To collect updated information on Brassica genetic resources, the following webpages were accessed: FAO-WIEWS, ITIS (Integrated Taxonomic Information System), POWO (Plants of the world online), Genesys, EURISCO, GRIN (Germplasm Resources Information Network), GBIS-IPK (Genebank Information System of the IPK - Institut Für Pflanzengenetik Und Kulturpflanzenforschung Gatersleben), UKVGB (United Kingdom Vegetable Gene bank), RDA (Rural Development Administration, S. Korea), NARO (National Agriculture and Food Research Organization), VIR (N.I. Vavilov All-Russian Scientific Research Institute of Plant Industry), WVC (World Vegetable Center), and CGN (Centre for Genetic Resources, the Netherlands) were accessed. For literature review the keywords used were “Brassica germplasm”, “Brassica conservation”, “Plant genetic resources”, “Brassica biodiversity” at Google scholar website (www.scholar.google.com). Only publications that discussed Brassica physiology or germplasm conservation strategies and/or policies were chosen. Preference was given to publications on policies and germplasms status updates published after the year 2017.

While WIEWS remains to be the global platform for information on conserved plant germplasm, two online data repositories, Genesys, managed by the Crop Trust and the European Search Catalogue for Plant Genetic Resources (EURISCO) also cater as major online directories for obtaining ordering information on specific accessions. Genesys database provides information on about 4.1 million accessions conserved in 450 genebanks all over the world of which approximately less than 1% (0.96%, 41,487 accessions as on 1^st December, 2022) belong to Brassica (Genesys, 2017) ( Figure 1 ). EURISCO stores data of about 2 million accessions of plants and their wild relatives, preserved in 400 institutes located in 43 member countries of the European Cooperative Programme for Plant Genetic Resources (ECPGR) (Weise et al., 2017). EURISCO stores only 26,321 accessions of Brassica (as on 1^st December, 2022) which constitute to only 1.3% of the total accessions in the database ( Figure 1 ). Brassica also does not appear among the list of top ten crops with at least 100,000 accessions conserved worldwide (Börner and Khlestkina, 2019). However, European nations are major contributors to the overall production of Broccoli, cauliflower and rapeseed/canola in the world with a combined output of over 7.6 million tons of agricultural crop production in 2021 ( Figure 1 ) (FAOSTAT, 1997). There is an evident underrepresentation of Brassica accessions among the conserved germplasm despite their nature of being such a diverse and anthropologically important genus of crop plants (Kellingray et al., 2017; Wijaya et al., 2020; Salehi et al., 2021).

A comparison of overall Brassica production in 2021 and major plant genetic resources centers involved in conserving Brassica biodiversity across the world indicate that Asia, the United States and Europe remain to be major producers of Brassica crops ( Figure 1 ). The WIEWS official data from the SDG 2.5.1.a indicates that a total of 71,378 accessions of Brassica are currently maintained ex situ all over the world ( Table S1 ). Primary observation may indicate that the highest number of accessions being conserved in genebanks in Europe and Asia, but in terms of species diversity, after Europe, North America (USDA-ARS) conserves diverse species of Brassica germplasm ( Table S1 ). Further analysis indicate that less than 10 accessions are conserved in genebanks from Africa (Niger, Nigeria, Eswatini, South Africa, Libya), Middle East (Cyprus, Tajikistan, Jordan, Lebanon), Eastern Europe (Georgia, Montenegro, Armenia) and South America (Argentina, Mexico) (WIEWS, 2022). However, these parts of the world also contribute significantly to the world Brassica production and fall among the top 5 producers of crops such as Broccoli, cauliflower, cabbage and mustard seeds ( Figure 1 ). Improving the available Brassica species biodiversity and dissemination could considerably advance further breeding and research in these parts of the world.

