An investigation on the softening and ripening process of tropical mango (Mangifera indica L.) with a particular focus on Rab GTPases

Mango (Mangifera indica L.) is an economically important fruit crop grown in the tropics. This crop species is popularly consumed as fruit in Malaysia. Once ripening is initiated, the process proceeds at a fast rate making postharvest life short. Excessive softening during ripening is a major challenge in the postharvest storage of fruits as it renders the fruits unfit for long term storage leading to heavy postharvest losses. The improved storage of mango fruit would greatly enhance the economic potential of this crop. Hence, an in-depth understanding of the ripening-related events is essential to facilitate the development of strategies to improve fruit quality and reduce post-harvest losses. Fruit softening during ripening involves the trafficking of cell wall polymers and enzymes. The Rab (Ras related proteins in brain) GTPase family are key players in vesicle trafficking. Therefore it is important to understand the linkage between the Rab (Ras related proteins in brain) GTPases and the differential softening rate in fruit varieties for effective postharvest management, hence this research. The first part of this research was conducted to characterize the ripening process of the mango varieties according to their postharvest quality attributes. The study was carried out on ‘Chokanan’ (CK), ‘Golden phoenix’(GP) and ‘Water lily’(WL) as exemplar mango varieties for which there are limited number of studies at the postharvest and molecular level respectively. Significant increase (P < 0.05) in the soluble solid concentration (SSC), ethylene production, respiration rate and weight loss coupled with significant decline in titratable acidity (TA) and fruit firmness occurred as ripening progressed. Significant differences were also found among the mango varieties. The analysis revealed that ‘Chokanan’ (CK) had greater fruit firmness (P < 0.05) than ‘Golden phoenix’ (GP) and ‘Water lily’ (WL) mango varieties. Multivariate analysis separated the unripe and ripe fruits according to their physicochemical attributes. The ripening stages (unripe and ripe) as defined based on the measured postharvest parameters were selected for further studies. Characterization of the mango Rab (Ras related proteins in brain) GTPase family by comparative analysis was performed. The study took advantage of the publicly available databases and transcriptome datasets to identify and conduct comprehensive comparison of the mango Rab (Ras related proteins in brain) GTPase family. A total of twenty-three members of the mango Rab (Ras related proteins in brain) GTPase family with similarity to those obtained from Arabidopsis thaliana and tomato (Solanum lycopersicum) were identified. Sequence similarity analyses and identification of conserved motifs, diagnostic of specific Rab family and subfamilies enabled the bona fide assignment of the deduced mango proteins. A transcriptomic approach by RNA-sequencing (RNA-seq) was performed to investigate the molecular basis of mango ripening using two ripening stages (unripe and ripe) and two mango groups with contrasting firmness (P 1 or log2 fold change (FC) < -1) respectively. On the other hand, comparison between mango groups at the same ripening stage identified 18,258 and 10,521 DEGs at the unripe and ripe stage respectively. Genes involved in the metabolism of energy, sugars, hormones and cell wall were differentially expressed. Notably, the Rab (Ras related proteins in brain) genes were also detected as differentially expressed suggesting the involvement of vesicle trafficking during ripening. Interaction analysis showed that the proteins involved in vesicle trafficking and cell wall softening were interconnected providing further evidence of the involvement of the Rab (Ras related proteins in brain) GTPases in fruit softening. The expression of ten genes evaluated by both RNA-sequencing (RNA-seq) and reverse transcription quantitative polymerase chain reaction (RT-qPCR) showed a good correlation (R2 = 0.769) indicating a good consistency between both techniques and the reliability of the RNA-sequencing data. Correlation analyses showed a significant relationship (P < 0.05) between the expression level of the RabA3 and RabA4 genes and fruit firmness at the unripe stage of ‘Chokanan’ (CK), ‘Golden phoenix’ (GP) and ‘Water lily’ (WL) suggesting that the differences in the Rab gene expression level may play an important role in the contrasting firmness of these varieties. The expression levels of these genes in other mango varieties were also analysed by reverse transcription quantitative polymerase chain reaction (RT-qPCR) but no consistent regularity was found suggesting the contribution of other factors to bring about softening in the varieties. In summary, these findings have provided insights into the molecular mechanisms underlying mango ripening and lay a foundation for the exploration of novel genes towards future development of strategies to improve fruit quality of mango and other non-model fleshy fruits

An investigation on the softening and ripening process of tropical mango (Mangifera indica L.) with a particular focus on Rab GTPases

