Oilseed rape (Brassica napus) resistance to growth of Leptosphaeria maculans in leaves of young plants contributes to quantitative resistance in stems of adult plants.

Yong-Ju Huang,Régine Delourme,Vinod Kumar,Graham J King,Bruce D L Fitt,Sophie Paillard,Harsh Raman

doi:10.1371/journal.pone.0222540

Abstract

Key message: One QTL for resistance against Leptosphaeria maculans growth in leaves of young plants in controlled environments overlapped with one QTL detected in adult plants in field experiments. The fungal pathogen Leptosphaeria maculans initially infects leaves of oilseed rape (Brassica napus) in autumn in Europe and then grows systemically from leaf lesions along the leaf petiole to the stem, where it causes damaging phoma stem canker (blackleg) in summer before harvest. Due to the difficulties of investigating resistance to L. maculans growth in leaves and petioles under field conditions, identification of quantitative resistance typically relies on end of season stem canker assessment on adult plants. To investigate whether quantitative resistance can be detected in young plants, we first selected nine representative DH (doubled haploid) lines from an oilseed rape DY ('Darmor-bzh' × 'Yudal') mapping population segregating for quantitative resistance against L. maculans for controlled environment experiment (CE). We observed a significant correlation between distance grown by L. maculans along the leaf petiole towards the stem (r = 0.91) in CE experiments and the severity of phoma stem canker in field experiments. To further investigate quantitative trait loci (QTL) related to resistance against growth of L. maculans in leaves of young plants in CE experiments, we selected 190 DH lines and compared the QTL detected in CE experiments with QTL related to stem canker severity in stems of adult plants in field experiments. Five QTL for resistance to L. maculans growth along the leaf petiole were detected; collectively they explained 35% of the variance. Two of these were also detected in leaf lesion area assessments and each explained 10-12% of the variance. One QTL on A02 co-localized with a QTL detected in stems of adult plants in field experiments. This suggests that resistance to the growth of L. maculans from leaves along the petioles towards the stems contributes to the quantitative resistance assessed in stems of adult plants in field experiments at the end of the growing season.

Highlights

Protecting crops from catastrophic yield losses caused by plant pathogens is a major goal of agriculture to safeguard global food security in response to growing concerns about food shortages and climate change [1,2,3]
This paper reports work on detection of quantitative trait loci (QTL) for resistance against L. maculans growth in leaves of young plants in controlled environment experiments and compares them with QTL detected in stems of adult plants in field experiments using a doubled haploid (DH) segregating population
For the nine DH lines, there were significant differences between DH lines for leaf lesion area (P < 0.001, 8 d.f.), distance grown along the leaf petiole (P < 0.001, 8 d.f.) and the amount of L. maculans DNA in the petiole (P < 0.001, 8 d.f.) (Table 1)

Summary

Introduction

Protecting crops from catastrophic yield losses caused by plant pathogens is a major goal of agriculture to safeguard global food security in response to growing concerns about food shortages and climate change [1,2,3]. Plant resistance used to control crop diseases is generally classified as either major genedetermined qualitative resistance or minor gene-determined quantitative resistance [8,9,10]. Qualitative resistance, considered as ‘complete’ resistance, is usually controlled by single, dominant resistance (R) genes which are often effective in preventing pathogens from colonising plants [11,12,13,14]. Quantitative resistance is considered as incomplete or partial resistance that does not prevent pathogens from colonising plants but is able to decrease symptom severity and/or epidemic progress over time [10, 20,21,22]. Combining resistance QTL (quantitative trait loci) with complementary modes of action or combining quantitative resistance with major R genes have proved to be valuable strategies for breeding effective, potentially durable resistance [23]. Understanding quantitative resistance will contribute to deployment of crop cultivars with more durable resistance

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