Differentiation between Flavors of Sweet Orange (<i>Citrus sinensis</i>) and Mandarin (<i>Citrus reticulata</i>)

Pioneering investigations referring to citrus flavor have been intensively conducted. However, the characteristic flavor difference between sweet orange and mandarin has not been defined. In this study, sensory analysis illustrated the crucial role of aroma in the differentiation between orange flavor and mandarin flavor. To study aroma, Valencia orange and LB8–9 mandarin were used. Their most aroma-active compounds were preliminarily identified by aroma extract dilution analysis (AEDA). Quantitation of key volatiles followed by calculation of odor activity values (OAVs) further detected potent components (OAV ≥ 1) impacting the overall aromatic profile of orange/mandarin. Follow-up aroma profile analysis revealed that ethyl butanoate, ethyl 2-methylbutanoate, octanal, decanal, and acetaldehyde were essential for orange-like aroma, whereas linalool, octanal, α-pinene, limonene, and (<i>E</i>,<i>E</i>)-2,4-decadienal were considered key components for mandarin-like aroma. Furthermore, an unreleased mandarin hybrid producing fruit with orange-like flavor was used to validate the identification of characteristic volatiles in orange-like aroma

Transcriptional and Microscopic Analyses of Citrus Stem and Root Responses to <i>Candidatus</i> Liberibacter asiaticus Infection

<div><p>Huanglongbing (HLB) is the most destructive disease that affects citrus worldwide. The disease has been associated with <i>Candidatus</i> Liberibacter. HLB diseased citrus plants develop a multitude of symptoms including zinc and copper deficiencies, blotchy mottle, corky veins, stunting, and twig dieback. <i>Ca</i>. L. asiaticus infection also seriously affects the roots. Previous study focused on gene expression of leaves and fruit to <i>Ca</i>. L. asiaticus infection. In this study, we compared the gene expression levels of stems and roots of healthy plants with those in <i>Ca</i>. L. asiaticus infected plants using microarrays. Affymetrix microarray analysis showed a total of 988 genes were significantly altered in expression, of which 885 were in the stems, and 111 in the roots. Of these, 551 and 56 were up-regulated, while 334 and 55 were down-regulated in the stem and root samples of HLB diseased trees compared to healthy plants, respectively. Dramatic differences in the transcriptional responses were observed between citrus stems and roots to <i>Ca</i>. L. asiaticus infection, with only 8 genes affected in both the roots and stems. The affected genes are involved in diverse cellular functions, including carbohydrate metabolism, cell wall biogenesis, biotic and abiotic stress responses, signaling and transcriptional factors, transportation, cell organization, protein modification and degradation, development, hormone signaling, metal handling, and redox. Microscopy analysis showed the depletion of starch in the roots of the infected plants but not in healthy plants. Collapse and thickening of cell walls were observed in HLB affected roots, but not as severe as in the stems. This study provides insight into the host response of the stems and roots to <i>Ca</i>. L. asiaticus infection.</p></div

Regulation of receptor-like kinase (RLK) genes by <i>Ca</i>. L. asiaticus infection in the stems of Valencia sweet orange (<i>Citrus sinensis</i>).

<p>Genes that were significantly up-regulated following <i>Ca</i>. L. asiaticus infection are displayed in blue, and down-regulated genes are displayed in red. Abbreviations/definitions: LRR, leucine-rich repeats; Extensin, RLK with extensin motif; LysM, RLKs with lysine motif; C-lectin, RLKs with lectin-like motifs; Crinkly4-like, RLKs with crinkly4-like domains; DUF26, domain of unknown function 26; LRK 10-like, RLK gene linked to Lr10 locus; L-lectin, RLKs with lectin-binding domains; PERK-like, proline-rich extensin-like kinase; S-locus, RLK with S-domain similar to S-locus glycoproteins; RKF3-like, receptor-like kinase in flowers 3; Thaumatin, RLK-like thaumatin protein; WAK, wall-associated kinase.</p

Differentially expressed genes related to secondary metabolism in the stems and roots of Valencia sweet orange (<i>Citrus sinensis</i>) caused by <i>Ca</i>. L. asiaticus infection.

