Impaginato 107 1. General Aspects Rocket salad also known as arugula is an annual herbaceous plant whose name encloses several species of the Brassicaceae family characterized by l ea v e s w i t h p ec u l i a r p u n g en t t a s t e a n d s t r o n g f l a v o u r . T h e c r o p h a s b e e n o r i g i n a t e d i n t h e Mediterranean and Near East, with a major centre of diversity in the regions of Western Mediterranean (Hall et al., 2015), which represent also the main areas of cultivation thanks to their growing condi- tions and climate. Several species, mainly belonging to the genus Eruca and Diplotaxis, are widely cultivated and recog- nized as rocket salad. The most common are Eruca vesicaria (L.) Cav. and Diplotaxis tenuifolia (L.) Dc. Eruca vesicaria includes four subspecies namely subsp. vesicaria, subsp. sativa (Miller), subsp. lon- girostris (Uechtr.) and subsp. pinnatifida (Desf.) (Gomez-Campo, 2003). Among these, the subsp sati- va, also called Eruca sativa, has been spreaded in dif- ferent part of the world as cultivated rocket and is the most consumed and economically relevant. This species is diploid with eleven chromosomes (2n = 22) (Padulosi and Pignone, 1996) with annual life cycle flowering at begin of spring and ending with the pro- d u c t i o n o f s e e d s i n l a t e s p r i n g / e a r l y s u m m e r . Nowadays it is cultivated in all continents in both marginal areas and/or fertile soils. Plants of this species are characterized by a height of about 15.0 cm, flowers with calyx caduceus, sepals only two cucullate and corolla cream or whitish (Gomez- Campo, 2003). D i p l o t a x i s g e n u s i n c l u d e s a b o u t 3 3 s p e c i e s (www.theplantlist.org) with great variability related to morphological traits and chromosome number (Table 1). The genus includes both annual and peren- nial plants with leaves of different shape, thickness, Adv. Hort. Sci., 2017 31(2): 107-113 DOI: 10.13128/ahs-21087 Mini review Rocket salad: crop description, bioactive compounds and breeding perspectives P. Tripodi (*), G. Francese, G. Mennella Consiglio per la Ricerca in Agricoltura e l’Analisi dell’Economia Agraria, Centro di Ricerca Orticoltura e Florovivaismo (CREA-OF), 84098 Pontecangnano-Faiano (SA), Italy. Key words: Diplotaxis tenuifolia, Eruca sativa, flavonols, genetic improvement, glucosinolates, phytochemicals, rocket salad. Abstract: Rocket salad is a plant member of the Brassicaceae family whose name encloses species of the Eruca and Diplotaxis genera characterized by leaves with peculiar pungent taste and strong flavour. It has been originated in the Mediterranean area and nowadays is worldwide cultivated and consumed as food condiment and in ready-to-use mixed salad packages. Several other uses are recognized in cosmetics and medicine. This crop represents a valuable source of health benefits due to the presence of a range of health-promoting phytochemicals including carotenoids, vitamin C, fiber, polyphenols, and glucosinolates. These compounds are potentially linked in the prevention of certain diseases and types of cancer. Glucosinolates, represent the major class of compounds in rocket, and their hydrolysis products are responsible of the typical pungent aromas and flavours. Despite the continuous increase of the global consumption dur- ing the recent years, few efforts have been carried out in genetic improvement programs aimed to constitute new vari- eties due to biological and reproductive barriers. In the present article is provided a brief overview of the principal species of rocket salad used in dietary and discussed the qualitative properties as well as the potentiality and constraints for breeding. (*) Corresponding author: pasquale.tripodi@crea.gov.it Received for publication 7 March 2017 Accepted for publication 21 June 2017 Copyright: © 2017 