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

Lawson, Tamunonengiyeofori (2019) An investigation on the softening and ripening process of tropical mango (Mangifera indica L.) with a particular focus on Rab GTPases. PhD thesis, University of Nottingham.

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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 < 0.05). It is worth pointing out that this is the first experiment reported on Southeast Asian mango varieties employing the RNA-sequencing (RNA-seq) approach. Pairwise comparison between the ripening stages (unripe and ripe) of Chokanan (‘CK’) and Pool (‘P’) mango groups identified 9,765 and 13,651 differentially expressed genes (DEGs) (adjusted P value < 0.05; log2 fold change (FC) > 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.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Chin, Chiew Foan
Lycett, Grantley W.
Ali, Asgar
Mayes, Sean
Keywords: Fruit, mango, ripening, transcriptomics, trafficking
Subjects: Q Science > QP Physiology
Faculties/Schools: UNMC Malaysia Campus > Faculty of Science > School of Biosciences
Item ID: 55915
Date Deposited: 25 Feb 2019 07:28
Last Modified: 25 Feb 2019 07:36

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