Transcriptome analysis of acerola fruit ripening: insights into ascorbate, ethylene, respiration, and softening metabolisms.

KEY MESSAGE: The first transcriptome coupled to metabolite analyses reveals major trends during acerola fruit ripening and shed lights on ascorbate, ethylene signalling, cellular respiration, sugar accumulation, and softening key regulatory genes. Acerola is a fast growing and ripening fruit that exhibits high amounts of ascorbate. During ripening, the fruit experience high respiratory rates leading to ascorbate depletion and a quickly fragile and perishable state. Despite its growing economic importance, understanding of its developmental metabolism remains obscure due to the absence of genomic and transcriptomic data. We performed an acerola transcriptome sequencing that generated over 600 million reads, 40,830 contigs, and provided the annotation of 25,298 unique transcripts. Overall, this study revealed the main metabolic changes that occur in the acerola ripening. This transcriptional profile linked to metabolite measurements, allowed us to focus on ascorbate, ethylene, respiration, sugar, and firmness, the major metabolism indicators for acerola quality. Our results suggest a cooperative role of several genes involved in AsA biosynthesis (PMM, GMP1 and 3, GME1 and 2, GGP1 and 2), translocation (NAT3, 4, 6 and 6-like) and recycling (MDHAR2 and DHAR1) pathways for AsA accumulation in unripe fruits. Moreover, the association of metabolites with transcript profiles provided a comprehensive understanding of ethylene signalling, respiration, sugar accumulation and softening of acerola, shedding light on promising key regulatory genes. Overall, this study provides a foundation for further examination of the functional significance of these genes to improve fruit quality traits.

Differential expression of recently duplicated PTOX genes in Glycine max during plant development and stress conditions.

Plastid terminal oxidase (PTOX) is a chloroplast enzyme that catalyzes oxidation of plastoquinol (PQH2) and reduction of molecular oxygen to water. Its function has been associated with carotenoid biosynthesis, chlororespiration and environmental stress responses in plants. In the majority of plant species, a single gene encodes the protein and little is known about events of PTOX gene duplication and their implication to plant metabolism. Previously, two putative PTOX (PTOX1 and 2) genes were identified in Glycine max, but the evolutionary origin and the specific function of each gene was not explored. Phylogenetic analyses revealed that this gene duplication occurred apparently during speciation involving the Glycine genus ancestor, an event absent in all other available plant leguminous genomes. Gene expression evaluated by RT-qPCR and RNA-seq data revealed that both PTOX genes are ubiquitously expressed in G. max tissues, but their mRNA levels varied during development and stress conditions. In development, PTOX1 was predominant in young tissues, while PTOX2 was more expressed in aged tissues. Under stress conditions, the PTOX transcripts varied according to stress severity, i.e., PTOX1 mRNA was prevalent under mild or moderate stresses while PTOX2 was predominant in drastic stresses. Despite the high identity between proteins (97%), molecular docking revealed that PTOX1 has higher affinity to substrate plastoquinol than PTOX2. Overall, our results indicate a functional relevance of this gene duplication in G. max metabolism, whereas PTOX1 could be associated with chloroplast effectiveness and PTOX2 to senescence and/or apoptosis.

Polymorphisms in plastoquinol oxidase (PTOX) from Arabidopsis accessions indicate SNP-induced structural variants associated with altitude and rainfall.

Plant plastoquinol oxidase (PTOX) is a chloroplast oxidoreductase involved in carotenoid biosynthesis, chlororespiration, and response to environmental stresses. The present study aimed to gain insight of the potential role of nucleotide/amino acid changes linked to environmental adaptation in PTOX gene/protein from Arabidopsis thaliana accessions. SNPs in the single-copy PTOX gene were identified in 1190 accessions of Arabidopsis using the Columbia-0 PTOX as a reference. The identified SNPs were correlated with geographical distribution of the accessions according to altitude, climate, and rainfall. Among the 32 identified SNPs in the coding region of the PTOX gene, 16 of these were characterized as non-synonymous SNPs (in which an AA is altered). A higher incidence of AA changes occurred in the mature protein at positions 78 (31%), 81 (31.4%), and 323 (49.9%). Three-dimensional structure prediction indicated that the AA change at position 323 (D323N) leads to a PTOX structure with the most favorable interaction with the substrate plastoquinol, when compared with the reference PTOX structure (Columbia-0). Molecular docking analysis suggested that the most favorable D323N PTOX-plastoquinol interaction is due to a better enzyme-substrate binding affinity. The molecular dynamics revealed that plastoquinol should be more stable in complex with D323N PTOX, likely due a restraint mechanism in this structure that stabilize plastoquinol inside of the reaction center. The integrated analysis made from accession geographical distribution and PTOX SNPs indicated that AA changes in PTOX are related to altitude and rainfall, potentially due to an adaptive positive environmental selection.

Genome-wide identification of ascorbate-glutathione cycle gene families in soybean (Glycine max) reveals gene duplication events and specificity of gene members linked to development and stress conditions.

Ascorbate-glutathione (AsA-GSH) cycle plays an important role in tuning beneficial ROS accumulation for intracellular signals and imparts plant tolerance to oxidative stress by detoxifying excess of ROS. Here, we present genome-wide identification of AsA-GSH cycle genes (APX, MDHAR, DHAR, and GR) in several leguminous species and expression analyses in G. max during stress, germination and tissue development. Our data revealed 24 genes in Glycine genus against the maximum of 15 in other leguminous species, which was due to 9 pars of duplicated genes mostly originated from sub/neofunctionalization. Cytosolic APX and MDHAR genes were highly expressed in different tissues and physiological conditions. Germination induced genes encoding AsA-GSH proteins from different cell compartments, whereas vegetative phase (leaves) stimulated predominantly genes related to chloroplast/mitochondria proteins. Moreover, cytosolic APX-1, 2, MDHAR-1a, 1b and GR genes were the primary genes linked to senescence and biotic stresses, while stAPX-a, b and GR (from organelles) were the most abiotic stress related genes. Biotic and abiotic stress tolerant genotypes generally showed increased MDHAR, DHAR and/or GR mRNA levels compared to susceptible genotypes. Overall, these data clarified evolutionary events in leguminous plants and point to the functional specificity of duplicated genes of the AsA-GSH cycle in G. max.