Author(s). This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. http://creativecommons.org/licenses/by/4.0/ http://creativecommons.org/licenses/by/4.0/ Adv. Hort. Sci., 2017 31(2): 107-113 108 indentation, and flower colour (white, yellow and purple). The most common species cultivated across all continents are Diplotaxis tenuifolia and Diplotaxis muralis. Both are perennial being cultivated in the winter and producing new sprouts in the spring. This aspect, combined to the dehiscence of the siliqua and the large number of viable seeds, helps to spread these species as weeds. Diplotaxis tenuifolia is the most used for human consumption; plants are char- acterized by average height of 80 cm, a deep tap root, fleshy leaves, and oblong, lobed with pointed apexes. Main differences occurred between Eruca and Diplotaxis genus in terms of plant architecture, leaf morphology, chromosomal number and phytochemi- cal compound contents. Eruca species, being annu- als, tends to have a higher growth rate, increased size of leaves and early flowering. These characteris- tics result in a high production of biomass, which make the system of cultivation different respect to the Diplotaxis spp. and requiring a lower seeding density and a lower number of harvests. Another trait discriminating the two genera is the larger seed dimension of the Eruca species (1.5 mm in length) with respect to Diplotaxis spp. (about 0.7 mm in length) (Padulosi and Pignone, 1996). A wider germi- nation temperature range and greater speed of ger- mination is also observed within the Eruca species, which is probably due to their annual nature, requir- ing greater energy of the plant to produce viable seeds (Hall et al., 2015). The consumption of rocket salad dates back since ancient time and included food and non food uses such as oil, deodorant, cosmetic and medical purpos- es (Hall et al., 2012). Aphrodisiac properties and medical uses related to anti inflammatory and depu- rative effects (Padulosi and Pignone, 1996) were emphasized by ancient poets during Greek and Roman times. Nowadays leaves are eaten fresh in salads or as topping of many dishes (e.g. pizza) or cooked in soups. Several recipes provide the prepara- tion of pureed, sauces and pesto. Other cosmetic uses concern the production of creams and lotions for body. Rocket salad is worldwide cultivated and commer- cialized in many countries as mix salad packages. In Europe, the needing of prepared products ready to use as well as the major attention given to a well bal- anced and assorted diet, composed of a variety of health-promoting compounds, has facilitated its con- sumption. In central and northern markets, over half of the rocket comes from Italy and Spain, which, Table 1 - List of Diplotaxis species recognized and their chromo- some number * na = not available. Species Chromosome number D. acris (Forsk.) Boiss. var. acris 11 var. duveyrieriana (Coss.) Coss. na * D. antoniensis Rustan na D. assurgens (Del.) Thell. 9 D. berthautii Br.-Bl. and Maire 9 D. brachycarpa Godr. 9 D. brevisiliqua (Coss.) Mart.-Laborde 8 D. catholica (L.) DC. var. catholica 9 var. rivulorum (Br.-Bl. And Maire) Maire na D. erucoides (L.) DC. subsp. Erucoides 7 subsp. longisiliqua (Coss.) Gómez-Campo 7 D. glauca (J.A. Schmidt) O.E. Schulz 13 D. gorgadensis Rustan subsp. gorgadensis na subsp. brochmanii Rustan na D. gracilis (Webb) O.E. Schulz 13 D. griffitthii (Hook.f. and Thomps.) Boiss. na D. harra (Forsk.) Boiss. subsp. crassifolia (Rafin.) Maire 13 subsp. harra 13 subsp. lagascana (DC.) O. Bolòs and Vigo 13 subsp. confusa Mart.-Lab. 13 D. hirta (Chev.) Rustan and Borgen 13 D. ibicensis (Pau) Gómez-Campo 8 D. ilorcitana (Sennen) Aedo et al. 8 D. kohlaanensis A. Miller and J. Nyberg na D. muralis (L.) DC. subsp. ceratophylla (Batt.) Mart.-Laborde na subsp. muralis 21 D. nepalensis Hara na D. ollivierii Maire na D. pitardiana Maire na D. scaposa DC. 9 D. siettiana Maire 8 D. siifolia Kunze subsp. bipinnatifida (Coss.) Mart.