Abstract Bean common mosaic virus (BCMV) and bean common mosaic necrosis virus (BCMNV) are among the most destructive aphid-transmitted viral pathogens affecting common beans and other cultivated legumes in India. Resistance to these viruses in common bean has traditionally been governed by the dominant I gene & some recessive genes. However, their effectiveness against the highly aggressive BCMNV-NL-3 (PG-VI) strain remains limited and it is necessary to accurately evaluate and deploy appropriate resistance genes either singly or in combinations for achieving effective NL-3 resistance. Therefore, in the present study, we performed in-depth screening of genotypes carrying the I gene in combination with different recessive resistance genes (bc-1, bc-3, bc-u, and bc-ud) for studying their effectiveness against the NL-3 resistance. Genotypes were mechanically inoculated with the BCMNV NL-3 strain maintained on Nicotiana benthamiana plants using infected leaf sap extract to characterize their resistance response. Subsequently, PCR assays employing SCAR, CAPS and Indel markers were used to identify both dominant and recessive resistance loci associated with the BCMNV resistance. Two distinct hypersensitive resistance (HR) groups were identified. The first plant group exhibited mild to severe systemic necrosis of both inoculated and upper uninoculated leaves and was identified as carrying either the I gene alone or in combination with I + bc-1 + bc-u genes. The second plant group displayed only localized vein necrosis of inoculated leaves and possessed the I + bc-1 + bc-ud gene combination. In contrast, bean accessions carrying bc-3 or I + bc-3 gene combinations exhibited resistance to the NL-3 strain without showing HR symptoms. These findings identified valuable resistance gene combinations such as Ibc-1bc-ud / Ibc-3 conferring effective protection against BCMNV-NL-3, which can be utilized in breeding programs aimed at developing virus-resistant bean cultivars. Furthermore, the study also highlights the current understanding of the bc-u locus in interacting with other recessive resistance genes to reduce the severity of BCMNV infection. Background Resistance to bean common mosaic necrosis virus (BCMNV) in common bean differs substantially from resistance to bean common mosaic virus (BCMV). Although the dominant I gene confers effective resistance against BCMV strains, its interaction with BCMNV frequently results in systemic necrosis, particularly under high temperature conditions, often leading to plant death. Therefore, the deployment of the I gene alone is not sufficient for protection against BCMNV. In this context, recessive resistance genes such as bc-1, bc-u and I bc-1 bc-ud play an important role by restricting the severe necrotic response triggered by BCMNV in I gene–carrying genotypes. Results In the present study, two distinct groups of resistant plants were identified based on their phenotypic response to the BCMNV. One group exhibited a hypersensitive resistance (HR) response, whereas the other group showed a resistant reaction without any visible HR symptoms. Among the plants displaying HR, two different types of necrotic responses were observed. One subgroup developed systemic vein necrosis, indicating the spread of necrotic symptoms beyond the inoculated tissues, while the other subgroup exhibited localized vein necrosis, with HR symptoms restricted only to the inoculated leaves. Conclusion The present study investigated the role of recessive resistance genes in protecting BCMNV-induced systemic necrosis and enhancing resistance stability in common bean cultivars carrying the I gene.

Phenotypic modulation of I gene-mediated resistance to bean common mosaic necrosis virus (BCMNV) through recessive gene interactions

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2026年8月4日

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Dasari Meghanath,
Sumiah Wani,
Shahjahan Rashid,
Arsalan Gulzar,
Mir Mehreen,
Sabiya Bashir,
Ajaz A. Lone,
Aflaq Hamid

摘要

Abstract

Bean common mosaic virus (BCMV) and bean common mosaic necrosis virus (BCMNV) are among the most destructive aphid-transmitted viral pathogens affecting common beans and other cultivated legumes in India. Resistance to these viruses in common bean has traditionally been governed by the dominant I gene & some recessive genes. However, their effectiveness against the highly aggressive BCMNV-NL-3 (PG-VI) strain remains limited and it is necessary to accurately evaluate and deploy appropriate resistance genes either singly or in combinations for achieving effective NL-3 resistance. Therefore, in the present study, we performed in-depth screening of genotypes carrying the I gene in combination with different recessive resistance genes (bc-1, bc-3, bc-u, and bc-u^d) for studying their effectiveness against the NL-3 resistance. Genotypes were mechanically inoculated with the BCMNV NL-3 strain maintained on Nicotiana benthamiana plants using infected leaf sap extract to characterize their resistance response. Subsequently, PCR assays employing SCAR, CAPS and Indel markers were used to identify both dominant and recessive resistance loci associated with the BCMNV resistance. Two distinct hypersensitive resistance (HR) groups were identified. The first plant group exhibited mild to severe systemic necrosis of both inoculated and upper uninoculated leaves and was identified as carrying either the I gene alone or in combination with I + bc-1 + bc-u genes. The second plant group displayed only localized vein necrosis of inoculated leaves and possessed the I + bc-1 + bc-u^d gene combination. In contrast, bean accessions carrying bc-3 or I + bc-3 gene combinations exhibited resistance to the NL-3 strain without showing HR symptoms. These findings identified valuable resistance gene combinations such as Ibc-1bc-u^d / Ibc-3 conferring effective protection against BCMNV-NL-3, which can be utilized in breeding programs aimed at developing virus-resistant bean cultivars. Furthermore, the study also highlights the current understanding of the bc-u locus in interacting with other recessive resistance genes to reduce the severity of BCMNV infection.

Background

Resistance to bean common mosaic necrosis virus (BCMNV) in common bean differs substantially from resistance to bean common mosaic virus (BCMV). Although the dominant I gene confers effective resistance against BCMV strains, its interaction with BCMNV frequently results in systemic necrosis, particularly under high temperature conditions, often leading to plant death. Therefore, the deployment of the I gene alone is not sufficient for protection against BCMNV. In this context, recessive resistance genes such as bc-1, bc-u and I bc-1 bc-u^d play an important role by restricting the severe necrotic response triggered by BCMNV in I gene–carrying genotypes.

Results

In the present study, two distinct groups of resistant plants were identified based on their phenotypic response to the BCMNV. One group exhibited a hypersensitive resistance (HR) response, whereas the other group showed a resistant reaction without any visible HR symptoms. Among the plants displaying HR, two different types of necrotic responses were observed. One subgroup developed systemic vein necrosis, indicating the spread of necrotic symptoms beyond the inoculated tissues, while the other subgroup exhibited localized vein necrosis, with HR symptoms restricted only to the inoculated leaves.

Conclusion

The present study investigated the role of recessive resistance genes in protecting BCMNV-induced systemic necrosis and enhancing resistance stability in common bean cultivars carrying the I gene.