The Arabidopsis chlorophyll a / b binding protein ( CAB ) gene underexpressed 1 ( cue1 ) mutant underexpresses light-regulated nuclear genes encoding chloroplast-localized proteins. cue1 also exhibits mesophyll-specific chloroplast and cellular defects, resulting in reticulate leaves. Both the gene underexpression and the leaf cell morphology phenotypes are dependent on light intensity. In this study, we determine that CUE1 encodes the plastid inner envelope phosphoenolpyruvate/phosphate translocator (PPT) and define amino acid residues that are critical for translocator function. The biosynthesis of aromatics is compromised in cue1 , and the reticulate phenotype can be rescued by feeding aromatic amino acids. Determining that CUE1 encodes PPT indicates the in vivo role of the translocator in metabolic partitioning and reveals a mesophyll cell-specific requirement for the translocator in Arabidopsis leaves. The nuclear gene expression defects in cue1 suggest that a light intensity-dependent interorganellar signal is modulated through metabolites dependent on a plastid supply of phosphoenolpyruvate.

The Arabidopsis chlorophyll a/b binding protein (CAB) gene underexpressed 1 (cue1) mutant underexpresses light-regulated nuclear genes encoding chloroplast-localized proteins. cue1 also exhibits mesophyll-specific chloroplast and cellular defects, resulting in reticulate leaves. Both the gene underexpression and the leaf cell morphology phenotypes are dependent on light intensity. In this study, we determine that CUE1 encodes the plastid inner envelope phosphoenolpyruvate/phosphate translocator (PPT) and define amino acid residues that are critical for translocator function. The biosynthesis of aromatics is compromised in cue1, and the reticulate phenotype can be rescued by feeding aromatic amino acids. Determining that CUE1 encodes PPT indicates the in vivo role of the translocator in metabolic partitioning and reveals a mesophyll cell-specific requirement for the translocator in Arabidopsis leaves. The nuclear gene expression defects in cue1 suggest that a light intensity-dependent interorganellar signal is modulated through metabolites dependent on a plastid supply of phosphoenolpyruvate.

A number of mutants have been described in Arabidopsis, whose leaf vascular network can be clearly distinguished as a green reticulation on a paler lamina. One of these reticulate mutants was named reticulata (re) by Rédei in 1964 and has been used for years as a classical genetic marker for linkage analysis. Seven recessive alleles of the RE gene were studied, at least four of which seem to be null. Contrary to many other leaf mutants studied in Arabidopsis, very little pleiotropy was observed in the external morphology of the re mutants, whose only aberration obvious at first sight is the reticulation exhibited by cotyledons and leaves. The re alleles caused a marked reduction in the density of mesophyll cells in interveinal regions of the leaf, which does not result from perturbed plastid development in specific cells, but rather from a dramatic change in internal leaf architecture. Loss of function of the RE gene seems to specifically perturb mesophyll cell division in the early stages of leaf organogenesis. The leaves of re mutants were nearly normal in shape in spite of their extremely reduced mesophyll cell density, suggesting that the epidermis plays a major role in regulating leaf shape in Arabidopsis. The RE gene was positionally cloned and found to be expressed in all the major organs studied. RE encodes a protein of unknown function and is identical to the LCD1 gene, which was identified based on the increased sensitivity to ozone caused by its mutant allele lcd1-1. Double mutant analyses suggest that RE acts in a developmental pathway that involves CUE1 but does not include DOV1.

Itti this study, we tested the hypothesis that elevated [CO2] shortens the cell cycle in meristems of Dactylis glomerata, more in a Portuguese population (38°53'N) than in a Swedish population (63°09'N). In the shoot meristem, the cell cycle shortened to about the same extent (= 26%) in both populations exposed to the elevated [CO2] treatment. In the root meristem, the cell cycle shortened by 17% in the Portuguese and by 8% in the Swedish population. However, the proportion of rapidly cycling cells increased in the Portuguese much more than in the Swedish population in both meristems. In the root meristem, there was a 1-86-fold increase in the Portuguese compared with a 1'31-fold increase in the Swedish. In the shoot meristem, the increases were 1-5-3-tbld for the Portuguese and 1-2-fold for the Swedish. The data are consistent in showing that a major response to the elevated [COj] treatment was an increase in the proportion of cells that were cycling and that this was more marked for the Portuguese population. A more general response to the elevated [CO^] treatment was a shortening of the cell cycle regardless of population.

Bundle sheath cells form a sheath around the entire vascular tissue in Arabidopsis leaves and constitute a distinct leaf cell type, as defined by their elongate morphology, their position adjacent to the vein and by differences in their chloroplast development compared to mesophyll cells. They constitute about 15% of chloroplast-containing cells in the leaf. In order to identify genes which play a role in the differential development of bundle sheath and mesophyll cell chloroplasts, a screen of reticulate leaf mutants of Arabidopsis was used to identify a new class of mutants termed dov (differential development of vascular-associated cells). The dov1 mutant clearly demonstrates a cell-specific difference in chloroplast development. Mutant leaves are highly reticulate with a green vascular pattern. The underlying bundle sheath cells always contain normal chloroplasts, whereas chloroplasts in mesophyll cells are abnormal, reduced in number per cell and seriously perturbed in morphology at the ultrastructural level. This demonstrates that differential chloroplast development occurs between the bundle sheath and mesophyll cells in the Arabidopsis leaf.