John C. Larkin

Arabidopsis GLABROUS1 Gene Requires Downstream Sequences for Function.

The Arabidopsis GLABROUS1 (GL1) gene is a myb gene homolog required for the initiation of trichome development. In situ hybridization revealed that the highest levels of GL1 transcripts were present in developing trichomes. In contrast, previous work had shown that putative promoter sequences from the 5[prime] noncoding region of the GL1 gene directed the expression of a [beta]-glucuronidase (GUS) reporter gene only in stipules. Deletion analysis of the 3[prime] noncoding region of GL1 has identified an enhancer that is essential for GL1 function. Sequences from the region containing the enhancer, in conjunction with GL1 upstream sequences, direct the expression of a GUS reporter gene in leaf primordia and developing trichomes in addition to stipules, indicating that the downstream enhancer is required for the normal expression pattern of GL1.

Roles of the GLABROUS1 and TRANSPARENT TESTA GLABRA Genes in Arabidopsis Trichome Development.

Arabidopsis trichomes are branched, single-celled epidermal hairs. These specialized cells provide a convenient model for investigating the specification of cell fate in plants. Two key genes regulating the initiation of trichome development are GLABROUS1 (GL1) and TRANSPARENT TESTA GLABRA (TTG). GL1 is a member of the myb gene family. The maize R gene, which can functionally complement the Arabidopsis ttg mutation, encodes a basic helix-loop-helix protein. We used constitutively expressed copies of the GL1 and R genes to test hypotheses about the roles of GL1 and TTG in trichome development. The results support the hypothesis that TTG and GL1 cooperate at the same point in the trichome developmental pathway. Furthermore, the constitutive expression of both GL1 and R in the same plant caused trichomes to develop on all shoot epidermal surfaces. Results were also obtained indicating that TTG plays an additional role in inhibiting neighboring cells from becoming trichomes.

The β-tubulin gene family in Zea mays: two differentially expressed β-tubulin genes

Maize β-tubulin are encoded by a large multigene family with at least nine members, as determined by Southern blot analysis. Two expressed genes, represented by the β1 genomic clone and the β2 cDNA clone, were examined in this study. The two genes encode β-tubulins which show 94% sequence identity at the amino acid level. Maize β1 transcript levels were highest in seedling root tip and tissue culture cells, which are both rapidly dividing tissues. No transcripts were detected in non-dividing leaf tissue. In contrast, β2 transcripts were present at relatively high levels in tissue culture cells and at lower levels in seedling root tip and leaf tissue. The electrophoretic mobility of the β2 polypeptide was examined in relation to the constellation of β-tubulin polypeptides on two-dimensional gel western blots of a maize pollen total protein extract. No evidence for post-translational modification of the β-tubulin polypeptides was found in pollen.

Isolation of a cytochrome P450 homologue preferentially expressed in developing inflorescences ofZea mays

Four cDNA clones exhibiting preferential hybridization to transcripts present in developing maize tassels were isolated by differential screening. One of these cDNA clones hybridizes to transcripts detectable only in the shoot apex. The abundance of this transcript is significantly higher in developing inflorescence apices than in vegetative apices. DNA sequence analysis of a 2107 nucleotide cDNA clone corresponding to this transcript revealed that the transcript encodes a polypeptide of 547 amino acids, with a molecular mass of 58.4 kDa. This polypeptide shares significant sequence similarity with members of the cytochrome P450 monooxygenase gene superfamily, including the conserved C-terminal domains typical of the cytochrome P450 monooxygenases.

Floral determination in the terminal bud of the short-day plant Pharbitis nil

Temporal and spatial aspects of floral determination in seedling terminal buds of the qualitative short-day plant Pharbitis nil were examined using a grafting assay. Floral determination in the terminal buds of 6-day-old P. nil seedlings is rapid; by 9 hr after the end of a 14-hr inductive dark period more than 50% of the induced terminal buds grafted onto uninduced stock plants produced a full complement of flower buds. When grafted at early times after the end of the dark period the terminal buds of induced plants produced three discrete populations of plants: plants with no flowers, plants with two axillary flowers at nodes 3 and 4 and a vegetative terminal shoot apex, and plants with five to seven flowers including a terminal flower. The temporal relationship among these populations of plants produced by apices grafted at different times indicates that under our conditions, the region of the terminal bud that will form the axillary buds at nodes 3 and 4 becomes florally determined prior to floral determination of the region of the terminal bud giving rise to the nodes above node 4.

A gene essential for viability and flagellar regeneration maps to the uni linkage group of Chlamydomonas reinhardtii

SummaryWe have isolated a mutant of Chlamydomonas reinhardtii that is both temperature sensitive for viability and temperature sensitive for flagellar regeneration. The mutation (designated tnr1, for temperature-sensitive nonregenerator) has been genetically mapped to a position near uni1 on the uni linkage group (ULG), an unusual genetically circular linkage group consisting primarily of mutations affecting flagellar assembly or function. tnr1 is the first essential gene identified on this linkage group, and is one of the few essential genes affecting flagellar function identified to date. We also find that tnr1 cells are not defective for induction of new tubulin transcripts or protein synthesis during flagellar regeneration at the nonpermissive temperature, and that at least a portion of the unassembled pool of flagellar proteins in mutant cells is assembly-competent at the nonpermissive temperature.