Significant progress has been made in elucidating the mechanism of abscisic acid (ABA)-regulated gene expression, including the characterization of an ABA-responsive element (ABRE), which is regulated by basic domain/Leu zipper transcription factors. In addition to the ABRE, a coupling element (CE1) has been demonstrated to be involved in ABAinduced expression. However, a trans factor that interacts with CE1 has yet to be characterized. We report the isolation of a seed-specific maize ABI4 homolog and demonstrate, using a PCR-based in vitro selection procedure, that the maize ABI4 protein binds to the CE-1 like sequence CACCG. Using electrophoretic mobility shift assays, we demonstrate that recombinant ZmABI4 protein binds to the CE1 element in a number of ABA-related genes. ZmABI4 also binds to the promoter of the sugar-responsive ADH1 gene, demonstrating the ability of this protein to regulate both ABA-and sugar-regulated pathways. ZmABI4 complements Arabidopsis ABI4 function, because abi4 mutant plants transformed with the ZmABI4 gene have an ABA-and sugar-sensitive phenotype. Identification of the maize ABI4 ortholog and the demonstration of its binding to a known ABA response element provide a link between ABA-mediated kernel development and the regulation of ABA response genes.

CesA genes are believed to encode the catalytic subunit of cellulose synthase. Identification of nine distinct CesA cDNAs from maize (Zea mays) has allowed us to initiate comparative studies with homologs from Arabidopsis and other plant species. Mapping studies show that closely related CesA genes are not clustered but are found at different chromosomal locations in both Arabidopsis and maize. Furthermore, sequence comparisons among the CesA-deduced proteins show that these cluster in groups wherein orthologs are often more similar than paralogs, indicating that different subclasses evolved prior to the divergence of the monocot and dicot lineages. Studies using reverse transcriptase polymerase chain reaction with gene-specific primers for six of the nine maize genes indicate that all genes are expressed to at least some level in all of the organs examined. However, when expression patterns for a few selected genes from maize and Arabidopsis were analyzed in more detail, they were found to be expressed in unique cell types engaged in either primary or secondary wall synthesis. These studies also indicate that amino acid sequence comparisons, at least in some cases, may have value for prediction of such patterns of gene expression. Such analyses begin to provide insights useful for future genetic engineering of cellulose deposition, in that identification of close orthologs across species may prove useful for prediction of patterns of gene expression and may also aid in prediction of mutant combinations that may be necessary to generate severe phenotypes.Evidence is accumulating to support the notion that some, if not all, of the members of the family of CesA genes in plants encode a glycosyltranferase that plays a key role in the process of cellulose synthesis (for recent reviews, see Brown et al., 1997; Kawagoe and Delmer, 1997, 1998; Delmer, 1999). The deduced proteins from members of this gene family are characterized by the presence of domains that share significant sequence homology with other family 2 glycosyltransferases that are characterized by having conserved motifs surrounding three conserved D residues and a QXXRW motif downstream of D 3 (Campbell et al., 1997). Recent crystallographic evidence supports a model in which the three D residues, in conjunction with a divalent cation, are involved in binding of the UDP-sugar substrate and in catalysis of glycosyltransfer (Charnock and Davies, 1999). In the deduced proteins encoded by most family 2 glycosyltransferases, the domains containing these conserved D residues are consecutive, but the predicted proteins in plants contain a plant-specific conserved and a hypervariable (HVR-2) domain that separate the domains containing these conserved residues. A conserved, extended N-terminal region containing two zinc fingers resembling LIM/Ring domains (Kawagoe and Delmer, 1997) followed by the HVR-1 region also characterizes the plant CesA proteins. Many of these glycosyltransferases, including the plant and bacterial CesA proteins, are predicted to be ancho...