<p>Accession No. is a unique identifier of EST sequences from several citrus species and hybrids linked to the NCBI. LFC is the ratio of the expression level in the infected samples compared to the healthy trees. The ratio is the mean of 3 replicates. The annotation is according to the latest available BLASTx search at non-redundant protein database at the NCBI. Metabolic pathway grouping is based on the gene ontology in the MapMan program (Thimm <i>et al</i>., 2004).</p

Microscopic analyses of stems of healthy and HLB affected Valencia sweet orange.

<p><b>A, B.</b> Light microscopy of cross-sections of healthy young stems showing the phloem, cambium and xylem cells. F-Phloem fibers. <b>C,D</b> Cross section of HLB affected young stems showing greater thickness of the phloem layer compared to the healthy. Arrows point to thickened cell walls.</p

Microscopic analyses of roots of healthy and HLB affected Valencia sweet orange on citrumelo rootstock (<i>Citrus paradisi</i> x <i>Poncirus trifoliata</i>).

<p><b>A, B.</b> Light microscopy of cross-sections of healthy fibrous roots showing secondary thickening. The cortex and some phloem parenchyma cells are shown containing starch (S). The cortex, phloem, cambium and xylem areas are delineated by lines. <b>C, D</b> are cross-sections of HLB affected roots. Note that the phloem layers of these roots are comparable in thickness to those of healthy above. The arrows point to collapse, thickened cell walls.</p

Regulation of biotic stress-related gene pathways by <i>Ca</i>. L. asiaticus infection in the stems and roots of Valencia sweet orange (<i>Citrus sinensis</i>).

<p><b>A</b> = stem and <b>B</b> = root. Genes that were significantly up-regulated following <i>Ca</i>. L. asiaticus infection are displayed in blue, and down-regulated genes are displayed in red.</p

Microscopic analyses of roots of healthy and HLB affected Valencia sweet orange on citrumelo rootstock (<i>Citrus paradisi</i> x <i>Poncirus trifoliata</i>).

<p><b>A–D.</b> Electron microscopy of healthy fibrous roots. <b>A & C</b> are relative low magnifications showing size and shape of normal phloem parenchyma (PP), sieve elements (SE) and companion cells (CC<b>). B</b> shows a higher magnification of same areas enhancing the sieve element areas. <b>D–F</b> are HLB affected roots. <b>D</b> is a low magnification comparable to <b>A & C</b> above showing the enriched cytoplasmic contents of these cells compared to the healthy above. <b>E</b> shows enlarged middle lamellas (ML). <b>F</b> shows the collapsed sieve elements (CSE). N, prominent nucleus and V, vacuole.</p

Differentially expressed genes related to signaling in the stems and roots of Valencia sweet orange (<i>Citrus sinensis</i>) caused by <i>Ca</i>. L. asiaticus infection.

<p>Accession No. is a unique identifier of EST sequences from several citrus species and hybrids linked to the NCBI. LFC is the ratio of the expression level in the infected samples compared to the healthy trees. The ratio is the mean of 3 replicates. The annotation is according to the latest available BLASTx search at non-redundant protein database at the NCBI. Metabolic pathway grouping is based on the gene ontology in the MapMan program (Thimm <i>et al</i>., 2004).</p

Differentially expressed genes related to carbohydrate metabolism and cell wall biogenesis in the stems and roots of Valencia sweet orange (<i>Citrus sinensis</i>) caused by <i>Ca</i>. L. asiaticus infection.

<p>Accession No. is a unique identifier of EST sequences from several citrus species and hybrids linked to the NCBI. LFC is the ratio of the expression level in the infected samples compared to the healthy trees. The ratio is the mean of 3 replicates. The annotation is according to the latest available BLASTx search at non-redundant protein database at the NCBI. Metabolic pathway grouping is based on the gene ontology in the MapMan program (Thimm <i>et al</i>., 2004).</p