Curing and low-temperature combined post-harvest storage enhances anthocyanin biosynthesis in blood oranges.

Anthocyanins are pigments present in blood oranges which can be enriched by post-harvest cold storage. Additionally, citrus fruits contain appreciable levels of other flavonoids, whose content increases under post-harvest heat treatments. Here, we investigated the effects of curing (37 °C for 3 days) and storage at low-temperature (9 °C) during 15, 30 and 45 days on accumulation of anthocyanins and other flavonoids in Moro and Sanguinelli Polidori blood oranges (Citrus sinensis L. Osbeck). Cured fruits reached up to 191.4 ± 1.4 mg/L of anthocyanins in their juice after cold storage and a 3-fold enrichment of other flavonoids such as flavones and flavanones, compared to 85.7 ± 3.3 mg/L anthocyanins from fruits with cold storage alone. Concomitantly, qPCR analysis showed that curing enhanced upregulation of the main structural and transcription factor genes regulating the flavonoid pathway. GC-MS analysis showed that no unpleasant compounds were generated in the cured plus cold-stored juice volatilome.

A transcriptional approach to unravel the connection between phospholipases A2 and D and ABA signal in citrus under water stress.

The effect of water stress on the interplay between phospholipases (PL) A2 and D and ABA signalling was investigated in fruit and leaves from the sweet orange Navelate and its fruit-specific ABA-deficient mutant Pinalate by studying simultaneously expression of 5 PLD and 3 PLA2-encoding genes. In general, expression levels of PLD-encoding genes were higher at harvest in the flavedo (coloured outer part of the peel) from Pinalate. Moreover, a higher and transient increase in expression of CsPLDα, CsPLDß, CsPLDδ and CsPLDζ was observed in the mutant as compared to Navelate fruit under water stress, which may reflect a mechanism of acclimation to water stress influenced by ABA deficiency. An early induction in CsPLDγ gene expression, when increase in peel damage during fruit storage was most evident, suggested a role for this gene in membrane degradation processes during water stress. Exogenous ABA on mutant fruit modified the expression of all PLD genes and reduced the expression of CsPLDα and CsPLDß by 1 week to levels similar to those of Navelate, suggesting a repressor role of ABA on these genes. In general, CssPLA2α and ß transcript levels were lower in flavedo from Pinalate than from Navelate fruit during the first 3 weeks of storage, suggesting that expression of these genes also depends at least partially on ABA levels. Patterns of expression of PLD and PLA2-encoding genes were very similar in Navelate and Pinalate leaves, which have similar ABA levels, when comparing both RH conditions. Results comparison with other from previous works in the same experimental systems helped to decipher the effect of the stress severity on the differential response of some of these genes under dehydration conditions and pointed out the interplay between PLA2 and PLD families and their connection with ABA signalling in citrus.

Citrus phenylpropanoids and defence against pathogens. Part I: metabolic profiling in elicited fruits.

Penicillium spp. are among the major postharvest pathogens of citrus fruit. Induction of natural resistance in fruits constitutes one of the alternatives to chemical fungicides. Here, we investigated the involvement of the phenylpropanoid pathway in the induction of resistance in Navelate oranges by examining changes in the metabolic profile of upon eliciting citrus fruits. By using both HPLC-PDA-FD and HPLC-PDA-QTOF-MS allowed the identification of several compounds that seem to be relevant for induced resistance. In elicited fruits, a greater diversity of phenolic compounds was observed in the flavedo (outer coloured part of the peel) when compared to the albedo (inner white part). Moreover, only small changes were detected in the most abundant citrus flavonoids. The coumarin scoparone was among the compounds with the highest induction upon elicitation. Two other highly induced compounds were identified as citrusnin A and drupanin aldehyde. All three compounds are known to exert antimicrobial activity. Our results suggest that phenylpropanoids and their derivatives play an important role in the induction of resistance in citrus fruit.

Transcriptomic profiling of citrus fruit peel tissues reveals fundamental effects of phenylpropanoids and ethylene on induced resistance.

Penicillium spp. are the major postharvest pathogens of citrus fruit in Mediterranean climatic regions. The induction of natural resistance constitutes one of the most promising alternatives to avoid the environmental contamination and health problems caused by chemical fungicides. To understand the bases of the induction of resistance in citrus fruit against Penicillium digitatum, we have used a 12k citrus cDNA microarray to study transcriptional changes in the outer and inner parts of the peel (flavedo and albedo, respectively) of elicited fruits. The elicitor treatment led to an over-representation of biological processes associated with secondary metabolism, mainly phenylpropanoids and cellular amino acid biosynthesis and methionine metabolism, and the down-regulation of genes related to biotic and abiotic stresses. Among phenylpropanoids, we detected the over-expression of a large subset of genes important for the synthesis of flavonoids, coumarins and lignin, especially in the internal tissue. Furthermore, these genes and those of ethylene biosynthesis showed the highest induction. The involvement of both phenylpropanoid and ethylene pathways was confirmed by examining changes in gene expression and ethylene production in elicited citrus fruit. Therefore, global results indicate that secondary metabolism, mainly phenylpropanoids, and ethylene play important roles in the induction of resistance in citrus fruit.