Proanthocyanidins and anthocyanins are produced by closely related branches of the flavonoid pathway and utilize the same metabolic intermediates. Previous studies have shown a flexible mechanism of flux diversion at the branch-point between the anthocyanin and proanthocyanidin pathways, but the molecular basis for this mechanism is poorly understood. Floral tissues in white clover plants (Trifolium repens) produce both proanthocyanidins and anthocyanins. This makes white clover amenable to studies of proanthocyanidin and anthocyanin biosynthesis and possible interactions within the flavonoid pathway. Results of this study show that the anthocyanin and proanthocyanidin pathways are spatially colocalized within epidermal cells of petals and temporally overlap in partially open flowers. A correlation between spatiotemporal patterns of anthocyanin and proanthocyanidin biosynthesis with expression profiles of putative flavonoid-related genes indicates that these pathways may recruit different isoforms of flavonoid biosynthetic enzymes. Furthermore, in transgenic white clover plants with downregulated expression of the anthocyanidin reductase gene, levels of flavan 3-ols, anthocyanins, and flavonol glycosides and the expression levels of a range of genes encoding putative flavonoid biosynthetic enzymes and transcription factors were altered. This is consistent with the hypothesis that flux through the flavonoid pathway may be at least partially regulated by the availability of intermediates.
C4 grasses are favoured as forage crops in warm, humid climates. The use of C4 grasses in pastures is expected to increase because the tropical belt is widening due to global climate change. While the forage quality of Paspalum dilatatum (dallisgrass) is higher than that of other C4 forage grass species, digestibility of warm-season grasses is, in general, poor compared with most temperate grasses. The presence of thick-walled parenchyma bundle-sheath cells around the vascular bundles found in the C4 forage grasses are associated with the deposition of lignin polymers in cell walls. High lignin content correlates negatively with digestibility, which is further reduced by a high ratio of syringyl (S) to guaiacyl (G) lignin subunits. Cinnamoyl-CoA reductase (CCR) catalyses the conversion of cinnamoyl CoA to cinnemaldehyde in the monolignol biosynthetic pathway and is considered to be the first step in the lignin-specific branch of the phenylpropanoid pathway. We have isolated three putative CCR1 cDNAs from P. dilatatum and demonstrated that their spatio-temporal expression pattern correlates with the developmental profile of lignin deposition. Further, transgenic P. dilatatum plants were produced in which a sense-suppression gene cassette, delivered free of vector backbone and integrated separately to the selectable marker, reduced CCR1 transcript levels. This resulted in the reduction of lignin, largely attributable to a decrease in G lignin.Electronic supplementary materialThe online version of this article (doi:10.1007/s11248-014-9784-1) contains supplementary material, which is available to authorized users.
BackgroundHistochemical staining of plant tissues with 4-dimethylaminocinnamaldehyde (DMACA) or vanillin-HCl is widely used to characterize spatial patterns of proanthocyanidin accumulation in plant tissues. These methods are limited in their ability to allow high-resolution imaging of proanthocyanidin deposits.ResultsTissue embedding techniques were used in combination with DMACA staining to analyze the accumulation of proanthocyanidins in Lotus corniculatus (L.) and Trifolium repens (L.) tissues. Embedding of plant tissues in LR White or paraffin matrices, with or without DMACA staining, preserved the physical integrity of the plant tissues, allowing high-resolution imaging that facilitated cell-specific localization of proanthocyanidins. A brown coloration was seen in proanthocyanidin-producing cells when plant tissues were embedded without DMACA staining and this was likely to have been due to non-enzymatic oxidation of proanthocyanidins and the formation of colored semiquinones and quinones.ConclusionsThis paper presents a simple, high-resolution method for analysis of proanthocyanidin accumulation in organs, tissues and cells of two plant species with different patterns of proanthocyanidin accumulation, namely Lotus corniculatus (birdsfoot trefoil) and Trifolium repens (white clover). This technique was used to characterize cell type-specific patterns of proanthocyanidin accumulation in white clover flowers at different stages of development.
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