Genome-Wide Association and Expression Analysis of the Lipoxygenase Gene Family in Passiflora edulis Revealing PeLOX4 Might Be Involved in Fruit Ripeness and Ester Formation

Abstract

Aroma is an important factor in fruit quality. Passiflora edulis (passion fruit) is popular among consumers because of its rich flavor and nutritional value. Esters are the main components of the volatile aroma of passion fruit. Lipoxygenase (LOX), as the first key enzyme upstream of esters, may play an important role in the formation of passion fruit aroma. In this study, a total of 12 passion fruit LOX (PeLOX) members were screened out based on the Passiflora edulis genome database, which were distributed unevenly on 6 chromosomes, all containing the highly conserved lipoxygenase domain and some containing the PLAT domain. The gene structure, evolutionary analysis and cis-acting elements of the family members were predicted in this study. Transcriptome analysis showed that 12 PeLOX genes had different degrees of response to different abiotic stresses (drought stress, salt stress, cold stress, and high temperature). PeLOX1, PeLOX2, PeLOX7, PeLOX11, and PeLOX12 responded significantly to various abiotic stresses, while PeLOX8 and PeLOX9 had little change in expression in all stresses. Quantitative real-time PCR (qRT-PCR) in six tissues revealed that the 12 PeLOX genes exhibited tissue expression specificity, and the relative expression of most genes were particularly high in the roots, stems, and fruits. Focusing on passion fruit ripening and ester synthesis, the transcriptomic analysis showed that with the increase in fruit development and fruit maturity, the expression levels of PeLOX1, PeLOX9, PeLOX11, and PeLOX12 showed downregulated expression, while PeLOX2 and PeLOX4 showed upregulated expression. In particular, the upregulation trend of PeLOX4 was the most obvious, and the qRT-PCR results were consistent with the transcriptome result. Pearson correlation analysis showed that with the development and ripening of fruit, the expression level of PeLOX4, LOX enzyme activity and total ester content all showed an increasing trend, in particular during the period when the peel was red and shrank (from T2 to T3 stage), the esters’ contents increased by 37.4 times; the highest expression levels were all in the T3 period. The results indicated that PeLOX4 may be a candidate gene involved in fruit ripeness and the formation of volatile aroma compounds, with the increase in fruit ripening, the expression level of PeLOX4 increased and the LOX enzyme activity increased accordingly, thereby promoting the synthesis of volatile esters in fruit pulp. Our discovery lays the foundation for the functional study of LOX in passion fruit.

Lipoxygenase (LOX) in Sweet and Hot Pepper (Capsicum annuum L.) Fruits during Ripening and under an Enriched Nitric Oxide (NO) Gas Atmosphere

“…The relevance of LOX activity in fruit ripening and postharvest storage has been studied in climacteric and non-climacteric fruits, where LOXs participate in aroma and flavor generation as well as in the generation of JA, a phytohormone implicated in the mechanism of wounding responses [ 35 , 36 , 37 , 38 ]. In the climacteric tomato ( Solanum lycopersicum ) fruits, 14 LOX genes have been identified [ 39 ].…”

“…This, therefore, places LOX IV in the crossroad of the regulatory roles attributed to this phytohormone. In this context, very recently it has been described that passion fruit ( Passiflora edulis ) has a total of 12 PeLOX genes whose expression change in the different organs and under several stress conditions [ 38 ]. However, it was remarkable that the PeLOX4 expression, as well as the total LOX activity in those fruits, increased significantly during the ripening process suggesting that this could be the candidate gene in the formation of volatile compounds that contribute to their flavor [ 38 ].…”

“…Pepper lipoxygenases show a complex regulation that depends on the pepper plant organ. In fruits, LOX regulation is associated with ripening [ 69 ] and it could contribute to some organoleptic properties such as aroma [ 38 , 70 ]. In pepper, the LOX regulation seems to take place at the transcriptional, translational, and post-translational levels.…”

Cotyledon Peeling Method for Passion Fruit Protoplasts: a versatile cell system for transient gene expression in passion fruit (Passiflora edulis)

“…It was not until 2021 that a chromosome-scale genome assembly of passion fruit (Passiflora edulis Sims) was just reported (Xia et al, 2021). This work enables several documentations for the identification of passion fruit genes, including eceriferm (CER), aquaporin (AQP), β-Ketoacyl-CoA synthase (KCS), peroxidase (POD), lateral organ boundary domain (LBD), and lipoxygenase (LOX) gene families (Rizwan et al, 2022a;Song et al, 2022;Rizwan et al, 2022b;Liang D. et al, 2022;Liang J. et al, 2022;Huang et al, 2022). However, one of the major barriers in gene functional studies is the lack of a simple and efficient transformation system in passion fruit.…”

Soybean (Glycine max L.) Lipoxygenase 1 (LOX 1) Is Modulated by Nitric Oxide and Hydrogen Sulfide: An In Vitro Approach

“…These products have numerous roles as growth regulators, antimicrobials and antifungals (aldehydes or divinyl ethers), flavors, and as signal molecules such as jasmonates [1][2][3]. Thus, LOXs are involved in diverse physiological plant functions including seed germination, plant growth and development, and fruit ripening and senescence [1,[4][5][6], but they are also involved in the mechanism of response to diverse environmental stresses [7,8].…”