Expression patterns of cell wall-modifying genes from banana during fruit ripening and in relationship with finger drop.

Few molecular studies have been devoted to the finger drop process that occurs during banana fruit ripening. Recent studies revealed the involvement of changes in the properties of cell wall polysaccharides in the pedicel rupture area. In this study, the expression of cell-wall modifying genes was monitored in peel tissue during post-harvest ripening of Cavendish banana fruit, at median area (control zone) and compared with that in the pedicel rupture area (drop zone). To this end, three pectin methylesterase (PME) and seven xyloglucan endotransglycosylase/hydrolase (XTH) genes were isolated. The accumulation of their mRNAs and those of polygalaturonase, expansin, and pectate lyase genes already isolated from banana were examined. During post-harvest ripening, transcripts of all genes were detected in both zones, but accumulated differentially. MaPME1, MaPG1, and MaXTH4 mRNA levels did not change in either zone. Levels of MaPME3 and MaPG3 mRNAs increased greatly only in the control zone and at the late ripening stages. For other genes, the main molecular changes occurred 1-4 d after ripening induction. MaPME2, MaPEL1, MaPEL2, MaPG4, MaXTH6, MaXTH8, MaXTH9, MaEXP1, MaEXP4, and MaEXP5 accumulated highly in the drop zone, contrary to MaXTH3 and MaXTH5, and MaEXP2 throughout ripening. For MaPG2, MaXET1, and MaXET2 genes, high accumulation in the drop zone was transient. The transcriptional data obtained from all genes examined suggested that finger drop and peel softening involved similar mechanisms. These findings also led to the proposal of a sequence of molecular events leading to finger drop and to suggest some candidates.

Effects of the Physiological Age of Bananas on Their Susceptibility to Wound Anthracnose Due to Colletotrichum musae.

Wound anthracnose, caused by Colletotrichum musae, and early ripening are the main problems affecting the quality of export bananas (Musa AAA Cavendish) from the Caribbean. These problems generally concern bananas grown in lowland plantations during the rainy season. Three experiments were carried out to study the influence of the physiological age of bananas, calculated on the basis of mean daily temperature sums, on their susceptibility to anthracnose. Stressful growing conditions, especially soil flooding, slowed fruit growth but had no direct effect on fruit susceptibility to C. musae or on the green life. However, fruit that had accumulated lower temperature sums were less susceptible to wound anthracnose. By varying the source-sink ratio, we show that bananas of the same grade but different physiological ages had markedly different susceptibility to C. musae. Bananas with the same temperature sum accumulation but grown in different soil-climate conditions had different levels of susceptibility. Fruit grown in cooler, highland areas were less susceptible to C. musae than fruit of the same physiological age from lowland plantations. Our results suggest that temperature sum accumulation rate is a critical factor affecting the susceptibility of bananas to the pathogen.

Elaboration of an Early Quantification Method of Quiescent Infections of Colletotrichum musae on Bananas.

Colletotrichum musae, the causal agent of banana anthracnose, forms quiescent infections on the surface of the fruit that lead to necrosis at maturity. This work was conducted to determine a practical way to quantify the inoculum level that is economically practicable for routine use and applicable to immature fruit, so that it can be used as a method of early prediction and subsequent decision support. The method that has been developed is based on treating the fruit with 1,200 µl of ethylene per liter for 24 h at 25°C. The fruit are then kept at 32°C for 5 days, maintaining the ethylene concentration, so that conditions are optimal for revealing the disease. The technique can be used on fruit whose age is 5 to 6 weeks after inflorescence emergence. A high CO2 concentration has an inhibitory effect on the development of lesions. The importance of the ethylene treatment on fruit maturation and breaking of appressorium dormancy is discussed, together with potential applications.