Mango (Mangifera indica L.) is an economically important fruit crop grown in the tropics. This crop species is popularly consumed as fruit in Malaysia. Once ripening is initiated, the process proceeds at a fast rate making postharvest life short. Excessive softening during ripening is a major challenge in the postharvest storage of fruits as it renders the fruits unfit for long term storage leading to heavy postharvest losses. The improved storage of mango fruit would greatly enhance the economic potential of this crop. Hence, an in-depth understanding of the ripening-related events is essential to facilitate the development of strategies to improve fruit quality and reduce post-harvest losses. Fruit softening during ripening involves the trafficking of cell wall polymers and enzymes. The Rab (Ras related proteins in brain) GTPase family are key players in vesicle trafficking. Therefore it is important to understand the linkage between the Rab (Ras related proteins in brain) GTPases and the differential softening rate in fruit varieties for effective postharvest management, hence this research. The first part of this research was conducted to characterize the ripening process of the mango varieties according to their postharvest quality attributes. The study was carried out on ‘Chokanan’ (CK), ‘Golden phoenix’(GP) and ‘Water lily’(WL) as exemplar mango varieties for which there are limited number of studies at the postharvest and molecular level respectively. Significant increase (P < 0.05) in the soluble solid concentration (SSC), ethylene production, respiration rate and weight loss coupled with significant decline in titratable acidity (TA) and fruit firmness occurred as ripening progressed. Significant differences were also found among the mango varieties. The analysis revealed that ‘Chokanan’ (CK) had greater fruit firmness (P < 0.05) than ‘Golden phoenix’ (GP) and ‘Water lily’ (WL) mango varieties. Multivariate analysis separated the unripe and ripe fruits according to their physicochemical attributes. The ripening stages (unripe and ripe) as defined based on the measured postharvest parameters were selected for further studies. Characterization of the mango Rab (Ras related proteins in brain) GTPase family by comparative analysis was performed. The study took advantage of the publicly available databases and transcriptome datasets to identify and conduct comprehensive comparison of the mango Rab (Ras related proteins in brain) GTPase family. A total of twenty-three members of the mango Rab (Ras related proteins in brain) GTPase family with similarity to those obtained from Arabidopsis thaliana and tomato (Solanum lycopersicum) were identified. Sequence similarity analyses and identification of conserved motifs, diagnostic of specific Rab family and subfamilies enabled the bona fide assignment of the deduced mango proteins. A transcriptomic approach by RNA-sequencing (RNA-seq) was performed to investigate the molecular basis of mango ripening using two ripening stages (unripe and ripe) and two mango groups with contrasting firmness (P 1 or log2 fold change (FC) < -1) respectively. On the other hand, comparison between mango groups at the same ripening stage identified 18,258 and 10,521 DEGs at the unripe and ripe stage respectively. Genes involved in the metabolism of energy, sugars, hormones and cell wall were differentially expressed. Notably, the Rab (Ras related proteins in brain) genes were also detected as differentially expressed suggesting the involvement of vesicle trafficking during ripening. Interaction analysis showed that the proteins involved in vesicle trafficking and cell wall softening were interconnected providing further evidence of the involvement of the Rab (Ras related proteins in brain) GTPases in fruit softening. The expression of ten genes evaluated by both RNA-sequencing (RNA-seq) and reverse transcription quantitative polymerase chain reaction (RT-qPCR) showed a good correlation (R2 = 0.769) indicating a good consistency between both techniques and the reliability of the RNA-sequencing data. Correlation analyses showed a significant relationship (P < 0.05) between the expression level of the RabA3 and RabA4 genes and fruit firmness at the unripe stage of ‘Chokanan’ (CK), ‘Golden phoenix’ (GP) and ‘Water lily’ (WL) suggesting that the differences in the Rab gene expression level may play an important role in the contrasting firmness of these varieties. The expression levels of these genes in other mango varieties were also analysed by reverse transcription quantitative polymerase chain reaction (RT-qPCR) but no consistent regularity was found suggesting the contribution of other factors to bring about softening in the varieties. In summary, these findings have provided insights into the molecular mechanisms underlying mango ripening and lay a foundation for the exploration of novel genes towards future development of strategies to improve fruit quality of mango and other non-model fleshy fruits

Transcriptome-wide identification and characterization of the Rab GTPase family in mango

