Development of carotenoid storage cells in Bixa orellana L. seed arils

Research progress on differentiation and regulation of plant chromoplasts

Carotenoids, natural tetraterpenoids found abundantly in plants, contribute to the diverse colors of plant non-photosynthetic tissues and provide fragrance through their cleavage products, which also play crucial roles in plant growth and development. Understanding the synthesis, degradation, and storage pathways of carotenoids and identifying regulatory factors represents a significant strategy for enhancing plant quality. Chromoplasts serve as the primary plastids responsible for carotenoid accumulation, and their differentiation is linked to the levels of carotenoids, rendering them a subject of substantial research interest. The differentiation of chromoplasts involves alterations in plastid structure and protein import machinery. Additionally, this process is influenced by factors such as the ORANGE (OR) gene, Clp proteases, xanthophyll esterification, and environmental factors. This review shows the relationship between chromoplast and carotenoid accumulation by presenting recent advances in chromoplast structure, the differentiation process, and key regulatory factors, which can also provide a reference for rational exploitation of chromoplasts to enhance plant quality.

Plant age-dependent dynamics of annatto pigment (bixin) biosynthesis in Bixa orellana

Age affects the production of secondary metabolites, but how developmental cues regulate secondary metabolism remains poorly understood. The achiote tree (Bixa orellana L.) is a source of bixin, an apocarotenoid used in diverse industries worldwide. Understanding how age-dependent mechanisms control bixin biosynthesis is of great interest for plant biology and for economic reasons. Here we overexpressed miRNA156 (miR156) in B. orellana to comprehensively study the effects of the miR156-SQUAMOSA PROMOTER BINDING PROTEIN-LIKE (SPL) module on age-dependent bixin biosynthesis in leaves. Overexpression of miR156 in annatto plants (miR156ox) reduced BoSPL transcript levels, impacted leaf ontogeny, lessened bixin production, and increased abscisic acid levels. Modulation of expression of BoCCD4-4 and BoCCD1, key genes in carotenoid biosynthesis, was associated with diverting the carbon flux from bixin to abscisic acid in miR156ox leaves. Proteomic analyses revealed an overall low accumulation of most secondary metabolite-related enzymes in miR156ox leaves, suggesting that miR156-targeted BoSPLs may be required to activate several secondary metabolic pathways. Our findings suggest that the conserved BomiR156-BoSPL module is deployed to regulate leaf dynamics of bixin biosynthesis, and may create novel opportunities to fine-tune bixin output in B. orellana breeding programs.

Advances in Tissue Culture and Transformation Studies in Non-model Species: Bixa orellana L. (Bixaceae)

Over the years, our team has dedicated significant efforts to studying a unique natural dye-producing species, annatto (Bixa orellana L.). We have amassed knowledge and established foundations that support the applications of gene expression analysis in comprehending in vitro morphogenic regeneration processes, phase transition aspects, and bixin biosynthesis. Additionally, we have conducted gene editing associated with these processes. The advancements in this field are expected to enhance breeding practices and contribute to the overall improvement of this significant woody species. Here, we present a step-by-step protocol based on somatic embryogenesis and an optimized transformation protocol utilizing Agrobacterium tumefaciens.

Dynamics of annatto pigment synthesis and accumulation in seeds of Bixa orellana L. revealed by integrated chemical, anatomical, and RNA-Seq analyses

Bixin is a commercially valuable apocarotenoid pigment found in the seed aril of Bixa orellana. The dynamics and regulation of its biosynthesis and accumulation during seed development remain largely unknown. Here, we combined chemical, anatomical, and transcriptomic data to provide stage-specific resolution of the cellular and molecular events occurring during B. orellana seed development. Seeds at five developmental stages (S1-S5) were used for analysis of bixin content and seed anatomy, and three of them (S1, S3, and S4) were selected for Illumina HiSeq sequencing. Bixin accumulated in large quantities in seeds compared with other tissues analyzed, particularly during the S2 stage, peaking at the S4 stage, and then decreasing slightly in the S5 stage. Anatomical analysis revealed that bixin accumulated in the large central vacuole of specialized cells, which were scattered throughout the developing mesotesta at the S2 stage, but enlarged progressively at later stages, until they occupied most of the parenchyma in the aril. A total of 13 million reads were generated and assembled into 73,381 protein-encoding contigs, from which 312 were identified as containing 1-deoxy-D-xylulose-5-phosphate/2-C-methyl-D-erythritol-4-phosphate (DOXP/MEP), carotenoid, and bixin pathways genes. Differential transcriptome expression analysis of these genes revealed that 50 of them were sequentially and differentially expressed through the seed developmental stages analyzed, including seven carotenoid cleavage dioxygenases, eight aldehyde dehydrogenases, and 22 methyltransferases. Taken together, these results show that bixin synthesis and accumulation in seeds of B. orellana are a developmentally regulated process involving the coordinated expression of DOXP/MEP, carotenoid, and bixin biosynthesis genes.

