Abstract
An analysis of key genes and enzymes of the betacyanin biosynthetic pathway in Amaranthus hypochondriacus (Ah) was performed. Complete cDNA sequence of Ah genes coding for cyclo-DOPA 5-O glucosyltransferase (AhcDOPA5-GT), two 4, 5-DOPA-extradiol-dioxygenase isoforms (AhDODA-1 and AhDODA-2, respectively), and a betanidin 5-O-glucosyltransferase (AhB5-GT), plus the partial sequence of an orthologue of the cytochrome P-450 R gene (CYP76AD1) were obtained. With the exception AhDODA-2, which had a closer phylogenetic relationship to DODA-like genes in anthocyanin-synthesizing plants, all genes analyzed closely resembled those reported in related Caryophyllales species. The measurement of basal gene expression levels, in addition to the DOPA oxidase tyrosinase (DOT) activity, in different tissues of three Ah genotypes having contrasting pigmentation levels (green to red-purple) was determined. Additional analyses were performed in Ah plants subjected to salt and drought stress and to two different insect herbivory regimes. Basal pigmentation accumulation in leaves, stems and roots of betacyanic plants correlated with higher expression levels of AhDODA-1 and AhB5-GT, whereas DOT activity levels coincided with pigment accumulation in stems and roots and with the acyanic nature of green plants, respectively, but not with pigmentation in leaves. Although the abiotic stress treatments tested produced changes in pigment levels in different tissues, pigment accumulation was the highest in leaves and stems of drought stressed betacyanic plants, respectively. However, tissue pigment accumulation in stressed Ah plants did not always correlate with betacyanin biosynthetic gene expression levels and/or DOT activity. This effect was tissue- and genotype-dependent, and further suggested that other unexamined factors were influencing pigment content in stressed Ah. The results obtained from the insect herbivory assays, particularly in acyanic plants, also support the proposal that these genes could have functions other than betacyanin biosynthesis.
Highlights
Amaranthus hypochondriacus, a grain amaranth, is a C4 dicot plant noted for its ability to tolerate stressful conditions and produce highly nutritious and health promoting seeds [1,2]
We describe the isolation of cDNA sequences of key genes of the betacyanin biosynthetic pathway in A. hypochondriacus
Relative expression levels3 were determined in leaves of A. hypochondriacus plants, with contrasting pigmentation patterns, subjected to insect herbivory
Summary
Amaranthus hypochondriacus, a grain amaranth, is a C4 dicot plant noted for its ability to tolerate stressful conditions and produce highly nutritious and health promoting seeds [1,2]. Grain amaranths are appreciated for their capacity to withstand drought stress and salinity in soils due to their remarkable water use efficiency, higher than many other C3 and C4 crops [3,4,5,6] Their drought-tolerance is attributed to the physiological advantages conferred by the C4 pathway, an indeterminate flowering habit, growth of long taproots and extensive lateral root systems in response to water shortage in the soil, osmolyte accumulation and the expression of genes coding for scavengers of reactive oxygen species (ROS), protein stabilizers and transcription factors [7,8,9,10,11,12]. Betalains are water-soluble, nitrogencontaining pigments with chemo-taxonomical value since, for still unresolved reasons, they have never been found jointly with anthocyanins in the same plant [13,15] They comprise the redviolet betacyanins and the yellow betaxanthins. They are 5-O-glucuroindoglucosides of two aglycons: betanidin and isobetanidin (its C-15 epimer), respectively, which are known to accumulate at different ratios, depending on the species [16,19,20,21,22]
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