Differential expression of genes in apical and basal tissues of expanding tobacco leaves

The leaves of dicots normally have no intercalary meristem active for a long time, as it commonly occurs in monocots. In dicots, the meristematic growth normally ends when the leaves are still very young and cell expansion takes over to ensure the subsequent leaf growth. The presence of more differentiated cells at the apex and less differentiated cells at the base has previously been demonstrated in young tobacco leaves. Such a base/tip gradient is made particularly evident by an overlapping gradient of chloroplast differentiation. By using a differential screening of tobacco "base" and "tip" cDNA libraries, a number of clones encoding putative regulatory proteins were isolated. The expression profile of two of them, encoding an Aux/IAA protein and a Ran-like GTP-binding protein, respectively, was analysed in detail and compared to that of two clones used as markers of base (a histone cDNA) and tip (a RUBISCO small subunit cDNA), respectively. The finding that the genes encoding regulatory proteins are differentially expressed in young growing leaves, while no gradient is found in fully mature leaves, supports the idea that the observed gradient of tissue differentiation might be due to the combined activity of these and other genes with regulatory functions