Biomedical subjects

James B Reid

Publications and source records attributed to James B Reid.

At least 19 recordsLinked to original sources

PubMed2006

Defective long-distance auxin transport regulation in the Medicago truncatula super numeric nodules mutant.

Long-distance auxin transport was examined in Medicago truncatula and in its supernodulating mutant sunn (super numeric nodules) to investigate the regulation of auxin transport during autoregulation of nodulation (AON). A method was developed to monitor the transport of auxin from the shoot to the root in whole seedlings. Subsequently, the transport was monitored after inoculation of roots with the nodulating symbiont Sinorhizobium meliloti. The sunn mutant showed an increased amount of auxin transported from the shoot to the root compared to the wild type. The auxin transport capacity of excised root segments was similar in wild type and sunn, suggesting that the difference in long-distance auxin transfer between them is due to loading in the shoot. After inoculation, wild-type seedlings showed decreased auxin loading from the shoot to the root; however, the sunn mutant failed to reduce the amount of auxin loaded. The time of reduced auxin loading correlated with the onset of AON. Quantification of endogenous auxin levels at the site of nodule initiation showed that sunn contained three times more auxin than wild type. Inoculation of sunn failed to reduce the level of auxin within 24 h, as was observed in the wild type. We propose a model for the role of auxin during AON of indeterminate legumes: 1) high levels of endogenous auxin are correlated with increased numbers of nodules, 2) inoculation of roots reduces auxin loading from the shoot to the root, and 3) subsequent reduction of auxin levels in the root inhibits further nodule initiation.

Biological Transport↗

PubMed2006

The impact of intragenic recombination on phylogenetic reconstruction at the sectional level in Eucalyptus when using a single copy nuclear gene (cinnamoyl CoA reductase).

Low copy number nuclear genes have been found to be useful for phylogenetic reconstruction at different taxonomic levels. This study investigated the utility of a single copy gene, cinnamoyl CoA reductase (CCR), for resolving phylogenetic relationships at the sectional level within Eucalyptus. The monophyly of sections Exsertaria and Latoangulatae was explored, using section Maidenaria as an outgroup, and the impact of intragenic recombination on phylogenetic reconstruction examined. Phylogenetic analysis did not resolve monophyletic groups. Latoangulatae and Maidenaria were polyphyletic or paraphyletic. Exsertaria species formed a clade but included a single Latoangulatae species (E. major). Recombination analysis identified two intragenic recombination events that involved species from different sections, which have probably been facilitated by inter-sectional hybridisation. One of the events most likely occurred prior to speciation, with several Latoangulatae species having the recombinant allele. The other event may have occurred after speciation, since only one of two E. globulus samples possessed the recombinant allele. This is the first detailed report of intragenic recombination in both CCR and Eucalyptus, and between species of different sections of a plant genus. The occurrence of intragenic recombination may explain the anomalous positions of some species within the phylogenetic tree, and indicates that phylogenetic analysis of Eucalyptus using nuclear genes will be problematic unless recombination is taken into account.

Aldehyde Oxidoreductases↗

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↗

PubMed2006

A role for ethylene in the phytochrome-mediated control of vegetative development.

Members of the phytochrome family of photoreceptors play key roles in vegetative plant development, including the regulation of stem elongation, leaf development and chlorophyll accumulation. Hormones have been implicated in the control of these processes in de-etiolating seedlings. However, the mechanisms by which the phytochromes regulate vegetative development in more mature plants are less well understood. Pea (Pisum sativum) mutant plants lacking phytochromes A and B, the two phytochromes present in this species, develop severe defects later in development, including short, thick, distorted internodes and reduced leaf expansion, chlorophyll content and CAB gene transcript level. Studies presented here indicate that many of these defects in phyA phyB mutant plants appear to be due to elevated ethylene production, and suggest that an important role of the phytochromes in pea is to restrict ethylene production to a level that does not inhibit vegetative growth. Mutant phyA phyB plants produce significantly more ethylene than WT plants, and application of an ethylene biosynthesis inhibitor rescued many aspects of the phyA phyB mutant phenotype. This deregulation of ethylene production in phy-deficient plants appears likely to be due, at least in part, to the elevated transcript levels of key ethylene-biosynthesis genes. The phytochrome A photoreceptor appears to play a prominent role in the regulation of ethylene production, as phyA, but not phyB, single-mutant plants also exhibit a phenotype consistent with elevated ethylene production. Potential interactions between ethylene and secondary plant hormones in the control of the phy-deficient mutant phenotype were explored, revealing that ethylene may inhibit stem elongation in part by reducing gibberellin levels.

