Gene expression analysis in citrus reveals the role of gibberellins on photosynthesis and stress

The effect of gibberellins (GA) on internode transcriptome was investigated in transgenic Carrizo citrange (Citrus sinensis x Poncirus trifoliata) plants overexpressing endogenous CcGA20ox1 (encoding a GA biosynthetic gene), and in non-transformed explants treated with GA(3), using a citrus cDNA microarray. Substantial modulation of gene expression was found in sense CcGA20ox plants. Extensive up-regulation of genes involved in photosynthesis and carbon utilization, and down-regulation of those involved in protein synthesis and ribosome biogenesis were shown for the first time in plants with higher GA content. Importantly, increase of net photosynthesis in attached leaves was also demonstrated. Expression of other genes belonging to functional groups not reported previously to be regulated by GA (mainly abiotic and biotic stresses, and cuticle biosynthesis), and genes involved in cell division and cell wall architecture were also differentially expressed. Culture of citrus explants for 24 h in GA(3) solution produced much lower changes in the transcriptome compared with CcGA20ox plants (1.6% versus 16%, respectively, of total genes in the microarray), suggesting that most of the changes observed in CcGA20ox plants were a consequence of a long-standing GA effect. Interestingly, genes related to abiotic and biotic stresses were similarly modulated in transgenics and GA(3)-treated explants.