-Laborde na subsp. Siifolia 10 subsp. vicentina (Samp.) Mart.-Laborde 10 D. simplex (Viv.) Sprengel 11 D. sundingii Rustan 13 D. tenuifolia (L.) DC. subsp. cretacea (Kotov) Sobr. Vesp. 11 subsp. tenuifolia 11 D. tenuisiliqua subsp. rupestris (J. Ball) Mart.-Laborde na subsp. tenuisiliqua 9 D. varia Rustan na D. villosa Boulos and Jallad na D. viminea (L.) DC. var. viminea and var. integrifo- 10 D. virgata (Cav. DC.) subsp. sahariensis Coss. na subsp. virgata 9 subsp. rivulorum (Br.-B1. and Maire) Mart.- na subsp. australis Mart.-Lab. na D. vogelii (Webb) O.E. Schulz na Tripodi et al. - Qualitative properties and breeding perspectives in rocket salad 109 thanks to their geographical position and mild climat- i c c o n d i t i o n s , r e p r e s e n t t h e m a i n p r o d u c e r s . Diplotaxis is much more cultivated, fitting better to the needs of the farmers and being better suited to commercial utilization, thanks to the possibility to perform several harvests per cycle with yield increas- ing after the first harvest (Hall et al., 2015). Arugula is recommended in diets, having a very low-calorie vegetables (25 calories per 100 grams of fresh leaves) and being a very good source of vita- mins and minerals (Table 2). Furthermore, it contains a r a n g e o f v i t a l c o m p o u n d s , w i t h i m p o r t a n t nutraceutical and anticancer properties, which are discussed in the next paragraph. 2. Bioactive Compounds Many studies associate a highly significant reduc- tion in the risk of cancer as well as a tumorogenesis inhibition and hepatoprotective effects with increas- ing consumption of Cruciferae (Lynn et al., 2006; Juge et al., 2007; Lamy et al., 2008; Alqasoumi et al., 2009). Rocket contains a range of health-promoting phytochemicals including carotenoids, vitamin C, fiber, polyphenols and glucosinolates (Bennett et al., 2006; Heimler et al., 2007). Glucosinolates (GLSs) represent the major class of compounds in rocket and their contents in this crop h a v e b e e n w e l l d o c u m e n t e d i n t h e l i t e r a t u r e (D’Antuono et al., 2008; Pasini et al., 2012). When g l u c o s i n o l a t e s a r e e x p o s e d t o m y r o s i n a s e ( E C 3.2.1.147, thioglucoside glucohydrolase) during tissue damage, glucose and an unstable intermediate are formed. This intermediate degrades to produce a sul- fate ion, and a variety of products including isothio- cyanates, nitriles and, to a lesser extent, thio- cyanates, epithionitriles and oxazolidines. The rela- tive proportion of these hydrolysis products depends on the plant species studied, on the glucosinolate itself (as side chain substitution), and reaction condi- tions like pH, metal ions or epithiospecifier protein (Bennett et al., 2007). Both Eruca and Diplotaxis species contain similar profiles of GLSs within the leaf tissue, the most prominent of which are glucosativin (4-mercapto- butyl-GLS), glucoerucin [4-(methylthio)butyl-GLS] and g l u c o r a p h a n i n [ 4 - ( m e t h y l s u l f i n y l ) b u t y l - G L S ] . Glucosativin and glucoerucin breakdown products are thought to contribute most to pungency and flavour in rocket (Pasini et al., 2012). Numerous other GLSs have also been identified within rocket tissue, for example diglucothiobeinin [4-(b-D-glucopyra- nosyldisulfanyl) butyl-GLS] (Kim et al., 2007), 4- hydroxyglucobrassicin (4-hydroxy-3-indolymethyl- GLS) (Cataldi et al., 2007) and 4-methoxyglucobras- sicin (4-methoxy-3-indolymethyl-GLS) (Kim and Ishii, 2006). Phenolics are the most abundant antioxidants in the human diet. Considerable evidence indicates that some of the protective effects of phenols in fruits and vegetables may be due to flavonoids (Clifford and Brown, 2006). Rocket species also contain large con- centrations of polyglycosylated flavonol compounds, which are known