© 2020, Springer Nature B.V. The Rab GTPase family plays a vital role in several plant physiological processes including fruit ripening. Fruit softening during ripening involves trafficking of cell wall polymers and enzymes between cellular compartments. Mango, an economically important fruit crop, is known for its delicious taste, exotic flavour and nutritional value. So far, there is a paucity of information on the mango Rab GTPase family. In this study, 23 genes encoding Rab proteins were identified in mango by a comprehensive in silico approach. Sequence alignment and similarity tree analysis with the model plant Arabidopsis as a reference enabled the bona fide assignment of the deduced mango proteins to classify into eight subfamilies. Expression analysis by RNA-Sequencing (RNA-Seq) showed that the Rab genes were differentially expressed in ripe and unripe mangoes suggesting the involvement of vesicle trafficking during ripening. Interaction analysis showed that the proteins involved in vesicle trafficking and cell wall softening were interconnected providing further evidence of the involvement of the Rab GTPases in fruit softening. Correlation analyses showed a significant relationship between the expression level of the RabA3 and RabA4 genes and fruit firmness at the unripe stage of the mango varieties suggesting that the differences in gene expression level might be associated with the contrasting firmness of these varieties. This study will not only provide new insights into the complexity of the ripening-regulated molecular mechanism but also facilitate the identification of potential Rab GTPases to address excessive fruit softening

Characterization of Southeast Asia mangoes (Mangifera indica L) according to their physicochemical attributes

Mango (Mangifera indica L.) is an economically important fruit crop grown in the tropics. One of the important traits of mango for successful commercial production is the storage quality of the fruit. This study was conducted to evaluate the postharvest qualities of three mango (Mangifera indica) varieties namely ‘Chokanan’, ‘Golden phoenix’ and ‘Water lily’ grown in Southeast Asia regions. The study found that variety and ripening stage had an impact on the postharvest qualities. In general, an increase in weight loss, L* value and soluble solids concentration (SSC) along with a reduction in titratable acidity (TA), firmness and hue value as ripening progressed were observed irrespective of the variety. Analysis of variance and multivariate analysis were used to characterize the ripening process. This study provides useful information for devising strategies in postharvest handling and implementation of breeding programs for mango crop improvement

Comparative proteomic analysis on fruit ripening processes in two varieties of tropical mango (Mangifera indica)

Mango (Mangifera indica L.) is an economically important fruit. However, the marketability of mango is affected by the perishable nature and short shelf-life of the fruit. Therefore, a better understanding of the mango ripening process is of great importance towards extending its postharvest shelf life. Proteomics is a powerful tool that can be used to elucidate the complex ripening process at the cellular and molecular levels. This study utilized 2-dimensional gel electrophoresis (2D-GE) coupled with MALDI-TOF/TOF to identify differentially abundant proteins during the ripening process of the two varieties of tropical mango, Mangifera indica cv. ‘Chokanan’ and Mangifera indica cv ‘Golden Phoenix’. The comparative analysis between the ripe and unripe stages of mango fruit mesocarp revealed that the differentially abundant proteins identified could be grouped into the three categories namely, ethylene synthesis and aromatic volatiles, cell wall degradation and stress-response proteins. There was an additional category for differential proteins identified from the ‘Chokanan’ variety namely, energy and carbohydrate metabolism. However, of all the differential proteins identified, only methionine gamma-lyase was found in both ‘Chokanan’ and ‘Golden Phoenix’ varieties. Six differential proteins were selected from each variety for validation by analysing their respective transcript expression using reverse transcription-quantitative PCR (RT-qPCR). The results revealed that two genes namely, glutathione S-transferase (GST) and alpha-1,4 glucan phosphorylase (AGP) were found to express in concordant with protein abundant. The findings will provide an insight into the fruit ripening process of different varieties of mango fruits, which is important for postharvest management

Plant Rabs and the role in fruit ripening

Fruit ripening is a complex developmental process that involves the synthesis and modification of the cell wall leading up to the formation of an edible fruit. During the period of fruit ripening, new cell wall polymers and enzymes are synthesized and trafficked to the apoplast. Vesicle trafficking has been shown to play a key role in facilitating the synthesis and modification of cell walls in fruits. Through reverse genetics and gene expression studies, the importance of Rab guanosine triphosphatases (GTPases) as integral regulators of vesicle trafficking to the cell wall has been revealed. It has been a decade since a rich literature on the involvement of Rab GTPase in ripening was published. Therefore, this review sets out to summarize the progress in studies on the pivotal roles of Rab GTPases in fruit development and sheds light on new approaches that could be adopted in the fields of postharvest biology and fruit-ripening research