Differential expression of putative 9-cis-epoxycarotenoid dioxygenases and abscisic acid accumulation in water stressed vegetative and reproductive tissues of citrus

The relationship between ABA accumulation and the expression of putative carotenoid cleavage dioxygenase (CCD) genes in water-stressed vegetative and reproductive tissues from Clemenules mandarin (Citrus clementina) scions grafted on Cleopatra mandarin (Citrus reshni) rootstocks was investigated. Four full-length cDNAs (CcNCED3, CcCCD4a, CcCCD4b, and CcNCED5) closely related to the CCD group and in particular to the 9-cis-epoxycarotenoid dioxygenase (NCED) subfamily of Arabidopsis were cloned. In citrus seedlings drought-induced progressive ABA accumulation in roots, leaves and developing ovaries while re-hydration provoked sudden reduction of its levels. In these organs expression of CcCCD and CcNCED genes in normal conditions was low and similar. During the drought/re-watering cycles expression of CcNCED3, the major drought-induced gene, paralleled the pattern of ABA accumulation. In contrast, either minor or no up-regulation changes of CcNCED5 and CcCCD4a transcripts were observed. In green fruits, transcript levels of CcNCED3 and CcNCED5 in the flavedo (coloured fruit layer) were rather similar to those observed in developing ovaries. However, CcNCED5 expression dramatically increased at colour break and remained high in ripe fruit. Consistent with this observation, in the flavedo of detached ripe fruits a 37 °C dehydration treatment increased in parallel levels of ABA and CcNCED5 transcripts. The above results showed that in Clemenules mandarin water stress induced specific up-regulation of CcNCED3 in photosynthetically active tissues such as leaves and developing ovaries. On the other hand, CcNCED5 was preferentially induced in Clemenules mandarin ripening fruits.