Investigation of genetic factors regulating chlorophyll and carotenoid biosynthesis in red pepper fruit

Pepper catalase: a broad analysis of its modulation during fruit ripening and by nitric oxide

Catalase is a major antioxidant enzyme located in plant peroxisomes that catalyzes the decomposition of H2O2. Based on our previous transcriptomic (RNA-Seq) and proteomic (iTRAQ) data at different stages of pepper (Capsicum annuum L.) fruit ripening and after exposure to nitric oxide (NO) enriched atmosphere, a broad analysis has allowed us to characterize the functioning of this enzyme. Three genes were identified, and their expression was differentially modulated during ripening and by NO gas treatment. A dissimilar behavior was observed in the protein expression of the encoded protein catalases (CaCat1-CaCat3). Total catalase activity was down-regulated by 50% in ripe (red) fruits concerning immature green fruits. This was corroborated by non-denaturing polyacrylamide gel electrophoresis, where only a single catalase isozyme was identified. In vitro analyses of the recombinant CaCat3 protein exposed to peroxynitrite (ONOO-) confirmed, by immunoblot assay, that catalase underwent a nitration process. Mass spectrometric analysis identified that Tyr348 and Tyr360 were nitrated by ONOO-, occurring near the active center of catalase. The data indicate the complex regulation at gene and protein levels of catalase during the ripening of pepper fruits, with activity significantly down-regulated in ripe fruits. Nitration seems to play a key role in this down-regulation, favoring an increase in H2O2 content during ripening. This pattern can be reversed by the exogenous NO application. While plant catalases are generally reported to be tetrameric, the analysis of the protein structure supports that pepper catalase has a favored quaternary homodimer nature. Taken together, data show that pepper catalase is down-regulated during fruit ripening, becoming a target of tyrosine nitration, which provokes its inhibition.

Characterization of leucine aminopeptidase (LAP) activity in sweet pepper fruits during ripening and its inhibition by nitration and reducing events

KEY MESSAGE

Pepper fruits contain two leucine aminopeptidase (LAP) genes which are differentially modulated during ripening and by nitric oxide. The LAP activity increases during ripening but is negatively modulated by nitration. Leucine aminopeptidase (LAP) is an essential metalloenzyme that cleaves N-terminal leucine residues from proteins but also metabolizes dipeptides and tripeptides. LAPs play a fundamental role in cell protein turnover and participate in physiological processes such as defense mechanisms against biotic and abiotic stresses, but little is known about their involvement in fruit physiology. This study aims to identify and characterize genes encoding LAP and evaluate their role during the ripening of pepper (Capsicum annuum L.) fruits and under a nitric oxide (NO)-enriched environment. Using a data-mining approach of the pepper plant genome and fruit transcriptome (RNA-seq), two LAP genes, designated CaLAP1 and CaLAP2, were identified. The time course expression analysis of these genes during different fruit ripening stages showed that whereas CaLAP1 decreased, CaLAP2 was upregulated. However, under an exogenous NO treatment of fruits, both genes were downregulated. On the contrary, it was shown that during fruit ripening LAP activity increased by 81%. An in vitro assay of the LAP activity in the presence of different modulating compounds including peroxynitrite (ONOO-), NO donors (S-nitrosoglutathione and nitrosocyteine), reducing agents such as reduced glutathione (GSH), L-cysteine (L-Cys), and cyanide triggered a differential response. Thus, peroxynitrite and reducing compounds provoked around 50% inhibition of the LAP activity in green immature fruits, whereas cyanide upregulated it 1.5 folds. To our knowledge, this is the first characterization of LAP in pepper fruits as well as of its regulation by diverse modulating compounds. Based on the capacity of LAP to metabolize dipeptides and tripeptides, it could be hypothesized that the LAP might be involved in the GSH recycling during the ripening process.

Enzyme-Based Biostimulants Influence Physiological and Biochemical Responses of Lactuca sativa L

Biostimulants (BSs) are natural materials (i.e., organic or inorganic compounds, and/or microorganisms) having beneficial effects on plant growth and productivity, and able to improve resilience/tolerance to biotic and abiotic stresses. Therefore, they represent an innovative alternative to the phyto- and agrochemicals, being environmentally friendly and a valuable tool to cope with extreme climate conditions. The objective of this study was to investigate the effects of several biomolecules (i.e., Xylanase, β-Glucosidase, Chitinase, and Tramesan), alone or in combinations, on lettuce plant growth and quality. With this aim, the influence of these biomolecules on biomass, pigment content, and antioxidant properties in treated plants were investigated. Our results showed that Xylanase and, to a lesser extent, β-Glucosidase, have potentially biostimulant activity for lettuce cultivation, positively influencing carotenoids, total polyphenols, and ascorbic acid contents; similar effects were found with respect to antioxidative properties. Furthermore, the effect of the more promising molecules (Xylanase and β-Glucosidase) was also evaluated in kiwifruit cultured cells to test their putative role as sustainable input for plant cell biofactories. The absence of phytotoxic effects of both molecules at low doses (0.1 and 0.01 µM), and the significantly enhanced cell biomass growth, indicates a positive impact on kiwifruit cells.

Regulation of Carotenoid Biosynthesis and Degradation in Lettuce (Lactuca sativa L.) from Seedlings to Harvest

Lettuce (Lactuca sativa L.) is one of the commercially important leafy vegetables worldwide. However, lettuce cultivars vary widely in their carotenoid concentrations at the time of harvest. While the carotenoid content of lettuce can depend on transcript levels of key biosynthetic enzymes, genes that can act as biomarkers for carotenoid accumulation at early stages of plant growth have not been identified. Transcriptomic and metabolomic analysis was performed on the inner and outer leaves of the six cultivars at different developmental stages to identify gene-to-metabolite networks affecting the accumulation of two key carotenoids, β-carotene and lutein. Statistical analysis, including principal component analysis, was used to better understand variations in carotenoid concentration between leaf age and cultivars. Our results demonstrate that key enzymes of carotenoid biosynthesis pathway can alter lutein and β-carotene biosynthesis across commercial cultivars. To ensure high carotenoids content in leaves, the metabolites sink from β-carotene and lutein to zeaxanthin, and subsequently, abscisic acid needs to be regulated. Based on 2-3-fold carotenoids increase at 40 days after sowing (DAS) as compared to the seedling stage, and 1.5-2-fold decline at commercial stage (60 DAS) compared to the 40 DAS stage, we conclude that the value of lettuce for human nutrition would be improved by use of less mature plants, as the widely-used commercial stage is already at plant senescence stage where carotenoids and other essential metabolites are undergoing degradation.