Production of plant growth inhibitors from xanthophylls: a possible source of dormin

Maize w3 disrupts homogentisate solanesyl transferase (ZmHst) and reveals a plastoquinone-9 independent path for phytoene desaturation and tocopherol accumulation in kernels

Maize white seedling 3 (w3) has been used to study carotenoid deficiency for almost 100 years, although the molecular basis of the mutation has remained unknown. Here we show that the w3 phenotype is caused by disruption of the maize gene for homogentisate solanesyl transferase (HST), which catalyzes the first and committed step in plastoquinone-9 (PQ-9) biosynthesis in the plastid. The resulting PQ-9 deficiency prohibits photosynthetic electron transfer and eliminates PQ-9 as an oxidant in the enzymatic desaturation of phytoene during carotenoid synthesis. As a result, light-grown w3 seedlings are albino, deficient in colored carotenoids and accumulate high levels of phytoene. However, despite the absence of PQ-9 for phytoene desaturation, dark-grown w3 seedlings can produce abscisic acid (ABA) and homozygous w3 kernels accumulate sufficient carotenoids to generate ABA needed for seed maturation. The presence of ABA and low levels of carotenoids in w3 nulls indicates that phytoene desaturase is able to use an alternate oxidant cofactor, albeit less efficiently than PQ-9. The observation that tocopherols and tocotrienols are modestly affected in w3 embryos and unaffected in w3 endosperm indicates that, unlike leaves, grain tissues deficient in PQ-9 are not subject to severe photo-oxidative stress. In addition to identifying the molecular basis for the maize w3 mutant, we: (1) show that low levels of phytoene desaturation can occur in w3 seedlings in the absence of PQ-9; and (2) demonstrate that PQ-9 and carotenoids are not required for vitamin E accumulation.

Intragenic enhancers and suppressors of phytoene desaturase mutations in Chlamydomonas reinhardtii

Photosynthetic organisms synthesize carotenoids for harvesting light energy, photoprotection, and maintaining the structure and function of photosynthetic membranes. A light-sensitive, phytoene-accumulating mutant, pds1-1, was isolated in Chlamydomonas reinhardtii and found to be genetically linked to the phytoene desaturase (PDS) gene. PDS catalyzes the second step in carotenoid biosynthesis—the conversion of phytoene to ζ-carotene. Decreased accumulation of downstream colored carotenoids suggested that the pds1-1 mutant is leaky for PDS activity. A screen for enhancers of the pds1-1 mutation yielded the pds1-2 allele, which completely lacks PDS activity. A second independent null mutant (pds1-3) was identified using DNA insertional mutagenesis. Both null mutants accumulate only phytoene and no other carotenoids. All three phytoene-accumulating mutants exhibited slower growth rates and reduced plating efficiency compared to wild-type cells and white phytoene synthase mutants. Insight into amino acid residues important for PDS activity was obtained through the characterization of intragenic suppressors of pds1-2. The suppressor mutants fell into three classes: revertants of the pds1-1 point mutation, mutations that changed PDS amino acid residue Pro64 to Phe, and mutations that converted PDS residue Lys90 to Met. Characterization of pds1-2 intragenic suppressors coupled with computational structure prediction of PDS suggest that amino acids at positions 90 and 143 are in close contact in the active PDS enzyme and have important roles in its structural stability and/or activity.

De novo transcriptomic analysis of an oleaginous microalga: pathway description and gene discovery for production of next-generation biofuels

Background

Eustigmatos cf. polyphem is a yellow-green unicellular soil microalga belonging to the eustimatophyte with high biomass and considerable production of triacylglycerols (TAGs) for biofuels, which is thus referred to as an oleaginous microalga. The paucity of microalgae genome sequences, however, limits development of gene-based biofuel feedstock optimization studies. Here we describe the sequencing and de novo transcriptome assembly for a non-model microalgae species, E. cf. polyphem, and identify pathways and genes of importance related to biofuel production.