to infer numerous beneficial health effects in humans and other animals. Particularly of note are their effects on the gastrointestinal tract and in cardiovascular health (Bjorkman et al., 2011; Traka and Mithen, 2011). Several studies in rocket have identified and quantified polyglycosylated flavonols, which belong to three core aglycones: Table 2 - Rocket salad nutritional values for 100 g of fresh leaves (USDA Nutrient Database *) *https://ndb.nal.usda.gov/ndb/foods/show/3569?manu=&fgcd= &ds= (z) Dietary folate equivalents. (y) Retinol activity equivalents. Nutrient Unit Value Energy kcal 25 Water g 91.71 Carbohydrate g 3.65 Protein g 2.58 Sugars g 2.05 Fiber g 1.6 Lipid g 0.66 Vitamins Vitamin C mg 15 Thiamin (Vitamin B 1 ) mg 0.044 Riboflavin (Vitamin B 2 ) mg 0.086 Niacin (Vitamin B 3 ) mg 0.305 Pyridoxine (Vitamin B 6 ) mg 0.073 Folate (Vitamin B 9 ), DFE (Z) µg 97 Vitamin A, RAE (y) µg 119 Vitamin A IU 2373 Vitamin E mg 0.43 Vitamin K µg 108.6 Minerals Calcium, Ca mg 160 Iron, Fe mg 1.46 Magnesium, Mg mg 47 Phosphorus, P mg 52 Potassium, K mg 369 Sodium, Na mg 27 Zinc, Zn mg 0.47 Adv. Hort. Sci., 2017 31(2): 107-113 110 isorhamnetin, kaempferol and quercetin (Bennett et al., 2006). Pasini et al. (2012) studied the glucosinolate and phenolic profiles of 37 rocket salad accessions (32 Eruca sativa and 5 Diplotaxis tenuifolia) obtained by liquid chromatography-mass spectrometry. The authors isolated eleven desulpho-glucosinolates (DS- GLSs) and the glucosinolate profiles did not differ between the two species. Total DS-GLS content, expressed as sinigrin equivalents (SE) revealed a cer- tain variability, ranging from 0.76 to 2.46 mg g-1 dry weight (dw) but, again, the quantitative analysis did not discriminate Eruca from Diplotaxis. Moreover, the polyphenol evaluation by HPLC-DAD-MS allowed the identification of two different classes of com- pounds in the two rocket salad species. Qualitative differences were observed between the polyphenol profiles at specific level: quercetin derivatives were the main phenolics of Diplotaxis, whereas kaempfer- ol derivatives characterised Eruca samples. The con- tents of total flavonoids determined as rutin equiva- lents (RE) ranged from 4.68 to 31.39 mg g-1 dw. K a e m p f e r o l - 3 , 4 ’ - d i g l u c o s i d e ( 7 1 . 4 - 8 2 . 2 % ) a n d isorhamnetin-3,4’-di-glucoside (7.8-18.4%) were always isolated as first and second more abundant phenolic compounds in Eruca samples. No marker phenolic compounds were isolated in Diplotaxis sam- ples. Durazzo et al. (2013) reported significant differ- ences in the quality of conventional and integrated cultivation practices on the nutritional properties and benefits of wild rocket [Diplotaxis tenuifolia (L.) DC.], while no influence on biological activity was evi- denced. The authors also determined the cytotoxicity and antiproliferative activity of rocket polyphenol extract on human colon carcinoma (Caco-2) cells, evi- dencing a significant accumulation of cells in G1 phase and a consequent reduction in the S and G2 + M phases in response to the treatment. Regarding antioxidant properties, they found FRAP (Ferric Reducing Antioxidant Power) values ranged from 4.44±0.11 mmol kg-1 fresh weight (fw) to 9.92±0.46 mmol kg -1 fw for conventional rocket and from 4.13±0.17 mmol kg-1 fw to 11.02±0.45 mmol kg-1 fw for integrated rocket. Villatoro-Pulido et al. (2013) analysing four E. sati- va accessions reported the total content of glucosi- nolates ranged from 6.12 to 12.33 mg g−1 of dw. Glucoraphanin represented up to 52% of the total glucosinolates in leaves of one accession. Accessions showed differences in the hydrolysis of gluco- raphanin to the isothiocyanate sulforaphane. No cor- relation between these compounds was observed, which insisted differences in the myrosinase activity within accessions. The same authors highlighted that rocket leaves