Zacarías-García Jaime, Cronje Paul J, Diretto Gianfranco, Zacarías Lorenzo, Rodrigo María Jesús
Departamento de Biotecnología de Alimentos, Instituto de Agroquímica y Tecnología de Alimentos (IATA), Consejo Superior de Investigaciones Científicas (CSIC), Valencia, Spain.
Citrus Research International (CRI), Department of Horticultural Sciences, University of Stellenbosch, Stellenbosch, South Africa.
Front Plant Sci. 2022 Oct 18;13:1034204. doi: 10.3389/fpls.2022.1034204. eCollection 2022.
Kirkwood Navel and Ruby Valencia are two spontaneous bud mutations of the respective parental lines of sweet orange () Palmer Navel and Olinda Valencia, showing an atypical red pigmentation of the pulp. These red-fleshed varieties are commercially available and highly attractive for consumers but their carotenoid metabolism and the basis of the mutation have not been investigated. The red colour of Kirkwood and Ruby pulp was observed from the very early stages of fruit development until full maturity and associated with an altered carotenoid profiling. The red-fleshed varieties accumulated from 6- up to 1000-times more total carotenoids compared to the standard oranges. Specifically, the pulp of Kirkwood and Ruby accumulated large amounts of phytoene and phytofluene, and moderate contents of lycopene. Moreover, the red-fleshed oranges contained other unusual carotenes as δ-carotene, and lower concentrations of downstream products such as β,β-xanthophylls, abscisic acid (ABA) and ABA-glucosyl ester. This peculiar profile was associated with chromoplasts with lycopene crystalloid structures and round vesicles likely containing colourless carotenes. The flavedo and leaves of Kirkwood and Ruby showed minor changes in carotenoids, mainly limited to higher levels of phytoene. The carotenoid composition in Kirkwood and Ruby fruits was not explained by differences in the transcriptional profile of 26 genes related to carotenoid metabolism, covering the main steps of biosynthesis, catabolism and other processes related to carotenoid accumulation. Moreover, sequence analysis of the lycopene cyclase genes revealed no alterations in those of the red-fleshed oranges compared to the genes of the standard varieties. A striking event observed in Kirkwood and Ruby trees was the reddish coloration of the inner side of the bark tissue, with larger amounts of phytoene, accumulation of lycopene and lower ABA content. These observation lead to the conclusion that the mutation is not only manifested in fruit, affecting other carotenogenic tissues of the mutant plants, but with different consequences in the carotenoid profile. Overall, the carotenoid composition in the red-fleshed mutants suggests a partial blockage of the lycopene β-cyclization in the carotenoid pathway, rendering a high accumulation of carotenes upstream lycopene and a reduced flow to downstream xanthophylls and ABA.
柯克伍德脐橙和鲁比巴伦西亚橙是甜橙（）帕尔默脐橙和奥林达巴伦西亚橙各自亲本系的两个自发芽变品种，其果肉呈现非典型的红色素沉着。这些红肉品种已投入商业生产，对消费者极具吸引力，但它们的类胡萝卜素代谢及突变的基础尚未得到研究。从果实发育的早期阶段直至完全成熟，都能观察到柯克伍德和鲁比果肉的红色，这与类胡萝卜素谱的改变有关。与标准橙子相比，红肉品种积累的总类胡萝卜素多6倍至1000倍。具体而言，柯克伍德和鲁比的果肉积累了大量的八氢番茄红素和六氢番茄红素，以及中等含量的番茄红素。此外，红肉橙还含有其他不寻常的胡萝卜素，如δ-胡萝卜素，以及较低浓度的下游产物，如β,β-叶黄素、脱落酸（ABA）和ABA-葡萄糖基酯。这种特殊的谱与具有番茄红素晶体结构和可能含有无色胡萝卜素的圆形小泡的有色体有关。柯克伍德和鲁比的外果皮和叶片在类胡萝卜素方面有微小变化，主要限于八氢番茄红素水平较高。柯克伍德和鲁比果实中的类胡萝卜素组成无法用与类胡萝卜素代谢相关的26个基因的转录谱差异来解释，这些基因涵盖了生物合成、分解代谢以及与类胡萝卜素积累相关的其他过程的主要步骤。此外，番茄红素环化酶基因的序列分析表明，与标准品种的基因相比，红肉橙的基因没有改变。在柯克伍德和鲁比树上观察到的一个显著现象是树皮组织内侧呈现微红颜色，含有大量八氢番茄红素、番茄红素积累且ABA含量较低。这些观察结果得出结论，该突变不仅表现在果实中，还影响突变植物的其他类胡萝卜素生成组织，但在类胡萝卜素谱方面有不同的结果。总体而言，红肉突变体中的类胡萝卜素组成表明类胡萝卜素途径中番茄红素β-环化存在部分阻断，导致番茄红素上游的胡萝卜素大量积累，而流向下游叶黄素和ABA的通量减少。
