Photosynthetic carbon metabolism during leaf ontogeny in Zea mays L.: Enzyme studies

Cellular pattern of photosynthetic gene expression in developing maize leaves

Leaf development in C4 plants such as maize involves the differentiation of two photosynthetic cell types [bundle sheath (BS) and mesophyll (M)] to form Kranz-type leaf anatomy. This cellular dimorphism partitions photosynthetic activities so that each enzyme of the C4 pathway accumulates only in the appropriate cell type. We have exploited this property to study BS and M cell interactions in developing maize leaves. Our previous studies showed that C4 proteins appear concurrently with the appearance of Kranz anatomy. To look at earlier events in BS and M cell development we have used three of the corresponding C4 mRNAs as cell-specific markers. We have followed, in situ, the accumulation of malic enzyme (ME), phosphoenolpyruvate carboxylase (PEPCase), and ribulose bisphosphate carboxylase (RuBPCase) mRNAs in developing leaves of both normal and mutant argentia (ar) maize. We have isolated a partial cDNA clone for maize ME to examine ME mRNA expression. We show that throughout the development of light-grown seedlings, all three mRNAs accumulate in a cell-specific fashion in both normal and ar leaves. The pattern of C4 mRNA accumulation longitudinally along the veins, laterally across the leaf, and locally around individual veins reveals the spatial and temporal sequence of BS and M cell development. BS cell-specific mRNAs accumulate around developing veins before Kranz anatomy is evident morphologically. Our analysis of the ar mutant, in which C4 mRNA appearance is delayed relative to the appearance of Kranz anatomy, demonstrates first that BS and M cells develop in clusters across the leaf blade and second that BS cells surrounding any individual vein are activated asynchronously. We discuss our results in relation to models and mechanisms of BS and M cell development.

The appearance of photosynthetic proteins in developing maize leaves

The appearance of photosynthetic proteins was directly measured in developing maize (Zea mays L.) leaves. Third leaves of 10-14-d-old seedlings were dissected into six successive sections from the basal meristem to the tip of the leaf. The membrane and soluble proteins were separated by polyacrylamide gel electrophoresis and then transferred onto cyanogen bromide paper. After transfer of membrane proteins the paper was reacted with antisera raised against the light-harvesting chlorophyll a/b protein of photosystem II, the chlorophyll a-binding protein of reaction center P-700 of photosystem I and the α-subunit of chloroplast-coupling factor 1. The blots of soluble proteins were reacted with antisera raised against the electron-transport proteins plastocyanin and Fe-NADP reductase (EC 1.6.7.1), the carbon-fixing enzymes ribulose-1,5-bisphosphate carboxylase (EC 4.1.1.39) and phosphoenolpyruvate carboxylase (EC 4.1.1.31), as well as pyruvate orthophosphate dikinase (EC 2.7.9.1). The schedule of appearance of proteins shows that the light-harvesting and ATP-generating proteins are present in the most immature segments at the leaf base and accumulate rapidly as the cells mature. The carbon-reducing enzymes, however, appear only in tissue that has differentiated into mesophyll and bundle-sheath cells.