Assessment of RNAi-induced silencing in banana (Musa spp.).

BACKGROUND: In plants, RNA- based gene silencing mediated by small RNAs functions at the transcriptional or post-transcriptional level to negatively regulate target genes, repetitive sequences, viral RNAs and/or transposon elements. Post-transcriptional gene silencing (PTGS) or the RNA interference (RNAi) approach has been achieved in a wide range of plant species for inhibiting the expression of target genes by generating double-stranded RNA (dsRNA). However, to our knowledge, successful RNAi-application to knock-down endogenous genes has not been reported in the important staple food crop banana. RESULTS: Using embryogenic cell suspension (ECS) transformed with ß-glucuronidase (GUS) as a model system, we assessed silencing of gusAINT using three intron-spliced hairpin RNA (ihpRNA) constructs containing gusAINT sequences of 299-nt, 26-nt and 19-nt, respectively. Their silencing potential was analysed in 2 different experimental set-ups. In the first, Agrobacterium-mediated co-transformation of banana ECS with a gusAINT containing vector and an ihpRNA construct resulted in a significantly reduced GUS enzyme activity 6-8 days after co-cultivation with either the 299-nt and 19-nt ihpRNA vectors. In the second approach, these ihpRNA constructs were transferred to stable GUS-expressing ECS and their silencing potential was evaluated in the regenerated in vitro plants. In comparison to control plants, transgenic plants transformed with the 299-nt gusAINT targeting sequence showed a 4.5 fold down-regulated gusA mRNA expression level, while GUS enzyme activity was reduced by 9 fold. Histochemical staining of plant tissues confirmed these findings. Northern blotting used to detect the expression of siRNA in the 299-nt ihpRNA vector transgenic in vitro plants revealed a negative relationship between siRNA expression and GUS enzyme activity. In contrast, no reduction in GUS activity or GUS mRNA expression occurred in the regenerated lines transformed with either of the two gusAINT oligo target sequences (26-nt and 19-nt). CONCLUSIONS: RNAi-induced silencing was achieved in banana, both at transient and stable level, resulting in significant reduction of gene expression and enzyme activity. The success of silencing was dependent on the targeted region of the target gene. The successful generation of transgenic ECS for second transformation with (an)other construct(s) can be of value for functional genomics research in banana.

Characterization and isolation of a T-DNA tagged banana promoter active during in vitro culture and low temperature stress.

BACKGROUND: Next-generation transgenic plants will require a more precise regulation of transgene expression, preferably under the control of native promoters. A genome-wide T-DNA tagging strategy was therefore performed for the identification and characterization of novel banana promoters. Embryogenic cell suspensions of a plantain-type banana were transformed with a promoterless, codon-optimized luciferase (luc+) gene and low temperature-responsive luciferase activation was monitored in real time. RESULTS: Around 16,000 transgenic cell colonies were screened for baseline luciferase activity at room temperature 2 months after transformation. After discarding positive colonies, cultures were re-screened in real-time at 26 degrees C followed by a gradual decrease to 8 degrees C. The baseline activation frequency was 0.98%, while the frequency of low temperature-responsive luciferase activity was 0.61% in the same population of cell cultures. Transgenic colonies with luciferase activity responsive to low temperature were regenerated to plantlets and luciferase expression patterns monitored during different regeneration stages. Twenty four banana DNA sequences flanking the right T-DNA borders in seven independent lines were cloned via PCR walking. RT-PCR analysis in one line containing five inserts allowed the identification of the sequence that had activated luciferase expression under low temperature stress in a developmentally regulated manner. This activating sequence was fused to the uidA reporter gene and back-transformed into a commercial dessert banana cultivar, in which its original expression pattern was confirmed. CONCLUSION: This promoter tagging and real-time screening platform proved valuable for the identification of novel promoters and genes in banana and for monitoring expression patterns throughout in vitro development and low temperature treatment. Combination of PCR walking techniques was efficient for the isolation of candidate promoters even in a multicopy T-DNA line. Qualitative and quantitative GUS expression analyses of one tagged promoter in a commercial cultivar demonstrated a reproducible promoter activity pattern during in vitro culture. Thus, this promoter could be used during in vitro selection and generation of commercial transgenic plants.

Improved T-DNA vector for tagging plant promoters via high-throughput luciferase screening.

Transferred DNA (T-DNA) tagging is a powerful tool for tagging and in planta characterization of plant genes on a genome-wide scale. An improved promoter tagging vector is described here, which contains the codon-optimized luciferase (luc+) reporter gene 31 bp from the right border of the T-DNA. Compared to the wild-type luciferase gene, this construct provides significantly increased reporter gene expression and a 40 times higher tagging frequency. The utility of the construct is demonstrated in banana, a tropical monocot species, by screening embryogenic cell colonies and regenerated plants with an ultrasensitive charged-coupled device (CCD) camera. The improved vector resulted in a luciferase activation frequency of 2.5% in 19,000 cell colonies screened. Detailed molecular analysis of flanking DNA sequences in a tagged line revealed insertion of the luciferase tag in a novel gene with near-constitutive expression.