Publications - Cell and Metabolic Biology

Plants have the ability to alleviate the harmful effects caused by abiotic and biotic stress. Phytohormones play a very important role in the acclimation to these stresses. To study the role of jasmonate in the acclimation to osmotic stress, an ALLENE OXIDE CYCLASE (AOC) mutant of rice (cpm2), disrupted in the biosynthesis of jasmonic acid (JA), and its wild type (WT) background were employed to investigate their responses to osmotic stress caused by treatment with polyethylene glycol (PEG) 6000. WT showed tolerance to osmotic stress, correlated with a fast transient increase of JA and JA-isoleucine (JA-Ile) in the shoots prior to an increase in abscisic acid (ABA), followed by a second increase in jasmonates when exposing to osmotic stress during 24 h. In roots, the pattern of hormonal increase was similar, but the response appeared to be faster, and remained transient, also with respect to low levels of jasmonates upon continuing osmotic stress. The mutant, which was containing extremely low levels of jasmonates, was hypersensitive to the stress. However, ABA accumulated in both, shoots and roots of cpm2, to similar (but not equal) levels as those seen in the WT, demonstrating that the biosynthesis or catabolism of ABA in response to osmotic stress is at least partially independent of JA, but can be modulated by JA. Our results suggest that jasmonates operate in parallel, presumably synergistically, to ABA, and are indispensable for osmotic stress tolerance in rice.

Soil pollutants may affect root growth through interactions among phytohormones like auxin and jasmonates. Rice is frequently grown in paddy fields contaminated by cadmium and arsenic, but the effects of these pollutants on jasmonates/auxin crosstalk during adventitious and lateral roots formation are widely unknown. Therefore, seedlings of Oryza sativa cv. Nihonmasari and of the jasmonate-biosynthetic mutant coleoptile photomorphogenesis2 were exposed to cadmium and/or arsenic, and/or jasmonic acid methyl ester, and then analysed through morphological, histochemical, biochemical and molecular approaches.In both genotypes, arsenic and cadmium accumulated in roots more than shoots. In the roots, arsenic levels were more than twice higher than cadmium levels, either when arsenic was applied alone, or combined with cadmium. Pollutants reduced lateral root density in the wild -type in every treatment condition, but jasmonic acid methyl ester increased it when combined with each pollutant. Interestingly, exposure to cadmium and/or arsenic did not change lateral root density in the mutant. The transcript levels of OsASA2 and OsYUCCA2, auxin biosynthetic genes, increased in the wild-type and mutant roots when pollutants and jasmonic acid methyl ester were applied alone. Auxin (indole-3-acetic acid) levels transiently increased in the roots with cadmium and/or arsenic in the wild-type more than in the mutant. Arsenic and cadmium, when applied alone, induced fluctuations in bioactive jasmonate contents in wild-type roots, but not in the mutant. Auxin distribution was evaluated in roots of OsDR5::GUS seedlings exposed or not to jasmonic acid methyl ester added or not with cadmium and/or arsenic. The DR5::GUS signal in lateral roots was reduced by arsenic, cadmium, and jasmonic acid methyl ester. Lipid peroxidation, evaluated as malondialdehyde levels, was higher in the mutant than in the wild-type, and increased particularly in As presence, in both genotypes.Altogether, the results show that an auxin/jasmonate interaction affects rice root system development in the presence of cadmium and/or arsenic, even if exogenous jasmonic acid methyl ester only slightly mitigates pollutants toxicity.

In oilseed rape (Brassica napus), the glucosyltransferase UGT84A9 catalyzes the formation of 1-O-sinapoyl-β-glucose, which feeds as acyl donor into a broad range of accumulating sinapate esters, including the major antinutritive seed component sinapoylcholine (sinapine). Since down-regulation of UGT84A9 was highly efficient in decreasing the sinapate ester content, the genes encoding this enzyme were considered as potential targets for molecular breeding of low sinapine oilseed rape. B. napus harbors two distinguishable sequence types of the UGT84A9 gene designated as UGT84A9-1 and UGT84A9-2. UGT84A9-1 is the predominantly expressed variant, which is significantly up-regulated during the seed filling phase, when sinapate ester biosynthesis exhibits strongest activity. In the allotetraploid genome of B. napus, UGT84A9-1 is represented by two loci, one derived from the Brassica C-genome (UGT84A9a) and one from the Brassica A-genome (UGT84A9b). Likewise, for UGT84A9-2 two loci were identified in B. napus originating from both diploid ancestor genomes (UGT84A9c, Brassica C-genome; UGT84A9d, Brassica A-genome). The distinct UGT84A9 loci were genetically mapped to linkage groups N15 (UGT84A9a), N05 (UGT84A9b), N11 (UGT84A9c) and N01 (UGT84A9d). All four UGT84A9 genomic loci from B. napus display a remarkably low micro-collinearity with the homologous genomic region of Arabidopsis thaliana chromosome III, but exhibit a high density of transposon-derived sequence elements. Expression patterns indicate that the orthologous genes UGT84A9a and UGT84A9b should be considered for mutagenesis inactivation to introduce the low sinapine trait into oilseed rape.

Resveratrol is a phytoalexin produced in various plants like wine, peanut or pine in response to fungal infection or UV irradiation, but it is absent in members of the Brassicaceae. Moreover, resveratrol and its glucoside (piceid) are considered to have beneficial effects on human health, known to reduce heart disease, arteriosclerosis and cancer mortality. Therefore, the introduction of the gene encoding stilbene synthase for resveratrol production in rapeseed is a tempting approach to improve the quality of rapeseed products. The stilbene synthase gene isolated from grapevine (Vitis vinifera L.) was cloned under control of the seed-specific napin promotor and introduced into rapeseed (Brassica napus L.) by Agrobacterium-mediated co-transformation together with a ds-RNA-interference construct deduced from the sequence of the key enzyme for sinapate ester biosynthesis, UDP-glucose:sinapate glucosyltransferase (BnSGT1), assuming that the suppression of the sinapate ester biosynthesis may increase the resveratrol production in seeds through the increased availability of the precursor 4-coumarate. Resveratrol glucoside (piceid) was produced at levels up to 361 μg/g in the seeds of the primary transformants. This value exceeded by far piceid amounts reported from B. napus expressing VST1 in the wild type sinapine background. There was no significant difference in other important agronomic traits, like oil, protein, fatty acid and glucosinolate content in comparison to the control plants. In the third seed generation, up to 616 μg/g piceid was found in the seeds of a homozygous T3-plant with a single transgene copy integrated. The sinapate ester content in this homozygous T3-plant was reduced from 7.43 to 2.40 mg/g. These results demonstrate how the creation of a novel metabolic sink could divert the synthesis towards the production of piceid rather than sinapate ester, thereby increasing the value of oilseed products.