Ethylenes↗

PubMed2005

Grapes on steroids. Brassinosteroids are involved in grape berry ripening.

Fruit ripening is a unique plant developmental process with direct implications for our food supply, nutrition, and health. In contrast to climacteric fruit, where ethylene is pivotal, the hormonal control of ripening in nonclimacteric fruit, such as grape (Vitis vinifera), is poorly understood. Brassinosteroids (BRs) are steroidal hormones, essential for normal plant growth and development but not previously implicated in the ripening of nonclimacteric fruit. Here we show that increases in endogenous BR levels, but not indole-3-acetic acid (IAA) or GA levels, are associated with ripening in grapes. Putative grape homologs of genes encoding BR biosynthesis enzymes (BRASSINOSTEROID-6-OXIDASE and DWARF1) and the BR receptor (BRASSINOSTEROID INSENSITIVE 1) were isolated, and the function of the grape BRASSINOSTEROID-6-OXIDASE gene was confirmed by transgenic complementation of the tomato (Lycopersicon esculentum) extreme dwarf (dx/dx) mutant. Expression analysis of these genes during berry development revealed transcript accumulation patterns that were consistent with a dramatic increase in endogenous BR levels observed at the onset of fruit ripening. Furthermore, we show that application of BRs to grape berries significantly promoted ripening, while brassinazole, an inhibitor of BR biosynthesis, significantly delayed fruit ripening. These results provide evidence that changes in endogenous BR levels influence this key developmental process. This may provide a significant insight into the mechanism controlling ripening in grapes, which has direct implications for the logistics of grape production and down-stream processing.

Cloning, Molecular↗

PubMed2005

Cryptochrome 1 contributes to blue-light sensing in pea.

Cryptochromes are widespread in higher plants but their physiological roles as blue-light photoreceptors have been examined in relatively few species. Screening in a phyA null mutant background has identified several blue-light response mutants in pea (Pisum sativum), including one that carries a substitution of a highly conserved glycine residue in the N-terminal photolyase-homologous domain of the pea CRY1 gene. Analyses of cry1, phyA, and phyB mutants show that all three photoreceptors contribute to seedling photomorphogenesis under high-irradiance blue light, whereas phyA is the main photoreceptor active under low irradiances. Triple phyA phyB cry1 mutants grown under high-irradiance blue light are indistinguishable from dark-grown wild-type plants in length and leaf expansion but show a small residual response to higher-irradiance white light. Monogenic cry1 mutants have little discernable phenotype at the seedling stage, but later in development are more elongated than wild-type plants. In addition, the loss of cry1 moderates the short-internode phenotype of older phyA mutants, suggesting an antagonism between phyA and cry1 under some conditions. Pea cry1 has a small inhibitory effect on flowering under long and short days. However, the phyA cry1 double mutant retains a clear promotion of flowering in response to blue-light photoperiod extensions, indicating a role for one or more additional blue-light photoreceptors in the control of flowering in pea.

Amino Acid Sequence↗

PubMed2005

Regulation of the early GA biosynthesis pathway in pea.

The early steps in the gibberellin (GA) biosynthetic pathway are controlled by single copy genes or small gene families. In pea (Pisum sativum L.) there are two ent-kaurenoic acid oxidases, one expressed only in the seeds, while ent-copalyl synthesis and ent-kaurene oxidation appear to be controlled by single copy genes. None of these genes appear to show feedback regulation and the only major developmental regulation appears to be during seed development. During shoot maturation, transcript levels do not change markedly with the result that all the three genes examined are expressed in mature tissue, supporting recent findings that these tissues can synthesise GAs. It therefore appears that the regulation of bioactive GA levels are determined by the enzymes encoded by the 2-oxoglutarate-dependent dioxygenase gene families controlling the later steps in GA biosynthesis. However the early steps are nonetheless important as a clear log/linear relationship exists between elongation and the level of GA1 in a range of single and double mutants in genes controlling these steps.

