In vitro selection for methomyl resistance in CMS-T maize

WHAT CHIMERAS CAN TELL US ABOUT PLANT DEVELOPMENT

▪ Abstract  The generation and analysis of plant chimeras and other genetic mosaics have been used to deduce patterns of cell division and cell fate during plant development and to demonstrate the existence of clonally distinct cell lineages in the shoot meristems of higher plants. Cells derived from these lineages do not have fixed developmental fates but rely on positional information to determine their patterns of division and differentiation. Chimeras with cells that differ genetically for specific developmental processes have been experimentally generated by a variety of methods. This review focuses on studies of intercellular interactions during plant development as well as of the coordination of cells during meristem function and organogenesis. Recent experiments combining mosaic analysis with molecular analysis of developmental mutants have begun to shed light on the nature of the signals involved in these processes and the mechanisms by which they are transmitted and received among cells.

Molecular interactions ofBipolaris maydisT-toxin and maize

The toxins (T-toxins) produced by the fungal pathogens Bipolaris maydis race T (BmT) and Phyllosticta maydis (Pm) target the mitochondrial receptor, URF13, in maize (Zea mays L.) plants containing the Texas male-sterile cytoplasm (cms-T). URF13, a 13-kDa protein, is the product of the maize mitochondrial gene T-urf13, which is found only in the mitochondrial genome of cms-T maize and is thought to be responsible for cytoplasmically inherited male sterility and disease susceptibility. Pm-toxin binds specifically to URF13 in a cooperative manner, and Pm- and BmT-toxins compete for the same, or overlapping, binding sites. The binding of T-toxin to URF13 causes rapid permcabilization of the inner mitochondrial membrane, which results in leakage of NAD+and other ions from the matrix. A pore consisting of at least six transmembrane α-helices is required for NAD+leakage. Cross-linking experiments showed that URF13 oligomers are present in the mitochondrial membrane. A model of the secondary structure of URF13 proposes that each monomer contains three transmembrane α-helices. Studies combining site-directed mutagenesis and chemical cross-linking of URF13 expressed by Escherichia coli cells indicate that the oligomers are composed of a central core of helices II that line the center of the URF13 pores. Key words: maize cytoplasmic male sterility, URF13, mitochondrial pores, T-toxin receptor, Bipolaris maydis race T, Phyllosticta maydis, Helminthosporium maydis.