Results

We performed the de novo assembly of E. cf. polyphem transcriptome using Illumina paired-end sequencing technology. In a single run, we produced 29,199,432 sequencing reads corresponding to 2.33 Gb total nucleotides. These reads were assembled into 75,632 unigenes with a mean size of 503 bp and an N50 of 663 bp, ranging from 100 bp to >3,000 bp. Assembled unigenes were subjected to BLAST similarity searches and annotated with Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) orthology identifiers. These analyses identified the majority of carbohydrate, fatty acids, TAG and carotenoids biosynthesis and catabolism pathways in E. cf. polyphem.

Conclusions

Our data provides the construction of metabolic pathways involved in the biosynthesis and catabolism of carbohydrate, fatty acids, TAG and carotenoids in E. cf. polyphem and provides a foundation for the molecular genetics and functional genomics required to direct metabolic engineering efforts that seek to enhance the quantity and character of microalgae-based biofuel feedstock.

The relationship between the methyl-erythritol phosphate pathway leading to emission of volatile isoprenoids and abscisic acid content in leaves

It was investigated whether the methyl-erythritol phosphate (MEP) pathway that generates volatile isoprenoids and carotenoids also produces foliar abscisic acid (ABA) and controls stomatal opening. When the MEP pathway was blocked by fosmidomycin and volatile isoprenoid emission was largely suppressed, leaf ABA content decreased to about 50% and leaf stomatal conductance increased significantly. No effect of fosmidomycin was seen in leaves with constitutively high rates of stomatal conductance and in plant species with low foliar ABA concentration. In all other cases, isoprene emission was directly associated with foliar ABA, but ABA reduction upon MEP pathway inhibition was also observed in plant species that do not emit isoprenoids. Stomatal closure causing a midday depression of photosynthesis was also associated with a concurrent increase of isoprene emission and ABA content. It is suggested that the MEP pathway generates a labile pool of ABA that responds rapidly to environmental changes. This pool also regulates stomatal conductance, possibly when coping with frequent changes of water availability. MEP pathway inhibition by leaf darkening, and its down-regulation by exposure to elevated CO2, was also associated with a reduction of foliar ABA content. However, stomatal conductance was reduced, indicating that stomatal aperture is not regulated by the MEP-dependent foliar ABA pool, under these specific cases.

Abscisic acid biosynthesis and catabolism

The level of abscisic acid (ABA) in any particular tissue in a plant is determined by the rate of biosynthesis and catabolism of the hormone. Therefore, identifying all the genes involved in the metabolism is essential for a complete understanding of how this hormone directs plant growth and development. To date, almost all the biosynthetic genes have been identified through the isolation of auxotrophic mutants. On the other hand, among several ABA catabolic pathways, current genomic approaches revealed that Arabidopsis CYP707A genes encode ABA 8'-hydroxylases, which catalyze the first committed step in the predominant ABA catabolic pathway. Identification of ABA metabolic genes has revealed that multiple metabolic steps are differentially regulated to fine-tune the ABA level at both transcriptional and post-transcriptional levels. Furthermore, recent ongoing studies have given new insights into the regulation and site of ABA metabolism in relation to its physiological roles.

Specific oxidative cleavage of carotenoids by VP14 of maize

The plant growth regulator abscisic acid (ABA) is formed by the oxidative cleavage of an epoxy-carotenoid. The synthesis of other apocarotenoids, such as vitamin A in animals, may occur by a similar mechanism. In ABA biosynthesis, oxidative cleavage is the first committed reaction and is believed to be the key regulatory step. A new ABA-deficient mutant of maize has been identified and the corresponding gene, Vp14, has been cloned. The recombinant VP14 protein catalyzes the cleavage of 9-cis-epoxy-carotenoids to form C25 apo-aldehydes and xanthoxin, a precursor of ABA in higher plants.

