The immutans (im) mutant of Arabidopsis shows a variegated phenotype comprising albino and green somatic sectors. We have cloned the IM gene by transposon tagging and show that even stable null alleles give rise to a variegated phenotype. The gene product has amino acid similarity to the mitochondrial alternative oxidase. We show that the IM protein is synthesized as a precursor polypeptide that is imported into chloroplasts and inserted into the thylakoid membrane. The albino sectors of im plants contain reduced levels of carotenoids and increased levels of the caro-tenoid precursor phytoene. The data presented here are consistent with a role for the IM protein as a cofactor for caro-tenoid desaturation. The suggested terminal oxidase function of IM appears to be essential to prevent photooxidative damage during early steps of chloroplast formation. We propose a model in which IM function is linked to phytoene de-saturation and, possibly, to the respiratory activity of the chloroplast.

The higher plant Arabidopsis thaliana (Arabidopsis) is an important model for identifying plant genes and determining their function. To assist biological investigations and to define chromosome structure, a coordinated effort to sequence the Arabidopsis genome was initiated in late 1996. Here we report one of the first milestones of this project, the sequence of chromosome 4. Analysis of 17.38 megabases of unique sequence, representing about 17% of the genome, reveals 3,744 protein coding genes, 81 transfer RNAs and numerous repeat elements. Heterochromatic regions surrounding the putative centromere, which has not yet been completely sequenced, are characterized by an increased frequency of a variety of repeats, new repeats, reduced recombination, lowered gene density and lowered gene expression. Roughly 60% of the predicted protein-coding genes have been functionally characterized on the basis of their homology to known genes. Many genes encode predicted proteins that are homologous to human and Caenorhabditis elegans proteins.

We used quantitative phase tomography with synchrotron radiation to elucidate the 3D structure of Arabidopsis seeds in their native state. The cells are clearly distinguished, and their internal structure is revealed through local variations in electron density. We visualized a 3D network of intercellular air space that might allow immediate gas exchange for energy supply during germination and͞or serve for rapid water uptake and distribution during imbibition.mature seed structure ͉ synchrotron radiation ͉ x-ray imaging ͉ embryo organs ͉ cell components O ur understanding of seed structure and germination depends critically on insights about the structural arrangement of organs and tissues within the seed. Germination starts with water uptake by the dry seed and is complete when the elongating radicle traverses the seed coat. One of the first changes during imbibition is reestablishment of respiration in mitochondria. This activity is connected to a remarkable initial rise in oxygen consumption that declines afterward until germination is complete; i.e., the radicle traverses the seed coat, and atmospheric oxygen can enter the seed and reach the young growing plantlet (1-3). In seeds, limited seed coat permeability is a strong obstacle to gas exchange (4, 5). Consequently, the oxygen needed for germination to start should be rapidly available within the seed. Oxygen might originate from inside the seed and͞or from water entering the seed at the beginning of imbibition. At present, no data are available concerning the internal oxygen content in mature seeds because the rigidity of the seed coat at that stage precludes measuring this content. However, stored internal oxygen should exist and be important for germination because oxygen production by endogenous photosynthesis is known to influence seed viability and germination. Unlike animals, plants lack specialized circulation systems for oxygen transport, and the question arises whether air (and oxygen) is stored in seeds and by which means it circulates.To answer these questions it is important to know the fine structure of the mature seeds. Histology of seeds is usually based on optical or electron microscopy of slices, the results from adjacent sections being then combined. The cutting and fixation steps alter the tissues, and only one-directional (i.e., 2D) processing is possible. Thus, methods based on slicing and fixation procedures cannot give reliable visualization of small cellular interspaces or indicate the existence of networks of spaces. We therefore looked for noninvasive tomographic techniques that could improve the visualization of cell-to-cell organization in the seed tissues. The tomographic approach consists of acquiring projections of an object along different directions and combining them computationally to obtain a 3D reconstruction of the object. This approach has been successfully used in soft x-ray microscopy, with Fresnel zone plates as objective lenses, providing unique visualization of a whole cell at 60-nm resolution (6, 7). The emer...