**6. Molecular markers to QTLian breeding**

Several economically important traits in radish are being identified. These traits are yield, insect resistance and disease resistance. Yield is a complex trait governed by polygenic characters. Identifying these traits using conventional breeding/traditional breeding is problematic because these traits depend on phenotypic expression and have environmental and genotypic interaction. The new tool molecular breeding overcomes these problems, identification of quantitative trait is being utilised with the help of DNA markers [7] and linkage mapping [63]. There are several DNA markers being used in breeding programmes, such as restriction fragment length polymorphism (RFLPs), random amplified polymorphism DNA (RAPD), simple sequence repeats (SSRs), single-nucleotide polymorphism (SNPs) [63–67]. Molecular markers such as RAPD have been used to establish the origin of hortensis var. sativus and var. niger, which formed from distinct progenitors and came from different sources [68]. Several Asian varieties show more incredible skin and flesh colour, size, length and weight of roots, var. hortensis' genetic variability is also not a surprise.

A genome-wide association study (GWAS) analysis identified 44 single-nucleotide polymorphisms (SNPs) and 20 putative candidate genes significantly associated with FW resistance. A total of four QTLs were identified from the F2 population derived from an FW resistant line and a susceptible line, one of which was co-located with the SNPs on chromosome 7. These markers are emerging tools for molecular breeding programmes and marker-assisted selection to develop FW-resistant varieties of *R. sativus* [69]. Moreover, for the identification of the disease Fusarium wilt, Yu et al. [70] constructed a genetic linkage map on the F2 population, they observed a total of eight loci conferring FW resistance that were distributed on 4LGs, namely 2, 3, 6 and 7 of the Raphanus genome. Synteny analysis using the linked markers QTL showed homology to A. thaliana chromosome 3, which contains disease-resistance gene clusters, suggesting the conservation of resistance genes between them. The list of significant QTLs is identified in the radish, and their location is provided in **Table 1**.

Moreover, identification of root shape and red pigmentation is performed, and it was observed that three quantitative trait loci for root shape, namely LG3, LG8 and LG9, two QTLs for root diameter, namely LG4 and LG8 and one for red pigmentation are identified with the help of using AFLP, SSR and SLG-CAPS [65]. Kamei et al. [71] constructed a genetic linkage map using AFLP and SSR markers and concluded that CR is governed by the single gene or closely linked gene loci, namely Crs1, Crs2 and Crr3. A genetic map was constructed using an F2 population by using markers SRAP, RAPD, SSR, ISSR, RAMP and RGA markers, and they found that a novel QTL qRCD9 is responsible for controlling root CD [63]. Resistance against cyst nematode (*Heterodera schachtii*) was identified using RAPD, dpRAPD, AFLP and SSR markers [66]. To identify quantitative traits in radish for morphological characters, namely ovule number per silique, seed number per silique, plant shape, pubescence, whole plant weight (g), upper part weight (g), whole root weight (g), main root weight (g) using recombinant inbred lines, they identified 8 and 10 quantitative traits in 2008 and 2009 respectively [67]. In the identified QTL regions, nine SNP markers were






#### **Table 1.**

*List of QTLs identified for important traits in radish.*

newly produced. Nucleotide sequences and expression of these genes suggested their possible function in 4MTB-GSL biosynthesis in radish roots. [72]. Whereas recently, it was discovered that the R2R3-MYB transcription factor responsible for anthocyanin pigment 2 (PAP2) production is located on chromosome 2. The amino acid sequence encoded by the RsPAP2 gene was entirely distinguishable from other previously published RsMYB genes responsible for the red skin colour of radish [73].