Alkyl and Aryl Transferases↗

PubMed2005

Nodulation phenotypes of gibberellin and brassinosteroid mutants of pea.

The initiation and development of legume nodules induced by compatible Rhizobium species requires a complex signal exchange involving both plant and bacterial compounds. Phytohormones have been implicated in this process, although in many cases direct evidence is lacking. Here, we characterize the root and nodulation phenotypes of various mutant lines of pea (Pisum sativum) that display alterations in their phytohormone levels and/or perception. Mutants possessing root systems deficient in gibberellins (GAs) or brassinosteroids (BRs) exhibited a reduction in nodule organogenesis. The question of whether these reductions represent direct or indirect effects of the hormone deficiency is addressed. For example, the application of GA to the roots of a GA-deficient mutant completely restored its number of nodules to that of the wild type. Grafting studies revealed that a wild-type shoot or root also restored the nodule number of a GA-deficient mutant. These findings suggest that GAs are required for nodulation. In contrast, the shoot controlled the number of nodules that formed in graft combinations of a BR-deficient mutant and its wild type. The root levels of auxin and GA were similar among these latter graft combinations. These results suggest that BRs influence a shoot mechanism that controls nodulation and that the root levels of auxin and GA are not part of this process. Interestingly, a strong correlation between nodule and lateral root numbers was observed in all lines assessed, consistent with a possible overlap in the early developmental pathways of the two organs.

Gene Expression Regulation, Plant↗

PubMed2005

Cochleata: getting to the root of legume nodules.

The homeotic mutant of Pisum sativum, cochleata, has stipules replaced by alternative leaf components, abnormal flowers and reduced fertility. Although the root system dry weight, root lengths and nodule numbers of cochleata are similar to those of its wild type, the nodulation phenotype of the mutant is unique. The nodules typically dichotomously branch and multiple callus and root structures emerge from their meristems. These nodule-roots incorporate a peripheral vascular bundle of the nodule into their own central vascular cylinder. Both the nodules and roots of the hybrid structures appear functional. Roles for COCHLEATA in development are discussed.

Mutation↗

PubMed2005

Genomic research in Eucalyptus.

Eucalyptus L'Hérit. is a genus comprised of more than 700 species that is of vital importance ecologically to Australia and to the forestry industry world-wide, being grown in plantations for the production of solid wood products as well as pulp for paper. With the sequencing of the genomes of Arabidopsis thaliana and Oryza sativa and the recent completion of the first tree genome sequence, Populus trichocarpa, attention has turned to the current status of genomic research in Eucalyptus. For several eucalypt species, large segregating families have been established, high-resolution genetic maps constructed and large EST databases generated. Collaborative efforts have been initiated for the integration of diverse genomic projects and will provide the framework for future research including exploiting the sequence of the entire eucalypt genome which is currently being sequenced. This review summarises the current position of genomic research in Eucalyptus and discusses the direction of future research.

Agriculture↗

PubMed2005

The cryptochrome gene family in pea includes two differentially expressed CRY2 genes.

The cryptochromes are a family of blue light photoreceptors that play important roles in the control of plant development. We have characterised the cryptochrome gene family in the model legume garden pea (Pisum sativum L.). Pea contains three expressed cryptochrome genes; a single CRY1 orthologue, and two distinct CRY2 genes that we have termed CRY2a and CRY2b. Genomic southern blots indicate that there are unlikely to be more CRY genes in pea. Each of the three genes encodes a full-length CRY protein that contains all the major domains characteristic of other higher plant cryptochromes. Database searches have identified Medicago truncatula expressed sequence tags (ESTs) corresponding to all three genes, whereas only a single CRY2 is represented in EST collections from the more distantly related legumes soybean and Lotus japonicus. The proteins encoded by the pea and Medicago CRY2b genes are distinguished from other CRY2 proteins by their shorter C-terminus. Expression analyses have identified marked differences in the regulation of the three genes, with CRY2b expression in particular distinguished by high-amplitude diurnal cycling and rapid repression in seedlings transferred from darkness to blue light.

