Breeding and Analysis of Two New Grapefruit-Like Varieties with Low Furanocoumarin Content

Furanocoumarins (FCs) are a group of related plant defense metabolites occurring in several plant families, including some species in the genus citrus, such as grapefruit and pummelo. FCs function as toxins against pathogens, insects and other plant pests and some are toxic to humans at high levels. Although the levels of FCs in grapefruits are non-toxic to humans, they inhibit the intestinal enzyme CYP3A, thus preventing degradation of medicines, such as statins, and causing dangerous overdose effects. This overdosing can cause devastating side effects, ranging from stomach bleeding to kidney problems, muscle aches and irregular heartbeats. In the present study, we utilize LC/MS to characterize the levels of FCs pathway intermediates and end products in twelve citrus cultivars, including mandarin (Citrus reticulata), orange [Citrus sinensis (L.) Osbeck], Pummelo [Citrus maxima (Burm.) Merr.], grapefruit (Citrus paradisi Macf.), and two newly selected grapefruit like varieties [(Citrus reticulate) X [Citrus maxima (Burm.) Merr]. The orange and mandarin varieties do not contain FCs or FCs precursor compounds suggesting that this biosynthetic pathway is absent or inactive in mandarins and oranges and therefore a good genetic source for null alleles to FCs biosynthesis. We report the selection and characterization of two new low FCs and seedless grapefruit-like varieties, “Aliza” and “Coocki”, developed by a cross between pummelo and mandarin. Fruits of these varieties resemble grapefruit and contain high levels of the flavanone naringin, typical of grapefruit, but contain only trace amounts of FCs (based on LCMS analysis). Based on the variability of FCs content and inheritance in citrus species, the results suggest that future development of new low-FCs grapefruit varieties is an achievable objective.

Arahidopsis CHLOROPHYLLASE 1 protects young leaves from long-term photodamage by facilitating FtsH-mediated D1 degradation in photosystemⅡrepair

The proteolytic degradation of the photodamaged D1 core subunit during the photosystemⅡ(PSⅡ)repair cycle is well understood,but chlorophyll turnover during D1 degradation remains unclear.Here,we report that Arabidopsis thaliana CHLOROPHYLLASE 1(CLH1)plays important roles in the PSII repair process.The abundance of CLH1 and CLH2 peaks in young leaves and is induced by high-light exposure.Seedlings of clh1 single and clh1-1/2-2 double mutants display increased photoinhibition after long-term high-light exposure,whereas seedlings overexpressing CLH1 have enhanced light tolerance compared with the wild type.CLH1 is localized in the developing chloroplasts of young leaves and associates with the PSⅡ-dismantling complexes RCC1 and RC47,with a preference for the latter upon exposure to high light.Furthermore,degradation of damaged D1 protein is retarded in young clh1-1/2-2 leaves after 18-h highlight exposure but is rescued by the addition of recombinant CLH1 in vitro.Moreover,overexpression of CLH1 in a variegated mutant(var2~2)that lacks thylakoid protease FtsH2,with which CLH1 interacts,suppresses the variegation and restores D1 degradation.A var2-2 clh1-1/2-2triple mutant shows more severe variegation and seedling death.Taken together,these results establish CLH1 as a long-sought chlorophyll dephytylation enzyme that is involved in PSⅡrepair and functions in long-term adaptation of young leaves to high-light exposure by facilitating FtsH-mediated D1 degradation.