RESUMO
Throughout most of its growth and development, Amaranthus tricolor produces fully green leaves. However, near the onset of flowering, unique leaves emerge that consist of three distinct color regions: green apices, yellow middle regions, and red basal regions. The green apices are identical to fully green leaves in terms of pigment composition, photosynthetic function, and C4 gene expression. The yellow and red regions possess greatly reduced levels of chlorophyll and they lack photosynthetic activity. The absence of photosynthetic capacity in the nongreen leaf regions was associated with three distinct alterations in C4 gene expression. First, there was a reduction in the translation of C4 polypeptides, and in the yellow regions synthesis of the ribulose-1,5-bisphosphate carboxylase small subunit occurred in the absence of large subunit synthesis. Second, there was a reduction in the relative transcription rates of two plastid-encoded photosynthetic genes, rbcL and psbA. Third, there was a loss of bundle-sheath cell-specific accumulation of the rbcL and RbcS mRNAs (but not the polypeptides, which remained bundle-sheath-specific). This study indicates that alterations in photosynthetic activity or developmental processes responsible for the loss of activity can influence C4 gene expression at multiple regulatory levels.
RESUMO
Plants that utilize the highly efficient C4 photosynthetic pathway possess two types of specialized leaf cells, the mesophyll and bundle sheath. In mature leaves of amaranth, a dicotyledonous C4 plant, ribulose 1,5-bisphosphate carboxylase (Rubisco) is localized specifically to the chloroplasts of bundle sheath cells, and is not present in the chloroplasts of mesophyll cells. The cell type-specific expression of the chloroplast-encoded Rubisco large subunit (rbcL) gene, and other representative chloroplastic genes, was investigated by using separated bundle sheath and mesophyll chloroplasts prepared from mature amaranth leaves. One-dimensional SDS-polyacrylamide gel electrophoresis revealed several differences in the polypeptide compositions of the two chloroplast types. Western analysis demonstrated that, as in the intact leaves, the Rubisco LSU polypeptide was present only in chloroplast preparations from bundle sheath cells. Pyruvate orthophosphate dikinase (PPdK), a nuclear-encoded chloroplastic enzyme, was found only in the mesophyll chloroplast preparations. rbcL mRNA was present only in the bundle sheath chloroplast preparations, whereas transcripts for the chloroplast-encoded psbA, psaA-B, and rpl2 genes were present in both chloroplast types. Although the rbcL message accumulated only in bundle sheath chloroplasts, run-on transcription analysis indicated that the rbcL gene was transcribed in both bundle sheath and mesophyll chloroplast preparations. Therefore, differential rbcL gene expression in the isolated C4 chloroplasts is regulated, at least in part, at the post-transcriptional level. Possibly this control is mediated by differential processing or stabilization of the rbcL transcript.