Currently, for ordering Brassica germplasm, the Germplasm Resources Information Network (GRIN) and Gene bank Information System (GBIS) from the US (maintained by Unites States Department of Agriculture - Agricultural Research Service (USDA-ARS) and Leibniz Institute of Plant Genetics and Crop Plant Research serve as channels where one can directly request Brassica germplasm for research purposes (Postman et al., 2010; Oppermann et al., 2015). In addition to these widely used genebanks, several other facilities are also available to source Brassica germplasm locally and internationally albeit the diversity being very limited (Liu et al., 2020). The current status of Brassica germplasm in major genebanks are given in the table below ( Table 1 ). A Principal Component Analysis (PCA) of the available Brassica germplasm information indicated that country such as India conserves very high number of accessions (accession) whereas Korea, Germany, Great Britain conserves high diversity of Brassica species (unique). At the same time, high number of accessions conserved in Korea and Japan possess data on their biological status (status) ( Figure 2A ). Comparison of species indicates that B. juncea, remains the highly conserved Brassica taxon followed by B. napus, B. rapa and unsegregated B. spp contribute to the highest number of accessions stored in genebanks ( Figure 2B ). A majority of these Brassica germplasm are maintained as ex situ collections by germplasm collections and materials are preserved in short, medium and long term storage and available as seeds, plant material, and/or DNA (Walters et al., 2004; Acker et al., 2017; Lusty et al., 2021). A search on Genesys and EURISCO webpages indicate that a majority of Brassica germplasm are stored as long term storage, followed by seed collections and mid-term storage ( Figure 2C ). The major plant material conserved in all these three strategies are of seed type and seeds are the easiest way for exchange or dissemination of Brassica germplasm.

Currently, comparison of the three online directories yields a total of 81,752 Brassica accessions stored in 81 countries across the world in addition to 3 regional and 3 international research centers (data summarized from WIEWS, Genesys & EURISCO). Additionally, few genebanks such as The National Crop Genebank of China (NCGC) and National Agrobiodiversity Center-RDA, South Korea also conserve Brassica germplasm but are limited for sharing of resources due to governmental policies. Historically, the six most common species of Brassica explained by the U triangle are well documented and are widely conserved all across the world (U, 1935) ( Figure S1 ). It can be observed that genebanks that conserve high number of accessions of species such as Brassica napus, B. oleracea and B. carinata are located in vicinity to their geographical origins ( Figure S2 ). Across Asia, green leafy vegetable crops of B. rapa (subsp. chinensis and pekinensis), oil producing rape B. napus and mustards of B. juncea with local origins are highly conserved in genebanks of India, Japan, Pakistan, Taiwan and South Korea ( Figure S2 ). Co-ordination and dissemination of Brassica germplasm have resulted in countries such as Australia, Canada and the United States of America (USA) which are geographically far from the natural areas of origin of Brassica species to accumulate in both number and diversity ( Table 1 ; Figure 3 ).

A country wise comparison of the WIEWS and Genesys databases for conserved Brassica accessions indicate that Australia, India, Germany, United Kingdom (UK), USA and Taiwan conserve the largest number of Brassica germplasm ( Figure 3A ). In terms of species diversity, the most common Brassica crop seeds conserved are B. oleracea (WIEWS: 17,389 accessions; Genesys 14,798 accessions); B. rapa (WIEWS: 15,132 accessions; Genesys 8,991 accessions) and B. juncea (WIEWS: 14,810 accessions; Genesys 4,698 accessions). However other Brassica species such as B. tyrrhena, B. taurica, B. bivoniana, B. atlantica, B. nivalis, and B. aucheri etc. are not highly conserved (<10 accessions). Moreover, there also exists disproportion in the biodiversity conserved at the plant genebanks. Among 146 plant genebanks recorded to conserve Brassica >38% geenbanks (56) have a Brassica biodiversity of less than 10 taxons (including subspecies and varieties) and largely conserve B. oleracea, B. rapa and B. juncea genetic resources. Data from FAO-WIEWS indicate that Australia, Germany, Spain, UK and USA conserve more than 25 representative species of Brassica including wild types ( Figure 3B ). Although the FAO promotes sharing of plant genetic resources for food and agriculture (PGRFA) through its sustainable developmental goals (SDGs), much work has to be done to ensure that the currently available biodiversity of Brassica species is not lost. Crop wild types are excellent sources for breeding for development of disease tolerant and abiotic stress tolerant plants that would be suitable for fresh challenges posed by the changing climate conditions.

