriptional activity of PPARgamma was upregulated. Addition of GW9662, which is a PPARgamma specific antagonist, could reserve the modulation action on CDK7. CONCLUSION: Our results support the hypothesis that cell cycle arrest induced by C2-ceramide may be mediated via accumulation of p21 and reduction of cyclinD1 and CDK7, at least partly, through PPARgamma activation. The ERK signaling pathway was involved in this process.