_________________________________________ABSTRACT

The response to cold storage of the flavedo of citrus fruits was approached using a transcriptomics approach. Citrus fruits are normally exposed to low temperatures to satisfy quarantine requirements. These treatment are needed to restrict pest from spreading to other countries where citrus fruits are exported. Unfortunately, fruits of sensitive varieties develop chilling injury under low temperature conditions, mainly in the flavedo (colored external surface).
For this work, we took advantage of Clementina de Nules (CN) mandarins, which are cold-tolerant, and Fortune (F) mandarins, which are cold-sensitive. In this experimental system, we analyzed, at the transcriptomic level, cold-adaption responses and chilling injury-development responses to low temperature conditions. To do this, (1) two cDNA libraries (one of them full length) were generated from flavedo of CN mandarins expossed to cold; (2) novel cDNAs obtained from these libraries were included for the design of the ‘Proyecto de Genómica de Cítricos’ expression microarray (CFGP1); and finally, (3) transcriptomic changes in CN and F flavedo was investigated along a time course of storage at 2ºC by means of the CFGP1 microarray.
The cold response libraries showed a considerable percentage of novelty, among which putative cold response specific genes were identified. The CFGP1 microarray is composed of 12672 cDNA probe sets, which represent 6875 unique genes. In total, a 10.3% of these cDNAs comes from the cold response libraries.
Transcriptomic analyses revealed a similar trend in the response during the first four weeks of cold storage in CN and F. A significant repression of biosynthetic processes took place in both varieties, mainly for protein biosynthesis, and lipids biosynthesis as well. The catabolism of this type of macromolecules was also activated. However, there was also an enrichment of genes involved in protein biosynthesis among the up-regulated ones, which may reflect a re-organization of the cellular functions at the translational level. Despite the similarities, there were some differences between the two varieties, since the changes in CN were more intense during the first week of cold exposure, while they took place steadier in F.
In contrast to the similar responses during cold exposure, major differences were identified between the transcript profiles of CN and F flavedo before their exposure to cold. Some of these differences may be responsible for the cold tolerance in CN, together with its faster adaptive response during cold storage. Finally, a late expression program specific for F was also identified, including processes probably related with the chilling injury development in the flavedo of this sensitive variety.