Submitted to: Plant Biology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/18/2001
Publication Date: 1/1/2002
Citation: Volk, G.M., V. Lynch-Holm, T.A. Kostman, and V.R. Franceshi. 2002. The role of druse and raphide calcium oxalate crystals in tissue calcium regulation in Pistia stratiotes leaves. Plant Biology 4:34-45. Interpretive Summary: Calcium oxalate crystals form in the leaves of the aquatic plant Pistia stratiotes. Excess calcium can be sequestered in crystals, limiting the deleterious effects of high calcium levels in plant cells. By withholding calcium from the nutrient solution, we altered the quantity and types of calcium oxalate crystals that formed. The formation of the spherical druse ecrystals was more dependent upon Ca supply than the formation of needle- like raphide crystals. Immature druse crystals are covered by microfacets; we believe these are the sites of facets initiation in developing crystals. These microfacets are not observed in crystals from mature leaves. We also show evidence that calcium is deposited on the entire surface of the druse crystal. This is in contrast to raphide crystals, which elongate bidirectionally from either end of the needle-like components. Druse crystals appear to dissolve when calcium was not present in the media. These studies demonstrate that excess calcium can be stored as calcium oxalate, the calcium can be remobilized under certain conditions, and different forms of calcium oxalate have different roles in the process of bulk calcium regulation
Technical Abstract: Ca oxalate crystal formation was examined in Pistia stratiotes L. leaves during excess Ca and Ca deficient conditions. Pistia produces druse crystal idioblasts in the adaxial mesophyll and raphide idioblasts in the abaxial aerenchyma. Raphide crystals were previously found to grow bidirectionally, and here we show that Ca is incorporated along the entire surfaces of developing druse crystals, which are coated with membrane boun microprojections. Leaves formed on plants grown on 0 Ca medium have fewer and smaller druse crystals than leaves formed under 5 mM Ca ("control") conditions, while raphide crystal formation is completely inhibited. When plants were moved from 0 to 15 mM ("high") Ca, the size and number of crystals in new leaves returned to (druse) or exceeded (raphide) control levels. High Ca also induced formation of druse, but not raphide, crystals in differentiating chlorenchyma cells. When plants were transferred from 15 mM Ca to 0 Ca, young druse crystals were preferentially partially dissolved. Oxalate oxidase, an enzyme that degrades oxalate, greatly increased under Ca deficient conditions and was localized to the crystal surfaces. The more dynamic nature of druse formation and dissolution may be due to hydration form and mechanism of Ca deposition. In contrast, raphide idioblasts have developmental constraints that interfere with a more flexible response to changing Ca. These studies demonstrate that excess Ca can be stored as Ca oxalate, the Ca can be remobilized under certain conditions, and different forms of Ca oxalate have different roles in this process of bulk Ca regulation.