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

Evidence of acidic Invertase as control step of sucrose level during ripening of two diploid banana fruit

We previously shown that diploid dessert cultivars accumulate more sucrose during ripening on plant than diploid cooking cultivars, and that Acid Invertase (AIV) might be in this case important by correlating the ratio sucrose / glucose + fructose of fruit (Fils-Lycaon et al. poster). To investigate the putative involved mechanisms, Sucrose Phosphate Synthase (SPS) and AIV, two enzymes involved in sucrose metabolism were studied in fruit of two diploid AA dessert (IDN110) and cooking (Sowmuk) bananas, during their development and postharvest ripening after acetylene treatment. From the green developmental to late ripening stages, SPS activity increased at comparable levels in both varieties from 107 to 222 µmoles of sucrose.h-1.g FW-1 for Sowmuk and from 68 to 168 and then after 106 µmoles of sucrose.h-1.g FW-1 for IDN110. Activity of AIV increased slowly in IDN110 fruit from 3 to 26 µmoles of glucose.h-1.g FW-1, and drastically in Sowmuk from 4 to 207 and then after 174 µmoles of glucose.h-1.gFW-1. At molecular level, the pattern of SPS and Cw-AIV mRNA accumulation was similar in both varieties during fruit development and ripening. SPS mRNA level increased transiently during the green developmental stage while that of Cw-AIV increased with ripening. Interestingly, at equivalent stages, the increase of Cw-AIV mRNA level was approximately 100-fold more in Sowmuk than in IDN110, which corroborates with the higher AIV activity measured for Sowmuk. As between the two varieties, no significant changes were observed for neutral invertase activity and for Susy gene expression and activity, our data strongly suggest that AIV activity constitutes a key step that controls the sucrose level during ripening of diploid banana fruit and that Cw-AIV gene contributes to this activity. Thus, Cw-AIV constitutes a target for the identification of molecular markers usable in breeding, and marker-assisted selection to improve quality traits of banana fruit. (Résumé d'auteur