Genetic analysis reveals opportunities and obstacles of tolerance to the Asian soybean rust fungus

Author(s)

Elesandro Bornhofen

• Document type: Doctoral Thesis
• Press: Piracicaba.
• Institution: Universidade de São Paulo (USP). Escola Superior de Agricultura Luiz de Queiroz (ESALA/BC)
• Defense date: 2019-08-30

Examining board members

Natal Antonio Vello; Luis Fernando Alliprandini; Jose Otavio Machado Menten; Jose Baldin Pinheiro

• Advisor: Natal Antonio Vello

Abstract

In the early 2000s, the first detection of Asia soybean rust in South America gave rise to what would become a multibillion-dollar battle to sustain the economic viability of the crop. While several strategies of control have been investigated and delivered to farmers, the rust management stills rely on fungicide sprays. However, global trends in reducing the use of agrochemicals, the continuous decrease in the efficiency of fungicide molecules against the causal agent of the disease, and economic reasons raise the need for other models of control. In this sense, genetic tolerance as an approach to sustain yield performance in the presence of rust stress can be an interesting alternative. Few studies have prospectively investigated the obstacles and benefits of soybean breeding for tolerance to P. pachyrhizi. The purpose of the this study was to design basis for the use of tolerance in soybean breeding programs, to identify valuable parents in terms of combing ability for this mechanism, to establish the importance of genotype × environment interaction (G × E) for tolerance and to investigate the existence of allocation cost, characteristic in which the metabolic costs of plant defense are not translated into performance in the absence of the pathogen. Adjacent experiments contrasting only for rust stress (presence and absence of rust) were carried out for three consecutive years and two locations. A total of 4,424 experimental plots were evaluated, including trials composed by 64 F5 biparental populations derived from an 8 × 8 partial diallel, 768 F5:6 and 256 F5:7 inbred lines, in addition to the 16 parental genotypes (coded from 21 to 36). In addition to the agronomic traits measured in all plots, the F5:6 inbreds and the parental genotypes had the samples of one hundred seeds evaluated through high-throughput phenotyping of digital imagery. All statistical analyses were performed via mixed models and for the trials containing the F5 populations and the F5:6 inbreds, an extra term was added to the model aiming the spatial correction through two-dimensional penalized spline regression. The rust stress reduced the overall seed yield in 16.18% for populations and 27.30% for inbred lines. The strategy of adjacent experiments with different fungicide management was efficient and differences were detected for the effect of the disease on all measured traits. The selection for tolerance should be practiced employing information collected in experiments carried out in multi-environment due to the relevance of the G × E interaction. The results show that increasing the period of seed filling in the presence of rust stress can benefit the breeding for tolerance due to a positive correlation with yield (P < 0.05). Non-additive genetic effects were detected and confirmed (P < 0.05) in late selfing generations, especially for the hundred seed weight trait. The finds indicate that USP 04-17.011 (30) and USP 231-2228-01 (31) are the best parents at generating tolerant descendants and, at the same time, able to deliver high yield in the absence of rust stress. In addition, there is allocation cost in the soybean × P. pachyrhizi pathosystem, however, there is sufficient genetic variability for the simultaneous selection of tolerant elite lines with high yield. (AU)

• FAPESP's process: 17/11235-0 - Strategies for selection and characterization of soybean lines with emphasis on rust tolerance
• Grantee: Elesandro Bornhofen
• Support Opportunities: Scholarships in Brazil - Doctorate