Genome-Wide Characterization of NBS-Encoding Genes in Watermelon and Their Potential Association with Gummy Stem Blight Resistance

Abstract

Watermelon (Citrullus lanatus) is a nutritionally rich and economically important horticultural crop of the Cucurbitaceae family. Gummy stem blight (GSB) is a major disease of watermelon, which is caused by the fungus Didymella bryoniae, and results in substantial economic losses in terms of yield and quality. However, only a few molecular studies have focused on GSB resistance in watermelon. Nucleotide binding site (NBS)-encoding resistance (R) genes play important roles in plant defense responses to several pathogens, but little is known about the role of NBS-encoding genes in disease resistance in watermelon. The analyzed NBS-encoding R genes comprises several domains, including Toll/interleukin-1 receptor(TIR), NBS, leucine-rich repeat (LRR), resistance to powdery mildew8(RPW8) and coiled coil (CC), which are known to be involved in disease resistance. We determined the expression patterns of these R genes in resistant and susceptible watermelon lines at different time points after D. bryoniae infection by quantitative RT-PCR. The R genes exhibited various expression patterns in the resistant watermelon compared to the susceptible watermelon. Only six R genes exhibited consistent expression patterns (Cla001821, Cla019863, Cla020705, Cla012430, Cla012433 and Cla012439), which were higher in the resistant line compared to the susceptible line. Our study provides fundamental insights into the NBS-LRR gene family in watermelon in response to D. bryoniae infection. Further functional studies of these six candidate resistance genes should help to advance breeding programs aimed at improving disease resistance in watermelons.

QTL associated with gummy stem blight resistance in watermelon

“…ClCG05G019540 is equivalent to the Cla020705 in the 97103_V1 genome (Guo et al 2013 ) and encodes proteins with both RPW8 and NBS-LRR domains. Cla020705 was differentially expressed between resistant PI 189225 and susceptible Charleston Gray inoculated with D. bryoniae (Hassan et al 2019 ). ClCG05G019540 (31,779,884—31,783,097 Mb) is 3,214 bp long and contains five exons.…”

“…Different modes of inheritance have been proposed for GSB resistance in cucurbits, including monogenic recessive (Norton 1979 ; Frantz and Jahn 2004 ; Hassan et al 2018 ), monogenic dominant (Zuniga et al 1999 ; Frantz and Jahn 2004 ; Wolukau et al 2007 ; Hu et al 2018 ), and polygenic (Lou et al 2013 ; Gusmini et al 2017 ; Liu et al 2017 ; Zhang et al 2017 ; Hassan et al 2019 ; Ren et al 2019 ) inheritance patterns. QTLs associated with GSB resistance have been described in cucumber (Lou et al 2013 ; Liu et al 2017 ; Zhang et al 2017 ) and watermelon (Ren et al 2019 ).…”

“…The NBS-LRR ClCG05G019540 gene found in ClGSB5.1 had thirteen synonymous SNPs in the exons. Hassan et al ( 2019 ) found that Cla020705 ( ClCG05G019540 ) exhibited higher expression in resistant (PI 189225) compared to the susceptible (Charleston Gray) watermelon, which could be due to mutations in the promoter region of this gene. Further functional analysis through expression studies of this gene between PI 482276 and Crimson Sweet may provide a better understanding of the association of ClCG05G019540 with GSB resistance in this genetic background.…”

“…Initial studies of GSB resistance in PI 189225 reported that resistance was mediated by a single gene, db (Norton 1979 ). However, later studies on PI 189225, PI 482283, and PI 526233 found that many genes with minor effects are most likely responsible for this trait (Gusmini et al 2017 ; Hassan et al 2019 ; Ren et al 2019 ). Recently, a quantitative trait locus (QTL) underlying GSB resistance in PI 189225 was described on chromosome 8 of watermelon (Ren et al 2019 ).…”

QTL Mapping for Gummy Stem Blight Resistance in Watermelon (Citrullus spp.)

“…Several different inheritance patterns have been described for controlling GSB resistance in cucurbits, such as a single recessive gene [ 16 , 37 ], single dominant [ 26 , 38 ], and multiple genes [ 11 , 17 , 28 , 29 , 37 ]. Inheritance of resistance to GSB in watermelon was previously described as a single recessive gene, db , from ‘PI 189225’ [ 16 ].…”