A major concern with worldwide Brassica germplasm collections often is the uncertainty associated with the naming of plant genetic resources. Descriptions of the origins, phenotypic and genotypic diversity of several Brassica members have been published (Dixon, 2006; Bonnema et al., 2011). However, naming of the Brassica members hitherto remains complex and often ambiguous due to differences in the naming conventions. Proper phylogenetic organization of the members of Brassica has been arduous due to extensive crossing and hybridization over centuries which occurred across geographically isolated microenvironments causing difficulties in tracing and comparison (Bonnema et al., 2011; Fahey, 2016). There is also an overlap of plant forms and morphological traits across species which further complicates phenotypical phylogenetic assignment and organization (OECD, 2016). The origins of modern Brassica vegetables and their taxonomy continues to be unsettled and regarded to be in a state of constant flux (Fahey, 2016).

At present, only 13 subordinate taxa in the rank of species are recognized by the Integrated Taxonomic Information System (ITIS) in the genus Brassica (ITIS, 2022) (as on 05^th October, 2022) ( Table S2 ). The ITIS is a taxonomic database maintained by the United States government which partners with other governmental agencies of Canada and Mexico as well as organizations dedicated to biodiversity such as GBIF (Global Biodiversity Information Facility) (GBIF, 2020). However, other well-recognized online catalogues as well as germplasm collections do not necessarily follow these naming conventions. For example, as on 05^th October, 2022, the Plants of the World Online-Royal Botanical Gardens, Kew in collaboration with World Flora Online project, provides taxonomic recognition of 41 species of Brassica (Borsch et al., 2020; POWO, 2022) ( Table S2 ). These names are accepted and added to its ‘World Checklist of Seed Plants’ (Govaerts et al., 2022). Another repository BrassiBase, dedicated towards developing taxonomic, evolutionary, systematics and germplasm knowledge systems specifically for Brassicaceae, reports 45 species of Brassica (Kiefer et al., 2013) ( Table S2 ). The GRIN-taxonomy resource which provides taxonomic information on plants conserved by the United States National Plant Germplasm System (USNPGS) remains as the only provider of taxonomic information along with conserving germplasm. The GRIN-taxonomy also currently recognizes 41 species of Brassica. But they also are not completely in agreement with the accepted list of Brassica species given the World Checklist of Seed plants. On a comparison of the Brassica taxa recognized by the plant taxonomy authorities, online PGR directories and plant genebanks we recognized issues related with the naming of Brassica taxa at species level ( Figure 4 ). There are 18 Brassica species that are present only among the plant taxonomy lists (ITIS, POWO, BrassiBase & GRIN-Taxonomy) which are not conserved under the designated name at any germplasm conservation center or present in the list of Brassica species in the online repositories (Genesys & EURISCO) ( Figure 4A ). Similarly, it can also be observed that there are 15 Brassica species which are listed only in the online PGR directories (Genesys & EURISCO). This could be solved as they all of them are subspecies of Brassica, particularly Brassica oleracea and B. rapa which have been misnamed or updated to a new taxon ( Figure 4B ). Same is the case of Brassica sylvestris which was formerly designated as Brassica rapa subsp. sylvestris which is now recognized only in plant genebanks but not by plant taxonomy authorities or online PGR directories. The NCBI taxonomy database under Brassica (Taxonomy ID:3705) indicates 129 distinct taxa which after removing species crosses and taxa marked ambiguous names such as groups provide 87 unique taxa ( Figure 5 ). These 87 unique taxa fail to include all the Brassica species conserved in plant genebanks.

