Pre- and Post-harvest Factors Affecting Glucosinolate Content in Broccoli

Abstract

Owing to several presumed health-promoting biological activities, increased attention is being given to natural plant chemicals, especially those frequently entering the human diet. Glucosinolates (GLs) are the main bioactive compounds found in broccoli (Brassica oleracea L. var. italica Plenck). Their regular dietary assumption has been correlated with reduced risk of various types of neoplasms (lung, colon, pancreatic, breast, bladder, and prostate cancers), some degenerative diseases, such as Alzheimer's, and decreased incidence of cardiovascular pathologies. GL's synthesis pathway and regulation mechanism have been elucidated mainly in Arabidopsis. However, nearly 56 putative genes have been identified as involved in the B. oleracea GL pathway. It is widely recognized that there are several pre-harvest (genotype, growing environment, cultural practices, ripening stage, etc.) and post-harvest (harvesting, post-harvest treatments, packaging, storage, etc.) factors that affect GL synthesis, profiles, and levels in broccoli. Understanding how these factors act and interact in driving GL accumulation in the edible parts is essential for developing new broccoli cultivars with improved health-promoting bioactivity. In this regard, any systematic and comprehensive review outlining the effects of pre- and post-harvest factors on the accumulation of GLs in broccoli is not yet available. Thus, the goal of this paper is to fill this gap by giving a synoptic overview of the most relevant and recent literature. The existence of substantial cultivar-to-cultivar variation in GL content in response to pre-harvest factors and post-harvest manipulations has been highlighted and discussed. The paper also stresses the need for adapting particular pre- and post-harvest procedures for each particular genotype in order to maintain nutritious, fresh-like quality throughout the broccoli value chain.

Keywords: Brassica oleracea L var. italica; aliphatic; biosynthetic pathways; broccoli; indole and aromatic glucosinolates; pre-harvest and post-harvest management.

Figure 1

Anatomy of broccoli (Brassica oleracea L., var. Italica).

Figure 2

Global trend of broccoli and cauliflower import (A) and export (B) by countries from 1999 to 2017.

Figure 3

General structure of glucosinolates.

Figure 4

The aliphatic and indole glucosinolate metabolic pathways and related genes in Brassica oleracea L. Amino acid chain elongation is the first step, followed by the biosynthesis of the core glucosinolate structure and finally, side chain elongation of the synthesized structure in parallel with secondary modifications. The enzymes implicated in the different steps are reported in green. MAM1-3, methylthioalkyl malate synthases; BCAT3/4, branched-chain aminotransferases; IPMDH1/3, isopropylmalate dehydrogenases; IPMI-SSU1/2/3, isopropylmalate isomerases; CYP79B2/3, CYP79F1/2, CYP81F1-4, CYP83B1 cytochrome P450 monooxygenases; GSTF10/11, glutathione S-transferases; CYP83A1, non-redundant cytochrome P450. Enzymes metabolizing oximes in the biosynthesis of glucosinolates; GGP1, γ-glutamyl peptidase, SUR1, tyrosine transaminase family protein; UGT74B1/C1, thiohydroximate S-glucosyltransferases; SOT16-18, sulfotransferases; FMOGS-OX1-5, flavin-containing monooxygenase; AOP2/3, 2-oxoglutarate-dependent dioxygenases; BZO1, benzoyl-CoA ligase; GS-OH, 2-oxoglutarate (2OG) and Fe(II)-dependent oxygenase superfamily protein; TGG1/2, myrosinases; TSB1, tryptophan synthesis gene; SUR1, C-S lyase SUPERROOT; UGT74B1, UDP-dependent glycosyl transferases; IGMT1/2, indole glucosinolate methyl tansferases; EN2, atypical myrosinase; GSH, γ-glutamylcysteine synthetase; MYB28/29/34/51/121 and R2R3, transcription factors.

Figure 5

Methionine derived aliphatic GLs biosynthesis in Brassica vegetables. Elong, methionine elongation enzyme; UDP-glu T, UDP-glucose thiohydroximate glucotansferase; Sulfo T, 3′PAPS-5′-phosphosulphate desulphoglucosinolate transferase.

Figure 6

Chemical structure of the main glucosinolates found in broccoli. (A) Aliphatic Gls, (B) Indole GLs, and (C) Aromatic GLs. Glucoerucin, 4-Methylthiobutyl-GL; Glucoiberin, 3-Methylsulphinylpropyl-GL; Sinigrin, 2-Propenyl-GL; Glucoibervirin, 3-Methylthiopropyl-GL; Glucoraphanin, 4-Methylsulphinylbutyl-GL; Progoitrin, 2(R)-2-Hydroxy-3-butenyl-GL; Gluconapin, 3-Butenyl-GL; Glucocheirolin, 3-Methylsufonylpropyl-GL; Glucoberteroin, 5-Methylthiopentyl-GL; Glucoalyssin, 5-Methylsulphinylpentenyl-GL; Gluconapoleiferin, 2-Hydroxy-4-pentenyl-GL; Glucobrassicanapin, 4-Pentenyl-GL; Glucobrassicin, 3-Indolylmethyl-GL; Neoglucobrassicin, 1-Methoxy-3-indolylmethyl-GL; 4-Hydroxyglucobrassicin, 4-Hydroxy-3-indolylmethyl-GL; 4-Methoxyglucobrassicin, 4-Methoxy-3-indolylmethyl-GL; Gluconasturtiin, 2-Phenylethyl-GL.

Figure 7

Breakdown products of some broccoli glucosinolates.

Figure 8

Major pre- and post-harvest factors positively affecting glucosinolate content in broccoli. MeJA, Methyl Jasmonate; 1-MCP, 1-Methylcyclopropene; UV, ultraviolet; UV-B, ultraviolet B; UVC, ultraviolet C; K₂SO₄, Potassium sulfate.

Figure 9

Major pre- and post-harvest factors negatively affecting glucosinolate content in broccoli. KCl, Potassium chloride; 1-MCP, 1-Methylcyclopropene, UV-B, ultraviolet B; CaCl₂, Calcium Chloride.