CALCIUM OXALATE IN PLANTS: Formation and Function

Abstract

Calcium oxalate (CaOx) crystals are distributed among all taxonomic levels of photosynthetic organisms from small algae to angiosperms and giant gymnosperms. Accumulation of crystals by these organisms can be substantial. Major functions of CaOx crystal formation in plants include high-capacity calcium (Ca) regulation and protection against herbivory. Ultrastructural and developmental analyses have demonstrated that this biomineralization process is not a simple random physical-chemical precipitation of endogenously synthesized oxalic acid and environmentally derived Ca. Instead, crystals are formed in specific shapes and sizes. Genetic regulation of CaOx formation is indicated by constancy of crystal morphology within species, cell specialization, and the remarkable coordination of crystal growth and cell expansion. Using a variety of approaches, researchers have begun to unravel the exquisite control mechanisms exerted by cells specialized for CaOx formation that include the machinery for uptake and accumulation of Ca, oxalic acid biosynthetic pathways, and regulation of crystal growth.

Certain Biominerals in Leaves Function as Light Scatterers

“…We note that the calcium oxalate crystals in C. illinoinensis differ from other calcium oxalate druses present in leaves, which are typically smaller (less than 20μm), unevenly distributed, and are present in different anatomical locations. [3] We performed modulated fluorometry experiments on C. illinoinensis leaves, and found that indeed light irradiated through the calcium oxalate druses causes slower fluorescence yield induction kinetics and lower steady-state fluorescence yield (Figure 5), similar to what was observed for cystoliths in F. microcarpa ( Figure 3A-B). Thus the calcium oxalate druses also scatter light into the mesophyll, reducing the light flux in the outer tissue and enriching the lower tissue with photons.…”

“…[1,2] Calcium oxalate and silica are known to take part in plant calcium regulation, heavy metal detoxification, mechanical support and plant protection. [3,4] Oval-shaped and relatively large calcified bodies, known as cystoliths, occur in few, albeit large families of angiosperms, and exhibit common features in different species. [5,6] Two centuries have elapsed since cystoliths were discovered, [7] and although physiological roles such as carbon or calcium reservoirs or pH regulators have been proposed, [5,8] no definitive evidence for cystolith function has been presented.…”

Foliar micro-morphology of <i>Gasteria bicolour</i> haw. (Asphodelaceae) from South Africa

“…A number of roles for crystal formation have been proposed, i.e. roles in cellular ion balance, in plant defense against herbivore, in tissue rigidity and support, in detoxification of oxalic acid or aluminium (Franceschi & Nakata, 2005). Hence, the co-occurrence of aluminum suggests the potential role of the crystals in detoxification of aluminum and heavy metals, as reported previously (Mazen, 2004).…”

“…Based on the relatively high carbon and oxygen peaks and the lower sulphur and obvious calcium peaks; calcium oxalate was considered as the major component and calcium sulfate as the minor component. Mineral formation in plants is common (Franceschi & Nakata, 2005) and the most abundant minerals formed by plants are silica, calcium carbonate and calcium oxalate. Although calcium is an essential plant nutrient, excess calcium is often precipitated in the form of calcium salts such as oxalate, carbonate, sulfate, phosphate, silicate, citrate, tartrate and malate (Weiner & Dove, 2003).…”

Plants and Light Manipulation: The Integrated Mineral System in Okra Leaves

“…Calcium oxalate (whewellite and weddellite) and to a lesser extent amorphous calcium carbonate (cystoliths) are widespread in a large variety of higher plant leaves 4. Many other roles, in addition to light manipulation, have been ascribed to calcium oxalates and cystoliths in leaves, including calcium regulation, leaf defense, detoxification of heavy metals, and an internal CO 2 source 5, 6, 7, 8. Silica deposition (phytoliths) can occur in any part of the leaf epidermis and in the vascular tissue 9.…”