The ambiguity of Brassica naming also continues to exist at the ground level. To begin with, the plant genetic resources storage centers around the world which are responsible for conservation and distribution of germplasm do not reflect the diverse species richness in Brassica germplasm conserved by them. The Gene bank Information System of the Leibniz Institute of Plant Genetics and Crop Plant Research (GBIS-IPK) indicates storing of accessions belonging to 31 species of Brassica whereas the United States Department of Agriculture- Germplasm Resource Information Network (USDA-GRIN) stores 24 different species of Brassica (excluding Brassica sp.) (Brassica-dataset, 2015; Oppermann et al., 2015) ( Table S2 ). It is understandable that plant genebanks concentrate towards conserving local resources and rely on crop nomenclature. However, as most of them follow the ex situ model of conservation, they can also be used as an effective tool in protecting and documenting the Brassica biodiversity. For example, as several of the accepted taxa of Brassica such as Brassica assyriaca, B. baldensis, B. beytepeensis etc. have been officially added as verified and accepted species (Govaerts et al., 2022), plant genetic resource centers around the world can attempt to work towards taxonomic reclassification and updating of information on the bioresources which they conserve.

The problem of taxonomic uncertainties becomes much more complex across genebanks all around the world when describing the status at much lower taxonomic levels such as subspecies, cultivars and varieties which are often interchanged depending on the author/gene bank accessed. Such issues at times can complicate the taxonomic assignment after breeding or during research. Introduction of universal code for naming at this juncture, would be a tedious as well as an incredible task. A polymerase chain reaction (PCR) based approach for identification of Brassica species has been proposed albeit its efficacy is limited within the species from the U triangle (Koh et al., 2017). There is a possibility that this problem could be solved by help of genetic sequencing approaches. Since the advancement of sequencing technologies, genomic component based typing followed by distinctive universal taxonomic assignments have helped to overcome such barriers. This would also shorten the time required by traditional identification using phenotyping. The current requirement on Brassica taxonomy would therefore involve a polyphasic approach for phylogenetic organization which would combine more than one of the following: phenotyping or studying morphological traits; use of cytological markers, karyotyping; phytochemical, storage protein profiling; studying isozyme markers; microsatellite markers such as SSRs (short tandem repeats), SNPs (single nucleotide polymorphisms); and sequencing methods such as specific-locus amplified fragment sequencing (SLAF-seq) and using large scale genomic analyses and fingerprinting to categorize Brassica species (Pino Del Carpio et al., 2011; Liu et al., 2013; El-Esawi, 2017; Fotev et al., 2018; Yang et al., 2018; Bhandari et al., 2020; Chao et al., 2020; He et al., 2021; Rakshita et al., 2021; Hong et al., 2022; Li et al., 2022)

The richness of both species and morphotypes in Brassica can be understood to have arisen after the whole genome triplication (WGT) event that has been accepted (Arias et al., 2014; Cheng et al., 2014). Among the speciation concepts of Brassica, the U triangle is the most famous and accepted concept of cytogenetic relationships between the species belonging to the genus Brassica ( Figure S1 ). The genome types of the ancestral diploid and tetraploid as explained in the triangle of U has been well discussed (Branca and Cartea, 2011). In addition to the six species of Brassica explained by the U triangle, other species of Brassica have also been continually described (Govaerts et al., 2022) ( Figure 6 ). These are morphologically distinct compared to the six species and not much information is available about them for scientific research. There is a marked morphological diversity among various species of Brassica where some form heads or curd or stem and lead modifications. This variation among the species can be observed in their leaves – size, shape, margins, presence of anthocyanin; stems; habit; flower color and roots. To give few examples, the Brassica species B. spinescens and B. balearica form slight succulent leaves which is unique among various Brassica and B. maurorum produces minute flowers which look like an inflorescence ( Figure 6 ). Some plants grow more than 2 meters in height (B. rapa) where are some (B. elongata, B. balearica and B. maurorum) do not grow more than 30 cm in height. Despite these plants having a long history of documentation and exhibiting significant difference in their morphology, habit and growth periods, they have not been studied in detail or conserved in large number in plant genebanks. In the recent decades, adequate emphasis has been given in literature to the importance of conservation of wild type species. These other Brassica species are vital as they can help to deal with the bottleneck problems associated with Brassica breeding which warrants the need for conservation of these wild type species of Brassica.

