A panel of monoclonal antibodies that recognize a class of cell wall proteins, related to the hydroxyproline‐rich glycoproteins, has been assembled and characterized in relation to their restricted patterns of binding amongst the cells comprising the carrot root apex. The occurrence of the epitopes at the surface of cells and intercellular spaces in the region of the apex between the meristematic initials and the region of cell expansion indicates dynamic patterns that reflect aspects of the development of the anatomical pattern. The monoclonal antibody JIM11 reacts with the surface of cells in the central root cap and the region of the meristem. As the cortex/stele boundary becomes established the reactivity is seen in the inner cortical layers and finally in the whole cortex. Later in development the JIM11 epitope is also expressed by two pairs of pericycle cell files adjacent to the phloem region and also by the epidermis. The JIM12 monoclonal antibody is unreactive with cells in the region of the root cap and the meristem but is reactive with intercellular spaces formed at the junction of the oblique and radial walls in the double‐layered sectors of the pericycle opposite the xylem poles. This epitope is also transiently expressed by the two phloem sieve tube element mother cells. Later in development JIM12 recognizes the future metaxylem cells. The antibody JIM20 recognizes all the cells and intercellular spaces recognized by JIM11 and JIM12. Immuno‐chemical analyses indicate cross‐reactivity with carrot taproot extensin and Solanaceous lectins.
Some kinetic properties of partially purified phosphoenolpyruvate carboxylase (PEPCase) from guard-cell and mesophyll-cell protoplasts of Commelina communis are described. The PEPCase activity inherent to each cell type was determined and the apparent K m (phosphoenolpyruvate) and K i (malate) were compared. Malate sensitivity was much higher (K i malate 0.4 mol m(-3)) in the extract of guard-cell protoplasts than in that of mesophyllcell protoplasts (K i malate 4.2 mol m(-3)). The stimulation of activity by glucose-6-phosphate in the presence of malate ('deinhibition') was also investigated in extracts from both cell types and was found to be similar to previously reported results with epidermal tissue. The effect of contamination of an extract of guard-cell protoplasts with mesophyll-cell protoplasts was measured in the presence and absence of malate. It was found that a small amount to mesophyll-cell contaminant appears to desensitize the malate inhibition of PEPCase from guard-cell protoplasts. It is concluded that experiments which use epidermal tissue to study guardcell PEPCase may give misleading information as a consequence of mesophyll contamination.
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