Molecular physiology of banana fruit ripening : improvement of fruit quality

It is now assumed that the quality of fruit is a complex trait in which a lot of mechanisms are involved, some of them being antagonistic. All of these mechanisms are the results the coercive action of differentially regulated genes. Understanding at molecular level of the mechanisms that control the target quality trait is an essential work beforehand to any plan of improvement of fruit quality trait using plant genetic strategies such as candidate-gene approach and or marker assisted selection. Presently we investigate at molecular level the ethylene responsiveness, and sugar and phenylpropanoïd metabolisms, three ripening aspects involved in functional, nutritional and organoleptic qualities of banana fruit. As a first part of this project, we report here the cloning and sequencing of genes that are differentially expressed during fruit ripening, as tools for functional genomic studies and putative molecular marker developments. Different molecular biology approaches haves been used to isolate ripening these ripening related-genes. They include cDNA amplification (RT- and RACE-PCR) and, construction of complete and subtractive suppressive cDNA libraries (SSH). Sequencing and BLAST analysis of some of these isolated cDNA clones revealed that 205 of them presented a high homology with different genes in database. Many of the predicted proteins encoded by these genes are putatively involved in the regulation of gene expression, hormonal metabolism, hormonal-signal transduction, sugar metabolism and other ripening process. Among these 205 genes, 11 are still unclassified since presenting homology to unknown proteins of rice or Arabidopsis. Finally, thirteen additional clones were putatively novel, since they failed to match with database sequences. Expressions of a few of the matching clones have been followed in relation with ethylene responsiveness of fruit. Our results show that the expression of these genes is under ethylene and development (or both) control. These cDNA clones provide us with a basis for future work that will combine physiological, genomic and genetic approaches to identify key candidate-genes involved in the expression of banana quality trait. (Résumé d'auteur

Expression patterns of ethylene biosynthesis genes from banana during fruit ripening and in relationship with finger drop

Banana finger drop is expressed as a dislodgement of individual fruits from the hand at the pedicel rupture area. As bananas fruit are marketed in hands of generally 4?9 fruits, this postharvest disorder considerably reduces the commercial value of the product. Together with a burst of ethylene production, finger drop phenomenon was found to be one of the main features closely associated with banana ripening. We have shown that finger drop process occur early after ripening induction and imply ethylene?regulated gene. In this study, we investigate at molecular level the putative relationship between ethylene and finger drop processes during ripening of Cavendish banana fruit. To this end, expression of ethylene biosynthesis genes (MaACO1, MaACO2, MaACS1, MaACS2, MaACS3 and MaACS4) was examined at median area (control zone) and compared to that in the pedicel rupture area (drop zone). During the 4 first days following the ripening induction, transcripts of all genes were detected in both zones, but accumulated differentially. MaACO2 mRNA levels did not change in either zone. Levels of MaACO1, MaACS1, MaACS2, MaACS4 mRNAs accumulated highly in the drop zone. A high the mRNA of MaACS3 gene accumulated highly in drop zone only at the harvest time. One day after ripening induction, this level decreased drastically at comparable level to that observed at median zone, and remain constant in both zones throughout postharvest ripening. The results demonstrate that finger drop process involved ripening ethylene biosynthesis. They also suggest that ethylene can be one of the regulator cues of finger drop process. (Résumé d'auteur

Expression patterns of ethylene biosynthesis genes from bananas during fruit ripening and in relationship with finger drop

Banana finger drop is defined as dislodgement of individual fruits from the hand at the pedicel rupture area. For some banana varieties, this is a major feature of the ripening process, in addition to ethylene production and sugar metabolism. The few studies devoted to assessing the physiological and molecular basis of this process revealed (i) the similarity between this process and softening, (ii) the early onset of related molecular events, between the first and fourth day after ripening induction, and (iii) the putative involvement of ethylene as a regulatory factor. This study was conducted with the aim of identifying, through a candidate gene approach, a quality-related marker that could be used as a tool in breeding programmes. Here we examined the relationship between ripening ethylene biosynthesis (EB) and finger drop in order to gain further insight into the upstream regulatory steps of the banana finger drop process and to identify putative related candidate genes. Methods Postharvest ripening of green banana fruit was induced by acetylene treatment and fruit taken at 1-4 days after ripening induction, and total RNA extracted from the median area [control zone (CZ)] and the pedicel rupture area [drop zone (DZ)] of peel tissue. Then the expression patterns of EB genes (MaACO1, MaACO2, MaACS1, MaACS2, MaACS3 and MaACS4) were comparatively examined in CZ and DZ via real-time quantitative polymerase chain reaction. Principal results Differential expression of EB gene was observed in CZ and DZ during the postharvest period examined in this study. MaACO1, MaACS2 and MaACS1 were more highly induced in DZ than in the control, while a slight induction of the MaACS4 gene was observed. No marked differences between the two zones were observed for the MaACO2 gene. Conclusions The finger drop process enhanced EB gene expression including developmental- and ripeninginduced genes (MaACO1), specific ripening-induced genes (MaACS1) and wound-induced. (Résumé d'auteur