Inhibition of carotenoid accumulation and abscisic acid biosynthesis in fluridone-treated dark-grown barley

Treatment of dark-grown barley with 0.1 mM fluridone inhibited carotenoid accumulation but did not alter plastid biogenesis. Plastids isolated from dark-grown control and dark-grown fluridone-treated plants were similar in size and protein compositions. Dehydration of dark-grown control barley caused abscisic acid levels to increase 30-40-fold in 4 h, while plants treated with 0.1 mM fluridone accumulated very little abscisic acid in response to dehydration. These results suggest that fluridone-treated plants do not accumulate abscisic acid because of carotenoid deficiency rather than plastid dysfunction. Dark-grown barley plants treated with 0.31 microM fluridone accumulated low levels of carotenoids. Dehydration of these plants resulted in a 4-8-fold increase in abscisic acid and a decrease in antheraxanthin, violaxanthin and neoxanthin, but no change in beta-carotene or lutein plus zeaxanthin levels. This result is consistent with the suggestion that xanthophylls are precursors to abscisic acid in dehydrated plants.

Enzymatic production of the plant growth inhibitor, xanthoxin

Incubation of violaxanthin with lipoxygenase and linoleate gave rise to the plant growth inhibitor, xanthoxin; and the yields were reduced to 1/10 and 1/20 by the omission of lipoxygenase and both lipoxygenase and linoleate respecively.

The detection and estimation of the growth inhibitor xanthoxin in plants

The plant growth inhibitor xanthoxin which can be prepared in vitro by the oxidation of certain xanthophylls has been identified in the ether extracts of the shoots of a wide variety of higher plants. Gas liquid chromatography of an acetylated derivative has been used for its quantitative estimation.Evidence is provided that xanthoxin is a true endogenous inhibitor and that violaxanthin or a related xanthophyll epoxide is its biogenetic precursor. The importance of xanthoxin and its relationship with abscisic acid and other plant growth inhibitors is discussed.

[Changes in the contents of abscisic acid, indoleacetic acid, and chloroplast pigments in pea seedlings treated with gibberellic acid]

Twelve-day old pea seedlings were treated with gibberellic acid (GA3). The hormone was applied to the apical bud in quantities of 0.05, 0.5, and 5 μg in 5 μl droplets.After 2 and 4 days the contents of indoleacetic acid (IAA) and abscisic acid (ABA) of the apical parts (apical bud including the 5th internode) and of the basal parts (3rd and 4th internode and leaves) were estimated by means of paper chromatography of ether extracts and bioassay with avena coleoptile segments. In addition, the contents of ABA were checked by spectropolarimetric measurement.Apical parts of the gibberellin treated plants contain more IAA, basal ones less IAA than the untreated controls. On the other hand, the contents of ABA are higher in basal parts and lower in apical parts. Only the seedlings treated with 5 μg GA3 and analysed after 2 days do not follow this rule. Their apical parts contain not only more IAA but also more ABA than the untreated plants, and their basal parts contain more IAA than the controls.It is concluded, therefore, that GA3 causes changes not only in the contents of these hormones but also in the distribution of hormones within the plant.Moreover, GA3 decreases the contents of chlorophyll a and b and of the carotenoids. The changes in carotenoid contents do not seem to be related to the changes in abscisic acid contents.

An increase in the "inhibitor-β" content of detached wheat leaves following a period of wilting

Wheat seedlings were grown under a 14-hour photoperiod and the first leaves excised at the end of the eighth dark period. The effect of treatments causing wilting on the "inhibitor-β" content of such leaves was studied.When leaves were rapidly wilted (i.e. to a 6% fresh weight loss) and extracted immediately, the amount of "inhibitor-β" per leaf was found to be the same as in fresh turgid leaves. However, when the leaves were maintained in a wilted condition in darkness for a period of 110 minutes, there was a marked increase in "inhibitor-β" content.The greater the degree of wilting (i.e. up to about a 9% loss in fresh weight) the greater the eventual "inhibitor-β" content. Moreover, the increment in "inhibitor-β" was shown to be temperature dependent.The time lapse requirement and the temperature dependency of the "inhibitor-β" formation suggest an enzymic conversion from a precursor.If a similar phenomenon occurs during the wilting of intact plants then the increase in this growth inhibitor might play a role in some of the physiological changes which accompany water stress.