Fine-mapping of angular leaf spot resistance gene Phg-2 in common bean and development of molecular breeding tools

Juanita Gil,Victor Mayor,Carlos Jara,Santos Barrera,Steve Beebe,Bodo Raatz,Diana Solarte,Juan David Lobaton

doi:10.1007/s00122-019-03334-z

Abstract

Key messageThe Common Bean Angular Leaf Spot Resistance Gene Phg-2 was fine-mapped to a 409-Kbp region, and molecular markers for breeders were developed and validated in field experiments.Common bean (Phaseolus vulgaris L.) is an important food legume in Latin America, Asia and Africa. It is an important source of protein, carbohydrates and micro-minerals, particularly for smallholder farmers. Common bean productivity is affected by angular leaf spot (ALS) disease caused by the pathogenic fungus Pseudocercospora griseola, resulting in significant yield losses, particularly in low-input smallholder farming systems in the tropics. The ALS resistance gene Phg-2, which was found in several highly resistant common bean genotypes, was investigated in crosses between Mesoamerican pre-breeding lines and elite Andean breeding lines. Next-generation sequencing (NGS) data sets were used to design new SNP-based molecular markers. The Phg-2 locus was confirmed to be the major locus providing ALS resistance in these crosses. The locus was fine-mapped to a 409-Kbp region on chromosome 8. Two clusters of highly related LRR genes were identified in this region, which are the best candidate genes for Phg-2. Molecular markers were identified that are closely linked to the Phg-2 resistance gene and also highly specific to the donor germplasm. Marker-assisted selection (MAS) was used to introgress the Phg-2 resistance locus into Andean breeding germplasm using MAB lines. The usefulness of molecular markers in MAS was confirmed in several field evaluations in complex breeding crosses, under inoculation with different ALS pathotypes. This project demonstrates that NGS data are a powerful tool for the characterization of genetic loci and can be applied in the development of breeding tools.

Highlights

Common bean (Phaseolus vulgaris L.) is the most important food legume for direct human consumption around the world (Broughton et al 2003)
To further introgress Angular leaf spot (ALS) resistance into beans with Andean seed type, MAB lines were crossed to CAL 96 and other elite Andean breeding germplasm
The MAB lines are pre-breeding lines that were selected for bush growth habit, small red seed and good agronomic performance, combined with repeated phenotypic ALS resistance under field conditions

Summary

Introduction

Common bean (Phaseolus vulgaris L.) is the most important food legume for direct human consumption around the world (Broughton et al 2003). In Africa and the Americas alone, production exceeds 13.5 million t/year (FAO 2016). Beans are of particular importance to smallholder farmers that depend on the staple for its calories, protein and nutritional value. Bean production is affected by many constraints including pests and diseases as well as abiotic stresses such as drought or low soil fertility. Smallholder farming systems with low inputs of agrochemicals are vulnerable to these stresses. Angular leaf spot (ALS), caused by Pseudocercospora griseola (Sacc.) Crous and Braun [previously known as Phaeoisariopsis griseola (Sacc.) Ferrari (Crous et al 2006)], is a major production constraint of common bean in the tropics (Wortmann et al 1998). Being a widespread and devastating disease, ALS can cause yield losses up to 80% (Schwartz et al 1981; Correa-Victoria et al 1989)

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