Identification of a new glucosinolate-rich cell type in Arabidopsis flowerstalk

Distribution of K, Ca, Cl, St and P in freeze-dried sections of Arabidopsis flower stalk was analyzed by energy dispersive x-ray imaging. Concentratio ns of these elements in different cell types were quantified by microanalys is of single-cell samples and phloem exudates. Results showed a differentia l pattern of distribution for all five elements. K concentration was found to be highest in the parenchymatous tissue around vascular bundles. Ca and Cl were present mainly in the central part of the flower stalk. P was large ly located in the bundles and in the parenchyma surrounding them. S signal was extraordinary high in groups of cells (S-cells) situated between the ph loem of every vascular bundle and the endodermis. Enzymatic hydrolysis by t hioglucosidase of cell sap collected from S-cells using a glass microcapill ary resulted in the release of glucose, indicating that these cells contain glucosinolates at high (> 100 mM) concentration, which is consistent with the concentration of S (> 200 mM) estimated by x-ray analysis of cell sap s amples. Since their position outside of the phloem is ideally suited for pr otecting the long-distance transport system from feeding insects, the possi ble roles of these cells as components of a plant defense system are discus sed.