Biomedical subjects

Anthony A Millar

Publications and source records attributed to Anthony A Millar.

5 recordsLinked to original sources

PubMed2006

Seed dormancy and ABA metabolism in Arabidopsis and barley: the role of ABA 8'-hydroxylase.

We have investigated the relationship between seed dormancy and abscisic acid (ABA) metabolism in the monocot barley and the dicot Arabidopsis. Whether dormant (D) or non-dormant (ND), dry seed of Arabidopsis and embryos of dry barley grains all had similarly high levels of ABA. ABA levels decreased rapidly upon imbibition, although they fell further in ND than in D. Gene expression profiles were determined in Arabidopsis for key ABA biosynthetic [the 9-cis epoxycarotenoid dioxygenasegene family] and ABA catabolic [the ABA 8'-hydroxylase gene family (CYP707A)] genes. Of these, only the AtCYP707A2 gene was differentially expressed between D and ND seeds, being expressed to a much higher level in ND seeds. Similarly, a barley CYP707 homologue, (HvABA8'OH-1) was expressed to a much higher level in embryos from ND grains than from D grains. Consistent with this, in situ hybridization studies showed HvABA8'OH-1 mRNA expression was stronger in embryos from ND grains. Surprisingly, the signal was confined in the coleorhiza, suggesting that this tissue plays a key role in dormancy release. Constitutive expression of a CYP707A gene in transgenic Arabidopsis resulted in decreased ABA content in mature dry seeds and a much shorter after-ripening period to overcome dormancy. Conversely, mutating the CYP707A2 gene resulted in seeds that required longer after-ripening to break dormancy. Our results point to a pivotal role for the ABA 8'-hydroxylase gene in controlling dormancy and that the action of this enzyme may be confined to a particular organ as in the coleorhiza of cereals.

Abscisic Acid↗

PubMed2005

Plant and animal microRNAs: similarities and differences.

Plant and animal microRNAs (miRNAs) are evolutionarily ancient small RNAs, approximately 19-24 nucleotides in length, that are generated by cleavage from larger highly structured precursor molecules. In both plants and animals, miRNAs posttranscriptionally regulate gene expression through interactions with their target mRNAs, and these targets are often genes involved with regulating key developmental events. Despite these similarities, plant and animal miRNAs exert their control in fundamentally different ways. Generally, animal miRNAs repress gene expression by mediating translational attenuation through (multiple) miRNA-binding sites located within the 3' untranslated region of the target gene. In contrast, almost all plant miRNAs regulate their targets by directing mRNA cleavage at single sites in the coding regions. These and other differences suggest that the two systems may have originated independently, possibly as a prerequisite to the development of complex body plans.

Animals↗

PubMed2005

The Arabidopsis GAMYB-like genes, MYB33 and MYB65, are microRNA-regulated genes that redundantly facilitate anther development.

The functions of the vast majority of genes encoding R2R3 MYB domain proteins remain unknown. The closely related MYB33 and MYB65 genes of Arabidopsis thaliana have high sequence similarity to the barley (Hordeum vulgare) GAMYB gene. T-DNA insertional mutants were isolated for both genes, and a myb33 myb65 double mutant was defective in anther development. In myb33 myb65 anthers, the tapetum undergoes hypertrophy at the pollen mother cell stage, resulting in premeiotic abortion of pollen development. However, myb33 myb65 sterility was conditional, where fertility increased both under higher light or lower temperature conditions. Thus, MYB33/MYB65 facilitate, but are not essential for, anther development. Neither single mutant displayed a phenotype, implying that MYB33 and MYB65 are functionally redundant. Consistent with functional redundancy, promoter-beta-glucuronidase (GUS) fusions of MYB33 and MYB65 gave identical expression patterns in flowers (sepals, style, receptacle, anther filaments, and connective but not in anthers themselves), shoot apices, and root tips. By contrast, expression of a MYB33:GUS translational fusion in flowers was solely in young anthers (consistent with the male sterile phenotype), and no staining was seen in shoot meristems or root tips. A microRNA target sequence is present in the MYB genes, and mutating this sequence in the MYB33:GUS fusion results in an expanded expression pattern, in tissues similar to that observed in the promoter-GUS lines, implying that the microRNA target sequence is restricting MYB33 expression. Arabidopsis transformed with MYB33 containing the mutated microRNA target had dramatic pleiotrophic developmental defects, suggesting that restricting MYB33 expression, especially in the shoot apices, is essential for proper plant development.

Alleles↗

PubMed2005

Dormancy release, ABA and pre-harvest sprouting.

Seed dormancy is an adaptive trait that enables the seeds of many species to remain quiescent until conditions become favorable for germination. Dormancy is normally initiated during seed maturation and maintained to seed maturity. In mature seeds, the loss of dormancy may be gradual (after-ripening) or can be terminated by chilling and other environmental triggers. Dormancy is an important trait for many important crop species: it inhibits pre-harvest spouting or vivipary, a widespread problem in many regions of the world. Too much dormancy, however, can lead to non-uniform germination in the field. Recent progress has been made in understanding the role of abscisic acid metabolism and dormancy release in both model plants and crop species. Advances in our understanding of the molecular mechanisms that are involved in dormancy, along with approaches using quantitative genetics, will provide new strategies through which the desired level of dormancy can be introduced into crop species.

Abscisic Acid↗

PubMed2002

Significance of the expression of the CER6 condensing enzyme for cuticular wax production in Arabidopsis.

To learn more about the role of the CER6 condensing enzyme in Arabidopsis surface wax production, we determined CER6 transcription domains and the timing of CER6 transcription in vegetative and reproductive structures from juvenile, mature, and senescing tissues. We found that CER6 is highly transcribed throughout development, exclusively in the epidermal cells in all tissues examined. The only exception to the epidermal expression was observed in anthers nearing maturity, in which CER6 mRNA was localized in the tapetum. To determine if environmental factors such as light and water deficit, which are known to stimulate wax accumulation, induce CER6 transcription, we examined the effects of these factors on CER6 transcript abundance. Our results demonstrate that light is essential for CER6 transcription, and that osmotic stress and the presence of abscisic acid enhance CER6 transcript accumulation. CER6 promoter-directed expression of the beta-glucuronidase reporter gene in transgenic plants demonstrated that the CER6 promoter was highly effective in directing epidermis-specific expression in Arabidopsis and tobacco (Nicotiana tabacum). Furthermore, CER6 promoter-driven CER6 overexpression resulted in increased wax deposition in Arabidopsis stems. These experiments indicate that the expression level of CER6 in the epidermis is one of the factors controlling wax accumulation on Arabidopsis stems.

Acyltransferases↗