Calcium oxalate crystal morphology

Plants invest considerable resources of carbon and calcium in crystal formation, indicating that it is an important basic process in growth and development. The diversity of crystal shapes, as well as their prevalence and spatial distribution, have led to several hypotheses regarding crystal function in plants. The proposed functions include roles in ion balance, in plant defense, in tissue support, in detoxification, and in light gathering and reflection [1xCalcium oxalate crystals in plants. Franceschi, V.R. and Horner, H.T. Bot. Rev. 1980; 46: 361–427Crossref | Scopus (480)See all References][1]. The primary role of crystal formation might vary depending on the plant in question. Although calcium oxalate crystal formation has intrigued scientist for many years, knowledge of many aspects of its formation and function are unknown. Crystal formation does not appear to be a simple random precipitation event [2xCell-mediated crystallization of calcium oxalate in plants. Webb, M.A. Plant Cell. 1999; 11: 751–761PubMedSee all References][2]. Instead, crystals of specific morphologies are formed in a controlled and defined fashion. Most crystals can be classified into one of five categories based on their morphology: crystal sand, raphide, druse, styloid and prismatic [1xCalcium oxalate crystals in plants. Franceschi, V.R. and Horner, H.T. Bot. Rev. 1980; 46: 361–427Crossref | Scopus (480)See all References][1]. Specific variations in crystal shape can occur within each classification. Such variations of the prismatic crystal morphology can be observed in the leaves of different legumes, such as Trifolium pratense, Vigna unguiculata and Vicia faba (Fig. 1Fig. 1). The mechanisms controlling the morphology of a crystal are unknown.Fig. 1A comparison of crystals from selected legumes. Scanning electron micrographs of calcium oxalate crystals from Trifolium pratense (a), Vigna unguiculata (b), Vicia faba (c), wild-type Medicago truncatula (d) and from M. truncatula crystal morphology defective mutatnts cmd 4 (e) and cmd 6 (f). Each of the crystal shapes from the M. truncatula plants (d–f) match those present in other legumes (a–c). Scale bar = 1 μm (a, b, d–f) and 10 μm (c).View Large Image | Download PowerPoint SlideAs a step toward elucidating the mechanisms regulating crystal morphologies, my laboratory has recently initiated a mutant screen using the model legume, Medicago truncatula. To date, the screen has led to the identification of several crystal morphology defective (cmd) mutants [3xSee all References][3]. Characterization of the cmd mutants has revealed that the introduction of a single point mutation, through EMS-mutagenesis, can lead to the generation of M. truncatula plants with crystal shapes that match those present in other legumes (Fig. 1Fig. 1). For example, the crystals present in cmd 4 and cmd 6 appear similar in overall morphology to those of Vigna unguiculata and Vicia faba, respectively (Fig. 1Fig. 1). Wild-type M. truncatula crystals look identical to Trifolium pratense (Fig. 1Fig. 1). Based on such comparisons, we hypothesize that simple point mutations contributed to the evolution of the various prismatic crystal morphologies commonly observed in legumes today. Such findings emphasize that the control of the morphology of a crystal is a tightly regulated genetic process and that a simple point mutation can drastically alter the size and shape of the crystal. The availability of the cmd mutants should open new avenues of investigation into the mechanisms regulating crystal shapes and sizes.