Amino Acid Sequence↗

PubMed2004

The brassinosteroid growth response in pea is not mediated by changes in gibberellin content.

The objective of this study was to increase our understanding of the relationship between brassinosteroids (BRs) and gibberellins (GAs) by examining the effects of BR deficiency on the GA biosynthesis pathway in several tissue types of pea (Pisum sativum L.). It was suggested recently that, in Arabidopsis, BRs act as positive regulators of GA 20-oxidation, a key step in GA biosynthesis [Bouquin et al. (2001) Plant Physiol 127:450-458]. However, this may not be the case in pea as GA20 levels were consistently higher in all shoot tissues of BR-deficient (lk and lkb) and BR-response (lka) mutants. The application of brassinolide (BL) to lkb plants reduced GA20 levels, and metabolism studies revealed a reduced conversion of GA19 to GA20 in epi-BL-treated lkb plants. These results indicate that BRs actually negatively regulate GA20 levels in pea. Although GA20 levels are affected by BR levels, this does not result in consistent changes in the level of the bioactive GA, GA1. Therefore, even though a clear interaction exists between endogenous BR levels and the level of GA20, this interaction may not be biologically significant. In addition to the effect of BRs on GA levels, the effect of altered GA1 levels on endogenous BR levels was examined. There was no significant difference in BR levels between the GA mutants and the wild type (wt), indicating that altered GA1 levels have no effect on BR levels in pea. It appears that the BR growth response is not mediated by changes in bioactive GA levels, thus providing further evidence that BRs are important regulators of stem elongation.

Arabidopsis↗

PubMed2004

A novel mutant with modified tropic responses in Pisum sativum L.

A single-gene recessive mutant which displays increased phototropic and gravitropic responses has been isolated in Pisum sativum L. cv. Torsdag and is provisionally named mtr-1, for its modified tropic response. Mutant plants attain a greater degree of bending during both phototropic and gravitropic induction due to an extension of the curvature phase. In addition to their increase in tropic curvature, mutant plants have longer and narrower leaves as mature plants, attenuated blue-light-induced ion flux responses, and lower levels of PsPK5 mRNA (a PHOT1 orthologue). Possible causes of these effects are discussed.

Cryptochromes↗

PubMed2004

Brassinosteroids do not undergo long-distance transport in pea. Implications for the regulation of endogenous brassinosteroid levels.

It is widely accepted that brassinosteroids (BRs) are important regulators of plant growth and development. However, in comparison to the other classical plant hormones, such as auxin, relatively little is known about BR transport and its potential role in the regulation of endogenous BR levels in plants. Here, we show that end-pathway BRs in pea (Pisum sativum) occur in a wide range of plant tissues, with the greatest accumulation of these substances generally occurring in the young, actively growing tissues, such as the apical bud and young internodes. However, despite the widespread distribution of BRs throughout the plant, we found no evidence of long-distance transport of these substances between different plant tissues. For instance, we show that the maintenance of steady-state BR levels in the stem does not depend on their transport from the apical bud or mature leaves. Similarly, reciprocal grafting between the wild type and the BR-deficient lkb mutants demonstrates that the maintenance of steady-state BR levels in whole shoots and roots does not depend on either basipetal or acropetal transport of BRs between these tissues. Together, with results from (3)H-BR feeding studies, these results demonstrate that BRs do not undergo long-distance transport in pea. The widespread distribution of end-pathway BRs and the absence of long-distance BR transport between different plant tissues provide significant insight into the mechanisms that regulate BR homeostasis in plants.

Biological Transport↗

PubMed2004

A dominant mutation in the pea PHYA gene confers enhanced responses to light and impairs the light-dependent degradation of phytochrome A.

