Amaranth calcium oxalate crystals are associated with chloroplast structures and proteins

• Published in: Microscopy research and technique 2022-11, Vol.85 (11), p.3694-3706
• Main Authors: Cerritos‐Castro, Ivan Takeshi, Patrón‐Soberano, Araceli, Bojórquez‐Velázquez, Esaú, González‐Escobar, Jorge Luis, Vargas‐Ortiz, Erandi, Muñoz‐Sandoval, Emilio, Barba de la Rosa, Ana Paulina
• Format: Article
• Language: English
• Subjects: Amaranth; Amaranthus; biomineralization; Calcium; Calcium homeostasis; Calcium oxalate; calcium oxalate crystals; Carbon; Carbon sources; Cell walls; chloroplast degradation; Chloroplasts; Crystal structure; Crystals; Electron microscopy; Homeostasis; Immunohistochemistry; Leaves; Mesophyll; nLC–MS/MS; Oxalic acid; Parenchyma; Proteins; Ribulose-bisphosphate carboxylase; RuBisCO; Thylakoid membranes; Ultrastructure
• ISSN: 1059-910X; 1097-0029
• DOI: 10.1002/jemt.24221

Calcium oxalate (CaOx) crystals in plants are formed in crystal idioblasts cells and have specific geometric shapes. Their proposed functions include calcium homeostasis and carbon source, among others. Amaranth is a plant that presents high tolerance to abiotic stresses and accumulates considerable amounts of CaOx crystals; however, few studies have focused on characterizing the crystals ultrastructure and none is related to identifying proteins bound to them. This information is of great interest to understand the mechanisms related to CaOx crystal formation and to support their proposed functions. Thus, this work aimed to characterize CaOx crystals in amaranth leaves. Crystals were purified and the proteins bound to them were isolated and identified by nLC–MS/MS. Leaf sections were analyzed by light and electron microscopy. The identified proteins were related to the chloroplast such as ATPb synthase, RuBisCO large subunit, and cell wall‐related proteins, which were validated by immunohistochemistry and immunogold labeling. In addition, it was observed that CaOx crystal idioblasts were formed from parenchyma cells associated with mesophyll and veins, in which the thylakoid membranes of degraded chloroplasts turned into crystal chambers. These results significantly advance our understanding of the mechanisms of CaOx crystal formation and the potential function as an alternative carbon source in leaves. Plants form calcium oxalate crystals with particular geometric shapes. These crystals grow in crystalline chambers within specialized cells. It has been hypothesized that these chambers may control crystal shape. Our results support this hypothesis and further suggest that the crystalline chambers are formed from degraded chloroplast thylakoids.