Gene expression profile during seed development of Bixa orellana accessions varying in bixin pigment

Diverse morphological, cellular and physiological changes occur during seed maturation in Bixa orellana when the seed tissues form specialized cell glands that produce reddish latex with high bixin amounts. Transcriptomic profiling during seed development in three B. orellana accessions (P12, N4 and N5) with contrasting morphologic characteristics showed enrichment in pathways of triterpenes, sesquiterpenes, and cuticular wax biosynthesis. WGCNA allows groups of all identified genes in six modules the module turquoise, the largest and highly correlated with the bixin content. The high number of genes in this module suggests a diversification of regulatory mechanisms for bixin accumulation with the genes belonging to isoprene, triterpenes and carotene pathways, being more highly correlated with the bixin content. Analysis of key genes of the mevalonate (MVA) and the 2C-methyl-D-erythritol-4-phosphate (MEP) pathways revealed specific activities of orthologs of BoHMGR, BoFFP, BoDXS, and BoHDR. This suggests that isoprenoid production is necessary for compounds included in the reddish latex of developing seeds. The carotenoid-related genes BoPSY2, BoPDS1 and BoZDS displayed a high correlation with bixin production, consistent with the requirement for carotene precursors for apocarotenoid biosynthesis. The BoCCD gene member (BoCCD4-4) and some BoALDH (ALDH2B7.2 and ALDH3I1) and BoMET (BoSABATH1 and BoSABATH8) gene members were highly correlated to bixin in the final seed development stage. This suggested a contributing role for several genes in apocarotenoid production. The results revealed high genetic complexity in the biosynthesis of reddish latex and bixin in specialized seed cell glands in different accessions of B. orellana suggesting gene expression coordination between both metabolite biosynthesis processes.

Isolation and functional characterization of two dioxygenases putatively involved in bixin biosynthesis in annatto (Bixa orellana L.)

Carotenoid cleavage dioxygenases (CCDs) are enzymes that have been implicated in the biosynthesis of a wide diversity of secondary metabolites with important economic value, including bixin. Bixin is the second most used pigment in the world's food industry worldwide, and its main source is the aril of achiote (Bixa orellana L.) seeds. A recent transcriptome analysis of B. orellana identified a new set of eight CCD members (BoCCD4s and BoCCD1s) potentially involved in bixin synthesis. We used several approaches in order to discriminate the best candidates with CCDs genes. A reverse transcription-PCR (RT-qPCR) expression analysis was carried out in five developmental stages of two accessions of B. orellana seeds with different bixin contents: (P13W, low bixin producer and N4P, high bixin producer). The results showed that three BoCCDs (BoCCD4-1, BoCCD4-3, and BoCCD1-1) had an expression pattern consistent with bixin accumulation during seed development. Additionally, an alignment of the CCD enzyme family and homology models of proteins were generated to verify whether the newly proposed CCD enzymes were bona fide CCDs. The study confirmed that these three enzymes were well-preserved and belonged to the CCD family. In a second selection round, the three CCD genes were analyzed by in situ RT-qPCR in seed tissue. Results indicated that BoCCD4-3 and BoCCD1-1 exhibited tissue-specific expressions in the seed aril. To test whether the two selected CCDs had enzymatic activity, they were expressed in Escherichia coli; activity was determined by identifying their products in the crude extract using UHPLC-ESI-QTOF-MS/MS. The cleavage product (bixin aldehyde) was also analyzed by Fourier transform infrared. The results indicated that both BoCCD4-3 and BoCCD1-1 cleave lycopene in vitro at 5,6-5',6'.

Carotenoid Metabolism in Plants: The Role of Plastids

Carotenoids are indispensable to plants and critical in human diets. Plastids are the organelles for carotenoid biosynthesis and storage in plant cells. They exist in various types, which include proplastids, etioplasts, chloroplasts, amyloplasts, and chromoplasts. These plastids have dramatic differences in their capacity to synthesize and sequester carotenoids. Clearly, plastids play a central role in governing carotenogenic activity, carotenoid stability, and pigment diversity. Understanding of carotenoid metabolism and accumulation in various plastids expands our view on the multifaceted regulation of carotenogenesis and facilitates our efforts toward developing nutrient-enriched food crops. In this review, we provide a comprehensive overview of the impact of various types of plastids on carotenoid biosynthesis and accumulation, and discuss recent advances in our understanding of the regulatory control of carotenogenesis and metabolic engineering of carotenoids in light of plastid types in plants.

Unraveling Massive Crocins Transport and Accumulation through Proteome and Microscopy Tools during the Development of Saffron Stigma

Crocins, the glucosides of crocetin, are present at high concentrations in saffron stigmas and accumulate in the vacuole. However, the biogenesis of the saffron chromoplast, the changes during the development of the stigma and the transport of crocins to the vacuole, are processes that remain poorly understood. We studied the process of chromoplast differentiation in saffron throughout stigma development by means of transmission electron microscopy. Our results provided an overview of a massive transport of crocins to the vacuole in the later developmental stages, when electron dense drops of a much greater size than plastoglobules (here defined "crocinoplast") were observed in the chromoplast, connected to the vacuole with a subsequent transfer of these large globules inside the vacuole. A proteome analysis of chromoplasts from saffron stigma allowed the identification of several well-known plastid proteins and new candidates involved in crocetin metabolism. Furthermore, expressions throughout five developmental stages of candidate genes responsible for carotenoid and apocarotenoid biogenesis, crocins transport to the vacuole and starch metabolism were analyzed. Correlation matrices and networks were exploited to identify a series of transcripts highly associated to crocetin (such as 1-Deoxy-d-xylulose 5-phosphate synthase (DXS), 1-Deoxy-d-xylulose 5-phosphate reductoisomerase (DXR), carotenoid isomerase (CRTISO), Crocetin glucosyltransferase 2 (UGT2), etc.) and crocin (e.g., ζ-carotene desaturase (ZDS) and plastid-lipid-associated proteins (PLAP2)) accumulation; in addition, candidate aldehyde dehydrogenase (ADH) genes were highlighted.