had variable phenolic profiles repre- sented by quercetin-3-glucoside, rutin, myricetin, quercetin and ferulic and p-coumaric acids. A high variability was observed for the total carotenoids ranged from 16.2 to 275 μg g-1 with lutein as the main carotenoid. Moreover, they found glucose was the predominant sugar, representing >70% of the total soluble carbohydrates. Bell and collaborators (2015) used Liquid chro- matography mass spectrometry (LC-MS) to obtain glucosinolate and flavonol content for 35 rocket accessions and commercial varieties. They identified 13 glucosinolates and 11 flavonol compounds; semi- quantitative methods were used to estimate concen- trations of both groups of compounds. Minor glucosi- n o l a t e c o m p o s i t i o n w a s f o u n d t o b e d i f f e r e n t between accessions; concentrations varied signifi- cantly. According to Pasini et al. (2012) they con- firmed flavonols differentiation between genera, with Diplotaxis accumulating quercetin glucosides and Eruca accumulating kaempferol glucosides. The authors detected several compounds in each genus that have only previously been reported in the other. Recently, we investigated the qualitative and quantitative profiles of glucosinolates and polyphe- nols, highlighting flavonoid glycoside compounds (flavonols), in 39 accessions of wild and cultivated rocket (Taranto et al., 2016). Seven DS-GLSs were detected in rocket leaves belonging to two chemical classes: five aliphatic compounds (glucoerucin, gluco- raphanin, progoitrin, glucoalyssin, and glucosativin) and two structurally related compounds containing one intermolecular disulfide linkage, 4-(β-D-glucopy- ranosyldisulfanyl)butyl-GLS and dimeric 4-mercapto- butyl-GLS. The species studied significantly differed for GLS content: total average concentrations being 29.61 and 19.41 mg g-1 dw for E. sativa (21 acces- sions) and D. tenuifolia (16 accessions), respectively. Total GLS content ranged from 2.10 to 40.96 mg g-1 dw and from 11.61 to 26.96 mg g-1 dw, for Eruca and Diplotaxis accessions, respectively. Additional acces- sions of D. muralis and Erucastrum spp. were evalu- ated exhibiting an average GLS content of 17.39 and 3.63 mg g-1 dw, respectively. Fifteen flavonol compounds were tentatively identified in the thirty-nine accessions studied. Diplotaxis accessions were characterized by nine dif- Tripodi et al. - Qualitative properties and breeding perspectives in rocket salad 111 ferent flavonols mainly represented by quercetin derivatives, total average content being 7.17 mg g-1 dw with a range from 4.91 to 8.57 mg g−1 dw. The most abundant flavonol compound in Diplotaxis was quercetin 3,4’-diglucoside-3’-(6-sinapoylglucoside). As regards Eruca accessions, the more abundant flavonoid group was represented by kaempferol derivatives, in agreement with a previous report (Martínez-Sánchez et al., 2007). The 21 Eruca sativa accessions showed a flavonol total average concen- tration of 8.13 mg g−1 dw, the lowest and the highest content being 0.82 and 10.16 mg g−1 dw, respectively. The most abundant flavonol was kaempferol 3,4’- diglucoside. According to previous research, isorhamnetin 3,4’-diglucoside was the only compound common to Diplotaxis and Eruca accessions studied (Martínez- Sánchez et al., 2008; Pasini et al., 2012; Bell et al., 2 0 1 5 ) . H o w e v e r , s o m e e x c e p t i o n s h a v e b e e n observed. Specific compounds mainly detected in Eruca such as kaempferol 3-glucoside and kaempfer- ol 3-diglucoside-7-glucoside have been reported also in Diplotaxis commercial varieties (Bell et al., 2015). O t h e r c o m p o u n d s s p e c i f i c f o r D i p l o t a x i s ( i . e . , quercetin 3,4’-diglucoside-3’-(6-caffeoylglucoside) and quercetin 3,4’-diglucoside-3’-(6-sinapoylgluco- side) have been also identified in Eruca (Bell et al., 2015). These inconsistencies could be related to the genetic material used. Overall the results of the analysis of glucosinolates and flavonols evidenced how the Eruca sativa gene pool contains potential candidates to use in breeding programs for quality. 