A transcriptional analysis reveals an extensive range of genes responsible for increasing the tolerance of Carrizo citrange to oxygen deficiency

Little information is available on the Citrus genus and its relatives with regard to their ability to tolerate oxygen deficiency, establishing physiological and structural modifications. In order to gain insight into how citrus rootstocks respond to low-oxygen stress, a transcriptomic analysis (using a custom microarray) was performed on Carrizo citrange (CC) seedlings. These seedlings were transformed with OsMybleu transcription factor (TF), known for inducing tolerance to oxygen deficiency, and compared with CC wildtype. They were flushed for 24 h with N2 and microarray, carrying out expressed sequence tags of Citrus and relatives isolated from the roots, was hybridized with RNA of roots before and after hypoxia treatment. The genes involved in fermentation, Krebs cycle, sugar metabolism, cell wall metabolism, hormones, and TFs all resulted significantly altered in response to hypoxia in both samples. Quantitative expression analysis was performed on 42 selected genes to validate microarray results. The outcome was that most of them were confirmed. The main results lead to the conclusion that CC is naturally tolerant to oxygen limitation. Transformed CC responded to hypoxia by activating the main genes which are known in other plants to be responsible for this type of tolerance such as pyruvate decarboxylase and alcohol dehydrogenase. Among TFs, several were also induced, such as an HDZipIII homologous to AtHB15, target of mir166, itself overexpressed exclusively in transformed CC under hypoxia compared with all other samples. The present manuscript represents one of the very few investigative works focused on hypoxia-responsive transcriptional networks in citrus