Phytochrome A (phyA) is an important photoreceptor controlling many processes throughout the plant life cycle. It is unique within the phytochrome family for its ability to mediate photomorphogenic responses to continuous far-red light and for the strong photocontrol of its transcript level and protein stability. Here we describe a dominant mutant of garden pea (Pisum sativum) that displays dramatically enhanced responses to light, early photoperiod-independent flowering, and impaired photodestruction of phyA. The mutant carries a single base substitution in the PHYA gene that is genetically inseparable from the mutant phenotype. This substitution is predicted to direct the replacement of a conserved Ala in an N-terminal region of PHYA that is highly divergent between phyA and other phytochromes. This result identifies a region of the phyA photoreceptor molecule that may play an important role in its fate after photoconversion.

Amino Acid Sequence↗

PubMed2004

Brassinosteroid deficiency due to truncated steroid 5alpha-reductase causes dwarfism in the lk mutant of pea.

The endogenous brassinosteroids in the dwarf mutant lk of pea (Pisum sativum) were quantified by gas chromatography-selected ion monitoring. The levels of castasterone, 6-deoxocastasterone, and 6-deoxotyphasterol in lk shoots were reduced 4-, 70-, and 6-fold, respectively, compared with those of the wild type. The fact that the application of brassinolide restored the growth of the mutant indicated that the dwarf mutant lk is brassinosteroid deficient. Gas chromatography-selected ion monitoring analysis of the endogenous sterols in lk shoots revealed that the levels of campestanol and sitostanol were reduced 160- and 10-fold, respectively, compared with those of wild-type plants. These data, along with metabolic studies, showed that the lk mutant has a defect in the conversion of campest-4-en-3-one to 5alpha-campestan-3-one, which is a key hydrogenation step in the synthesis of campestanol from campesterol. This defect is the same as that found in the Arabidopsis det2 mutant and the Ipomoea nil kbt mutant. The pea gene homologous to the DET2 gene, PsDET2, was cloned, and it was found that the lk mutation would result in a putative truncated PsDET2 protein. Thus it was concluded that the short stature of the lk mutant is due to a defect in the steroidal 5alpha-reductase gene. This defect was also observed in the callus induced from the lk mutant. Biosynthetic pathways involved in the conversion of campesterol to campestanol are discussed in detail.

3-Oxo-5-alpha-Steroid 4-Dehydrogenase↗

PubMed2004

The pea gene LH encodes ent-kaurene oxidase.

The pea (Pisum sativum) homolog, PsKO1, of the Arabidopsis GA3 gene was isolated. It codes for a cytochrome P450 from the CYP701A subfamily and has ent-kaurene oxidase (KO) activity, catalyzing the three step oxidation of ent-kaurene to ent-kaurenoic acid in the gibberellin (GA) biosynthetic pathway when expressed in yeast (Saccharomyces cerevisiae). PsKO1 is encoded by the LH gene because in three independent mutant alleles, lh-1, lh-2, and lh-3, PsKO1 has altered sequence, and the lh-1 allele, when expressed in yeast, failed to metabolize ent-kaurene. The lh mutants of pea are GA deficient and have reduced internode elongation and root growth. One mutant (lh-2) also causes a large increase in seed abortion. PsKO1 (LH) is expressed in all tissues examined, including stems, roots, and seeds, and appears to be a single-copy gene. Differences in sensitivity to the GA synthesis inhibitor, paclobutrazol, between the mutants appear to result from the distinct nature of the genetic lesions. These differences may also explain the tissue-specific differences between the mutants.

Alternative Splicing↗

PubMed2004

Phytochrome regulation of pea phototropin.

Type 1 phototropin, one of the blue light receptors responsible for phototropism, is encoded in peas by at least two genes, PsPHOT1A and PsPHOT1B (formerly PsPK4 and PsPK5), both of which are more similar to Arabidopsis PHOT1 than to Arabidopsis PHOT2. We show here that PsPHOT1B encodes a full-length phototropin, whose expression pattern suggests that Psphot1b is the predominant phot1-type phototropin in etiolated seedlings. The gene encoding the other type 1 phototropin, PsPHOT1A, is expressed at low levels, with its highest levels in the leaves and stems of more mature, light-grown plants. Studies with phyA, phyB and the phyAphyB double mutants show that phyA and phyB have partially redundant roles in the reduction of PsPHOT1B expression under red light.

Arabidopsis Proteins↗