Multiple quantitative trait loci contribute to resistance to bacterial canker incited by Pseudomonas syringae pv. actinidiae in kiwifruit (Actinidia chinensis)

Abstract

Pseudomonas syringae pv. actinidiae (Psa) biovar 3, a virulent, canker-inducing pathogen is an economic threat to the kiwifruit (Actinidia spp.) industry worldwide. The commercially grown diploid (2×) A. chinensis var. chinensis is more susceptible to Psa than tetraploid and hexaploid kiwifruit. However information on the genetic loci modulating Psa resistance in kiwifruit is not available. Here we report mapping of quantitative trait loci (QTLs) regulating resistance to Psa in a diploid kiwifruit population, derived from a cross between an elite Psa-susceptible ‘Hort16A’ and a resistant male breeding parent P1. Using high-density genetic maps and intensive phenotyping, we identified a single QTL for Psa resistance on Linkage Group (LG) 27 of ‘Hort16A’ revealing 16–19% phenotypic variance and candidate alleles for susceptibility and resistance at this loci. In addition, six minor QTLs were identified in P1 on distinct LGs, exerting 4–9% variance. Resistance in the F1 population is improved by additive effects from ‘Hort16A’ and P1 QTLs providing evidence that divergent genetic pathways interact to combat the virulent Psa strain. Two different bioassays further identified new QTLs for tissue-specific responses to Psa. The genetic marker at LG27 QTL was further verified for association with Psa resistance in diploid Actinidia chinensis populations. Transcriptome analysis of Psa-resistant and susceptible genotypes in field revealed hallmarks of basal defense and provided candidate RNA-biomarkers for screening for Psa resistance in greenhouse conditions.

QTL Mapping for Resistance to Cankers Induced by Pseudomonas syringae pv. actinidiae (Psa) in a Tetraploid Actinidia chinensis Kiwifruit Population

“…The distribution of Stab_Psa_score and Stem_necrosis across the population was Normal, whilst data for ooze, wilt and leaf spot were distributed in a non-Normal manner (Figure 6b-f). Normal distribution of the Stab_Psa_score was previously reported in a diploid A. chinensis population [38]. The analysis showed that greenhouse inoculation results are highly dependent on time of inoculation, similar to the earlier results for the diploid population [38], with clonal repeatabilities substantially lower than field repeatabilities.…”

“…Normal distribution of the Stab_Psa_score was previously reported in a diploid A. chinensis population [38]. The analysis showed that greenhouse inoculation results are highly dependent on time of inoculation, similar to the earlier results for the diploid population [38], with clonal repeatabilities substantially lower than field repeatabilities. The best inoculated repeatability values were found for the sets inoculated in June (0.5) (winter) and the lowest for the sets inoculated in January (0.3) (summer).…”

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“…savastanoi (Caballo-Ponce et al, 2016). Tissue-specific roles of T3Es have yet to be found in P. syringae, although varying resistance responses can occur between tissues (Tahir et al, 2019).…”

“…Broad-acting partial resistance to all clades exists in certain cultivars, notably Merton Glory (Hulin et al, 2018b). Perennial plants provide challenging study systems as differences in tissue-specific resistance, plant age, seasonal environment conditions, and nutrient status impact on susceptibility (Kus et al, 2002;Mur et al, 2017;Velásquez et al, 2018;Tahir et al, 2019).…”

“…The candidate genes identified are involved in cell wall metabolism and signalling (Tahir et al, 2019). Interestingly, this study used multiple pathogenicity assays to identify resistance QTLs that were tissue-specific and associated with particular disease symptomology.…”