Members of the family Brassicaceae family especially the vegetable crops, prefer to grow under cooler temperate conditions (15–20°C), with high humidity, but some species are also found to grow also under subtropical and even tropical zones (Fahey, 2016; Lin et al., 2018; Razzaq et al., 2021). For example Chinese cabbage plants grow well at a temperatures range of 13–20°C but prefer warmer growing conditions and often bolt at lower temperatures (Salehi et al., 2021). Brassica juncea or mustard plants widely grown on northern parts of India also prefer warmer temperatures as high as 27°C and can tolerate temperatures as low as 6°C (Shekhawat et al., 2012). Among other species, Brussels sprouts and kale are the most resistant to low temperatures, as they can tolerate temperatures as low as −12°C. In addition to preference of longer photoperiods for optimized growth, Brassica vegetables such as kohlrabi, Chinese cabbage, cabbage have a weaker root system and require humus rich or loamy soil with pH near neutral (6.2-7.0) and high water retention properties (Dhaliwal, 2017; Razzaq et al., 2021). Therefore, propagation of diverse Brassica species requires environment controllable facilities and not all of them can be multiplied in outdoor fields in all countries.

It has been reported that some Brassica species are self-fertile but most of them are self-incompatible and may require cross pollination to obtain seed (Stewart, 2002). This pollination can occur through wind, animals or insects or through human intervention. Though the human intervention aspects are related to breeding, cross-pollination by wind or insects such as bees are undesirable and can contaminate varieties. In case of wind pollination, isolation of plants is very helpful to avoid cross-pollination and incase of B. napus outcross could be reduced to less than 4% on separating plants by 120 cm; in oil seed rape almost 10% of pollen could be detected at a distance of even 360 m from the edge of the field; in radish also, the minimal distance for 0% crossing was at least 50 m (Stewart, 2002). The plant B. napus is highly prone to cross-pollination and spontaneous inter-specific hybridization of B. napus is possible with sexually compatible species (Meglic and Pipan, 2018). To produce pure seeds, Brassica plants need to be grown in isolation and cross-pollination risk reduces with the distance or introducing physical barriers in the flowering plants. However, as explained earlier, cross pollination is not a common phenomenon and rates are very low among Brassica spp. (Stewart, 2002). Avoidance of cross pollination can be carried by using a number of strategies including creating topographical barriers such as edges that would restrict or limit wind and insects, shifting of bee hives, establishment of border zones at the edges of the where seeds will not be collected from the plants in the zones and planting fields in such a way that the flowering time and period do not overlap (Stewart, 2002).

Almost all the Brassica vegetables are relatively susceptible to the same or similar diseases as well as insect pests (Fahey, 2016). Overall, pests are a serious threat to agricultural productivity and lately, with the exponential growth of Brassica cultivation, the distribution of Brassica vegetable pests have also increased (Witzel et al., 2021). The common microbial diseases occurring in Brassica species are tabulated ( Table S3 ). The most common microbial infections in Brassica species are most caused by fungi especially Leptosphaeria (Blackleg disease), Sclerotinia (rot) and Verticillium (wilt). However, clubroot disease caused by Plasmodiophora is the highly reported disease among Brassica plants. Among bacterial pathogens Pesudomonas and Pectobacterium are the most common and Turnip yellow virus is the most common viral pathogen. In addition to diseases, Brassica crops are also attacked by a variety of pests that affect the growth as well as yield of Brassica members ( Table S4 ). These causes a variety of issues ranging from feeding on the leaf tissues to killing the whole plants by feeding on plant roots. The most common insect pests of Brassica include aphids, worms, mites and weevils. However, there are also several beneficial microorganisms and insects that aid growth of Brassica species. These help the plants grow and prevent predators from feeding on the plants. Microorganisms including bacteria and fungi have been reported to help Brassica species by stimulating plant growth through mechanisms such as nitrogen fixation, siderophore production, alleviate chilling and heavy metal stress, and more importantly act as biocontrol agents against several bacterial and fungal diseases (Card et al., 2015). In addition to microbial interactions leading to enhanced growth of Brassica plants, some of the insects have also been reported to aid Brassica plants by acting as major predators of pests and feed on eggs of pests, aphids, small caterpillars, thrips and mites that attack the plants ( Table S5 ).