GSE191235
Taro Raphide-Associated Proteins: Profilin, Allergens and Crystal Growth
Calcium oxalate raphide crystals are found in bundles in intravacuolar membrane chambers of specialized idioblasts cells of most plant families including many crop plants and are found in most tissues. Raphides are needle-shaped, often with barbed ends, and their morphology and biomineralization pattern have been a focus of much research. In the family Araceae, raphides have been proposed to cause acridity in crops such as taro (Colocasia esculenta (L.) Schott). Acridity is irritation that causes swelling of lips, mouth and throat, as well as itchiness and pain when raw or insufficiently cooked tissues are eaten; this response varies among taro consumers. Since raphides in some other foods do not cause acridity, and since acridity can be inactivated by cooking and/or protease treatment, it is possible that a toxin or allergen-like compound is associated with the crystals. Using two-dimensional (2D) gel electrophoresis and mass spectrometry (MS) peptide sequencing of selected peptides from purified raphides and the results from taro apex transcriptome sequencing, we have shown that profilins, known allergens, are present on raphides. One of the five raphide profilins genes was highly expressed in the apex and had a 17-amino-acid insert that significantly increased that profilin's epitope peak. This insert was predicted to be coiled and exposed on the surface of the folded peptide. A second profilin had a 2-amino-acid insert that also had a greater B-cell epitope prediction. The taro profilins showed 83 to 92% similarity to known characterized profilins while other raphide peptides showed greater than 90% similarity to other higher plants. The presence on the raphides of peptides normally associated with mitochrondria (ATP synthase), chloroplasts (chaperonin cpn 60 kDa), cytoplasm (actin, profilin) and vacuole (V-type ATPase) indicates a multistage biocrystallation process ending with possible invagination of the tonoplast and addition of mucilage that may be derived from the Golgi. Actin might play a crucial role in the generation of the needle-like raphides. We also showed that commercial allergen test strips for hazelnuts, where profilin is a secondary allergen, have potential for screening in a plant breeding program to reduce acridity and during food processing to avoid over-cooking. Overall design: Transcriptome of four taro samples from apex leaves were sequenced.