EXPRESSION OF THE C-4 PATTERN OF PHOTOSYNTHETIC ENZYME ACCUMULATION DURING LEAF DEVELOPMENT IN ATRIPLEX ROSEA (CHENOPODIACEAE)

Immunolocalization of the bundle sheath-specific enzyme, ribulose-1,5- bisphosphate carboxylase/oxygenase (RuBPCase), and of the mesophyll-sp ecific enzyme, phosphoenolpyruvate carboxylase (PEPCase), was used to follow development of the C-4 pattern of photosynthetic enzyme express ion during leaf growth in Atriplex rosea. The leaf tissue used for thi s characterization was also used in a parallel ultrastructural study, so that the temporal coordination of developmental changes in enzyme e xpression and cell structure could be monitored. Bundle sheath-specifi c accumulation of RuBPCase occurs early, at the time that bundle sheat h tissue is delimited from the ground meristem, and follows the order of vein initiation. PEPCase proteins were detected 2-4 days after the first appearance of RuBPCase. PEPCase accumulation is restricted to gr ound meristem cells that are in direct contact with bundle sheath tiss ue and that will become C, mesophyll; PEPCase was never found in more distant ground tissue. This pattern suggests that, while bundle sheath -specific accumulation of RuBPCase coincides with formation of the app ropriate precursor cells, PEPCase expression is delayed until mesophyl l tissue reaches a critical developmental stage. Cell-specific express ion of both photosynthetic enzymes occurs well before the striking ana tomical divergence of bundle sheath and mesophyll tissues, suggesting that biochemical compartmentation might serve as a developmental signa l for subsequent structural differentiation.