3. Potentiality and Perspectives for Breeding Despite the global consumption of rocket salad has increased in the recent years, little efforts have been spent by both private and public breeding pro- grams aimed to constitute new varieties. The impor- tance in phytochemicals has been above discussed and novel knowledge as source of resistances have b e e n r e c e n t l y d e s c r i b e d ( P a n e e t a l . , 2 0 1 7 ) . N o w a d a y s , c o n s t r a i n t s a r e m a i n l y c a u s e d b y pathogens, nitrate accumulation, early flowering and physiological disorders due to intensive culture sys- tem. Accessions of Eruca sativa are reported to be late-bolting (Kenigsbuch et al., 2014) and to accumu- late less nitrate than Diplotaxis tenuifolia (Cavaiuolo and Ferrante, 2014), being good candidates for the improvement with respect to the latter. However, several limitations for the transfer of these useful traits are linked to the failure of intergeneric crosses between Eruca and Diplotaxis due to post-zygotic barriers (Tripodi unpublished) resulting in the absence of a cost effective hybridization system available for rocket. Moreover, interspecific crosses among Diplotaxis species are difficult due to their dif- ferent chromosomes number (Table 1). E r u c a s a t i v a x B r a s s i c a r a p a a n d D i p l o t a x i s tenuifolia x Brassica rapa hybrids are instead possi- ble using embryo rescue (Agnihotri et al., 1990; Jeong et al., 2009) and somatic hybridization (Zhang et al., 2008) techniques, making the two rocket salad species a good source to use for the improvement of Brassica rapa. The possibility of intercross has been applied in the development of cytoplasmic male ster- ile (CMS) Eruca sativa plants transferring a male ster- ile cytoplasm from Brassica oleracea or Brassica napus (Merete et al., 2012). Two approaches have been used: one requiring the application of embryo rescue after the first cross hybridization, subsequent chromosome doubling and backcrossing of the resulting hybrid to Eruca sativa, another, using proto- plast fusion from cytoplasmic male sterile Brassica, subsequent regeneration of allogenic cells and cross- ing of the regenerated plant with pollen from Eruca s a t i v a . T h e s a m e a p p r o a c h h a s b e e n u s e d b y Hosemans and Leviell (2012) by transferring cyto- plasmic male sterility from Raphanus sativus to Diplotaxis tenuifolia. Raphanus sativus has been also used to transfer CMS in Eruca sativa (Nothangel et al., 2016). Despite these achievements, breeding activities are still carried out by means of traditional selection schemes such as mass selection or single seed descent. New possibilities may be obtained by TILL- ING (McCallum et al., 2000) in order to select mutants for gene of interest or genome wide associ- ation approaches (GWAS) (Huang and Han, 2014) for the dissection of the genetic basis of complex traits and the development of markers for breeding assist- e d s e l e c t i o n . M u t a g e n e s i s m e d i a t e d b y e t h y l methanesulfonate (EMS) is already reported with success in Diplotaxis tenuifolia (Kenigsbuch et al., 2014), resulting in a mutant showing late flowering a n d d e l a y e d p o s t h a r v e s t s e n e s c e n c e . T h e s e a p p r o a c h e s s u c c e s s f u l i n B r a s s i c a s p e c i e s (Stephenson et al., 2010; Xu et al., 2016) may be also applied in rocket salad for a better exploitation of the genetic potentiality of this crop, and further- more, to address the challenges of the modern agri- culture that demands major security and quality of foods. Adv. Hort. 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