Restriction fragment length polymorphisms have become powerful tools for genetic investigations in plant species. They allow a much greater degree of genome saturation with neutral markers than has been possible with isozymes or morphological loci. A previous investigation employed isozymes as genetic markers to infer the location of genetic factors influencing the expression of quantitative traits in the maize population: (CO159×Tx303)F2. This investigation was conducted to examine the inferences that might be derived using a highly saturated map of RFLP markers and isozymes to detect quantitative trait loci (QTLs) in the same maize F2 population. Marker loci that were associated with QTL effects in this investigation generally corresponded well with previous information where such comparisons were possible. Additionally, a number of previously unmarked genomic regions were found to contain factors with large effects on some plant traits. Availability of numerous marker loci in some genomic regions allowed: more accurate localization of QTLs, resolution of linkage between QTLs affecting the same traits, and determination that some chromsome regions previously found to affect a number of traits are likely to be due to linkage of QTLs affecting different traits. Many of the factors that affected plant height quantitatively in this investigation were found to map to regions also including known sites of major genes influencing plant height. Although the data are not conclusive, they suggest that some of the identified QTLs may be allelic to known major genes affecting plant height.

Retinoblastoma (RB-1) is a tumor suppressor gene that encodes a 105-kDa nuclear phosphoprotein. To date, RB genes have been isolated only from metazoans. We have isolated a cDNA from maize endosperm whose predicted protein product (ZmRb) shows homology to the "pocket" A and B domains of the Rb protein family. We found ZmRb behaves as a pocket protein based on its ability to specifically interact with oncoproteins encoded by DNA tumor viruses (E7, T-Ag, E1A). ZmRb can interact in vitro and in vivo with the replication-associated protein, RepA, encoded by the wheat dwarf virus. The maize Rb-related protein undergoes changes in level and phosphorylation state concomitant with endoreduplication, and it is phosphorylated in vitro by an S-phase kinase from endoreduplicating endosperm cells. Together, our results suggest that ZmRb is a representative of the pocket protein family and may play a role in cell cycle progression. Moreover, certain plant monopartite geminiviruses may operate similarly to mammalian DNA viruses, by targeting and inactivating the retinoblastoma protein, which otherwise induces G1 arrest.Retinoblastoma (RB-1) is a tumor suppressor gene that encodes a nuclear phosphoprotein with a molecular mass of about 105 kDa (1, 2). Inactivation of the RB-1 gene contributes to both familial and sporadic forms of cancer (3). Retinoblastoma and its related p107 and p130 proteins are among the negative regulators of the cell cycle (4, 5). While hypophosphorylated, the Rb protein exerts a growth suppressive effect and arrests cells in G1 phase.Hyperphosphorylation of Rb, or its interaction with viral oncoproteins, prevents Rb from performing its normal functions at G1 phase and enables cells to begin DNA synthesis (6).

Gibberellins (GAs) are phytohormones required for normal growth and development in higher plants. The Dwarf3 (03) gene of maize encodes an early step in the GA biosynthesis pathway. We transposon-tagged the 0 3 gene using Robertson's Mutator (Mu) and showed that the mutant allele d3-2::Mu8 is linked to a Mu8 element. The DNA flanking the Mu8 element was cloned and shown to be linked to the d3 locus by mapping in a high-resolution population developed by selecting for recombination between d3 and linked genetic marken. To establish unambiguously the identity of the cloned gene as 03, a second mutant allele of 0 3 (d3-4) was also cloned and characterized using the d3-2::MuB sequences as a probe. d3-4 was found to have a nove1 insertion element, named Sleepy, inserted into an exon. A third mutant allele, d3-1, which has the same size 3' restriction fragments as d3-4 but different 5' restriction fragments, was found to contain a Sleepy insertion at the same position as d3-4. On the basis of the pedigree, Sleepy insertion, and restriction map, d3-1 appears to represent a recombinational derivative of d3-4. The 0 3 gene encodes a predicted protein with significant sequence similarity to cytochrome P450 enzymes. Analysis of D3 mRNA showed that the 03 transcript is expressed in roots, developing leaves, the vegetative meristem, and suspension culture cells. We detected reduced 0 3 mRNA levels in the mutant allele d3-5.