MAPPING OF FUSARIUM WILT RESISTANCE IN WATERMELON (Citrullus lanatus (Thunb.) Matsum and Nakai) GERMPLASM AND ASSESSING THEIR POTENTIAL AS ROOTSTOCKS
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Date
2020
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: ICAR-IIHR, Bengaluru,
Abstract
The present investigation was carried out to designate two selected virulent Indian
Fusarium oxysporum f. sp. niveum (Fon) isolates collected from different regions of the country
into races, identify stable source resistance against these races, to study the mode of inheritance
of resistance, mapping of the resistance employing candidate QTL and QTL-sequencing
approach and to study the influence of the resistant accessions on yield and quality of grafted
watermelon at ICAR-Indian Institute of Horticultural Research, Hesaraghatta Lake Post,
Bengaluru during 2016-2020. In this study, we have designated two virulent Indian isolates into
races 1 and 2 of Fon by studying their ability to infect a specific set of host differentials
inoculated with both pipette and root-dip inoculation methods. Two consecutive assays were
conducted through artificial inoculation of spore suspension of the pure cultures of Fon races 1
and 2. Among all the accessions, 44 Citrullus sp., six bottle gourd and three C. maxima × C.
moschata hybrids were resistant to Fon race 1 while a total of 29 Citrullus sp., nine bottle gourd
and three C. maxima × C. moschata hybrids were resistant to Fon race 2. 55.17% (n=16/29) of
the Citrullus sp. accessions possessing resistance to Fon race 2 also possessed resistance to Fon
race 1. To study the mode of inheritance of resistance to Fon race 2, five resistant Citrullus
lanatus accessions were crossed with a common susceptible parent (BIL-53) to develop six
generations for each combination. Based on the reaction of the segregating generations, the
resistance in these accessions (IC0523047, EC794455 and EC759804) was found to be governed
by two major complementary dominant genes with the involvement of modifying genetic
factors. The F2 and backcross progenies of the other two accessions (IIHR-9 and EC794421) did
not follow any of the Mendelian/epistatic ratios tested. Generation mean analysis revealed
presence of epistasis in BIL-53×EC794455 and BIL-53×EC759804 while in the other three
crosses, simple additive interaction was observed. These findings were confirmed by studying
the skewness and kurtosis of the F2 populations derived from these crosses. Allelism test
revealed the absence of any allelic relationship between IIHR-9 and IC0523047 for resistance.
The genetic diversity and population structure of a germplasm panel comprising of 336 Citrullus
sp. accessions were studied and a core collection of Indian Citrullus sp. accessions was
formulated. Data from 23 highly polymorphic microsatellite markers were used for genetic
diversity and population structure analysis while both molecular and phenotypic data from 17
traits were used to formulate the core set. The accessions clustered into two populations and an
admixture group. Intra-population analysis revealed three and two statistically distinct
subpopulations in Pop I and Pop II respectively. The exotic collections were predominant in Pop
I-A, Pop II-A and Pop II-B while the Indian accessions were preponderant in Pop I-B and Pop IC. Pop I-B recorded the maximum magnitude of gene diversity and the highest number of
alleles. Validation of six InDel markers, reported earlier by Li et al. (2017), located within a
candidate QTL (Fon-1) region governing resistance to Fon race 1, was also carried out in
the same germplasm panel through association analysis. Among them, four markers
were significantly associated with the resistance and the marker InDel-4 explained
maximum phenotypic variations, thus can successfully be utilized for marker-assisted
introgression of Fon race 1 into elite commercial backgrounds. Employing QTLsequencing technique, the resistance to Fon race 2 was mapped in a highly resistant Citrullus
lanatus accession EC794421. Phenotyping was carried out through artificial inoculation of
conidial suspension (1×106 conidia per ml; 5 ml per plant) in F2:3 progenies for two consecutive
assays. Genotyping through whole-genome sequencing was carried out in the corresponding
resistant and susceptible bulks of F2 and both the parents. Two statistically significant genomic
regions were identified each on chromosome 1 (spanned to 5.5 MB) and chromosome 7
(spanned to 2.5 MB) governing resistance to Fon race 2 in EC794421. The latter QTL on
chromosome 7 is novel and was not reported earlier. Further, two loci each on chromosomes 6
and 10 were just falling short of threshold significance. The markers viz., BVWS02309 and
BVWS01116 within QTL on chromosome 1 and BVWS00358 within QTL on chromosome 7
were found to be significantly associated with resistance. The resistant accessions identified and
the genomic information generated in the current study will be useful for the watermelon
breeders to incorporate the Fon race 2 resistance in elite commercial backgrounds through
marker-assisted selection/backcrossing without facing linkage drag of undesirable fruit-quality
traits. Based on the level of resistance and graftable hypocotyl diameter, eighteen Citrullus sp.
accessions, along with three hybrid squashes, two bottle gourd, one C. maxima and one tolerant
check (BIL-53) to sudden wilt disease were selected as rootstocks. These were characterized for
their root parameters and the vigour, earliness, quality and yield were studied by grafting a
susceptible scion (cv. Suprit) onto these. Significant variations were observed among the
rootstocks for all the root traits under study. The length and diameter of the rootstock hypocotyl
were found to be an indicator of the robustness of the root system and can be used as indirect
selection criteria. A significant influence on vine length and earliness of the grafted crop was
observed across different rootstocks. The grafts on Citrullus sp. rootstocks could maintain the
oblong fruit shape of the scion compared to those onto Cucurbita hybrid and Lagenaria sp.
rootstocks which yielded flat-globe to spherical shaped fruits. A wide variation of TSS and
carotenoid content was observed upon grafting while the pulp pH remained largely unaffected.
An increase of average fruit weight by 50.69%, 55.37% and 82.37% and yield per plant by
112.15%, 89.54% and 107.06% was observed in grafts onto RS-10, RS-11 and RS-18
respectively. These resistant rootstocks can be deployed for environment-friendly management
of Fusarium wilt disease in watermelon without compromising the fruit yield and quality.
Keywords: Watermelon, Fusarium wilt, physiological races, genetics, allelism, genetic
diversity, population structure, Indian core collection, association mapping, QTL-seq,
rootstocks, grafting
Description
T-10425