SummaryThe rth3 (roothairless 3) mutant is specifically affected in root hair elongation. We report here the cloning of the rth3 gene via a PCR-based strategy (amplification of insertion mutagenized sites) and demonstrate that it encodes a COBRA-like protein that displays all the structural features of a glycosylphosphatidylinositol anchor. Genes of the COBRA family are involved in various types of cell expansion and cell wall biosynthesis. The rth3 gene belongs to a monocot-specific clade of the COBRA gene family comprising two maize and two rice genes. While the rice (Oryza sativa) gene OsBC1L1 appears to be orthologous to rth3 based on sequence similarity (86% identity at the protein level) and maize/rice synteny, the maize (Zea mays L.) rth3-like gene does not appear to be a functional homolog of rth3 based on their distinct expression profiles. Massively parallel signature sequencing analysis detected rth3 expression in all analyzed tissues, but at relatively low levels, with the most abundant expression in primary roots where the root hair phenotype is manifested. In situ hybridization experiments confine rth3 expression to root hair-forming epidermal cells and lateral root primordia. Remarkably, in replicated field trials involving near-isogenic lines, the rth3 mutant conferred significant losses in grain yield.

SummaryEtched1 (et1) is a pleiotropic, recessive mutation of maize that causes ®ssured and cracked mature kernels and virescent seedlings. Microscopic examinations of the et1 phenotype revealed an aberrant plastid development in mutant kernels and mutant leaves. Here, we report on the cloning of the et1 gene by transposon tagging, the localization of the gene product in chloroplasts, and its putative function in the plastid transcriptional apparatus. Several alleles of Mutator (Mu)-induced et1 mutants, the et1-reference (et1-R) mutant, and Et1 wild-type were cloned and analyzed at the molecular level. Northern analyses with wildtype plants revealed that Et1 transcripts are present in kernels, leaves, and other types of tissue, and no Et1 expression could be detected in the et1 mutants analyzed. The ET1 protein is imported by chloroplasts and has been immunologically detected in transcriptionally active chromosome (TAC) fractions derived from chloroplasts. Accordingly, the relative transcriptional activity of TAC fractions was signi®cantly reduced in chloroplasts of et1-R plants. ET1 is the ®rst zinc ribbon (ZR) protein shown to be targeted to plastids. With regard to its localization and its striking structural similarity to the eukaryotic transcription elongation factor TFIIS, it is feasible that ET1 functions in plastid transcription elongation by reactivation of arrested RNA polymerases.Keywords: etched1, plastid nucleoids, transcriptional active chromosome, transposon tagging, plastid transcription, TFIIS. IntroductionThe analysis of genes involved in the development of the maize endosperm is of particular interest in order to understand the structural and regulatory features of seed growth. In maize, many mutants and a number of genes affecting tissue and organ development have been isolated. Numerous mutants have been described, which exhibit abnormal endosperm development (for review: Coe et al., 1988). Among these, a few are pleiotropic and thus of particular interest because they affect the development of different tissue types. Etched1 (et1) is one such mutation, which affects the development of kernels as well as of seedlings.Etched1 is a recessive mutation that was ®rst identi®ed and described by Stadler (1940). The mutant reference allele (et1-reference (et1-R)) was isolated from the progeny of a population of maize plants pollinated with X-ray irradiated pollen. Kernels homozygous for the et1-R allele are ®ssured because of depressions and crevices on the endosperm surface (Figure 1a±c). Prior biochemical and structural analyses of the et1-R kernels revealed that starch synthesis is reduced and starchless endosperm cells are present around the cracks and scars in the kernels (Figure 1; Sangeetha and Reddy, 1991). The et1 kernel phenotype becomes visible approximately 15 days after pollination (DAP). The degree of etching differs among the kernels on an ear and can vary from a weak to a very severe phenotype. This phenotypic variation is apparently not correlated with any speci®c genetic background ...