An Imaging Approach to Identify Mechanisms of Resistance to Pineapple Fruitlet Core Rot

Abstract

Fruitlet core rot is one of the major postharvest disease of pineapple (Ananas comosus var. comosus). In the past, control strategies were designed to eliminate symptoms without addressing their causes or mechanisms, thus achieving only moderate success. In this study, (i) we focused on the anatomy of the fruitlets in the resistant “MD-2” and susceptible “Queen” pineapple cultivars; (ii) we identified the key role of the carpel margin in the infection process; (iii) we identified the key role of the sinuous layer of thick-walled cells in the inhibition of Fusarium ananatum colonization; and (iv) we linked the anatomy of the fruitlets with the phenolic content of cell walls. The fruitlet anatomy of the two cultivars was studied using X-ray, fluorescence, and multiphoton microscopy. Sepals and bracts were not perfectly fused with each other, allowing the pathogen to penetrate the fruit even after flowering. In fact, the fungi were found in the blossom cups of both cultivars but only became pathogenic in the flesh of the “Queen” pineapple fruit under natural conditions. The outer layer of the “MD-2” cavity was continuous with thick cell walls composed of ferulic and coumaric acids. The cell walls of the “Queen” blossom cup were less lignified at the extremities, and the outer layer was interspersed with cracks. The carpel margins were fused broadly in the “MD-2” pineapple, in contrast to the “Queen” pineapple. This blemish allows the fungus to penetrate deeper into the susceptible cultivar. In pineapple fruitlets, the hyphae of F. ananatum mainly progressed directly between cell walls into the parenchyma but never reached the vascular region. A layer of thick-walled cells, in the case of the resistant cultivar, stopped the colonization, which were probably the infralocular septal nectaries. Anatomical and histochemical observations coupled with spectral analysis of the hypodermis suggested the role of lignin deposition in the resistance to F. ananatum. The major phenolics bound to the cell walls were coumaric and ferulic acids and were found in higher amounts in the resistant cultivar postinoculation. The combination of fruitlet anatomy and lignification plays a role in the mechanism of host resistance to fruitlet core rot.

Diversity and Toxigenicity of Fungi that Cause Pineapple Fruitlet Core Rot

“…This browning is the result of the oxidation of phenolic compounds into quinones by the enzymes polyphenol oxidase and laccase [35]. Soluble and cell wall-bound phenolic acids are found in the pineapple fruit parenchyma [36,37] and increase dramatically after the onset of the first symptoms of FCR [38,39]. These compounds provide antifungal activity on several species of fungi.…”

Potential Use of Essentials Oils to Control Fruitlet Core Rot (FCR) in Pineapple (Queen Victoria Variety) in Reunion Island

“…This result is contrary to what was obtained on mangoes [13] [14] and avocado [15] where the synthesis of polyphenolic compounds involved in the resistance mechanisms of fruits was elicited. The HBA contents generally remain stable or decrease lightly when necrosis develops, and the HCA contents increase strongly [6]. Thymol treatment disrupted the polyphenol biosynthesis chain characterized by a drop in levels.…”

“…The fungus was isolated from naturally infected "Queen" pineapple on Réunion Island and purified in selective media. The Fusarium-specific primer elongation factor tef-1α and tef-2α made it possible to identify Clp001 as a Fusarium ananatum [6].…”

“…Once the fungus reaches the floral cavity, it remains latent during fruit growth [4]. There are physiological changes following the pathogen attack, and the phenylpropanoid pathway is solicited on pineapple [5] [6]. This author showed an accumulation of free coumaroylisocitrate and caffeoylisocitrate in pineapple fruitlets following infection with F. ananatum.…”

An overview of non-destructive approaches for quality determination in pineapples

“…Numerous fast and nondestructive methods have been used in tandem with the quality evaluation of pineapples. These non-destructive methods include computer vision system (Angel et al, 2015), imagingbased approaches (Barral et al, 2019), spectroscopy-based approaches (Khatiwada et al, 2016), acoustic impulse (Valente et al, 2001), and electronic nose (Kalpana & Lakshmi Baghyam, 2019).…”