Joint Action of Pb1 and Pb2 Provides Dominant Complementary Resistance Against New Races of Peronospora belbahrii (Basil Downy Mildew)

Abstract

Sweet basil (Ocimum basilicum, 2n=4x=48) is susceptible to downy mildew caused by Peronospora belbahrii. Pb1 gene exhibit complete resistance to the disease. However, Pb1 became prone to disease due to occurrence of a new virulent races. Here we show that Zambian accession PI 500950 (O. americanum var pilosum) is highly resistant to the new races. From an interspecies backcross between PI 500950 and the susceptible cv ‘Sweet Basil’ we obtained, by embryo rescue, a population of 131 BC1F1 plants. This population segregated 73 Resistant: 58 Susceptible (1: 1, P=0.22), suggesting the resistance is controlled by one incompletely dominant gene called Pb2. To determine whether allelic relationship is existing between Pb1 and Pb2, we used two differential races, race-0, avirulent to both PI 500945 (Pb1) and PI 500950 (Pb2) and race-1, virulent to PI 500945 but avirulent to PI 500950. F1 plants obtained from ‘12-4-6’ (BC6F3 derived from PI 500945) and ‘56’ (BC3F3 derived from PI 500950) showed resistant superiority to both races due to dominant complementary interaction. F2 plants segregated to race-0 as follow; 12:3:1, immune: incomplete resistant: susceptible, as against to 9:3:4 to race-1, indicating Pb1 and Pb2 are not alleles. Since joint action is contributed in F1 plants and in advanced (BC3F3(56) x BC6F3(12-4-6) F4) populations who carrying both genes, it can be assumed that both accessions carry two unlinked genes but share a common signal transduction pathway which leading to dominant complementation superiority of the resistance against different races of BDM.

Identification of novel basil downy mildew resistance genes using de novo comparative transcriptomics

“…Other BDM resistant cultivars have been produced through interspecific hybridization of O. basilicum with O. americanum var. pilosum , leading to dominant resistance against two races of P. belbahrii (Ben-Naim & Weitman, 2021). We have observed that the downy mildew-resistant basil cultivars succumb to infection in production systems in different regions, consistent with the report of emerging pathogen races (Ben-Naim & Weitman, 2021).…”

“…pilosum , leading to dominant resistance against two races of P. belbahrii (Ben-Naim & Weitman, 2021). We have observed that the downy mildew-resistant basil cultivars succumb to infection in production systems in different regions, consistent with the report of emerging pathogen races (Ben-Naim & Weitman, 2021). Therefore, understanding the physiological and molecular bases of host-pathogen interactions is critical to rapidly developing improved cultivars, monitoring strategies, and management practices.…”

A comparison of transporter gene expression in three species of Peronospora plant pathogens during host infection

“…Synthetic fungicides are the primary means to control downy mildew disease in basil [12][13][14][15]. Control of downy mildew disease in basil was achieved in several cases through the action of two or more genes, but the mechanism(s) of host resistance is undetermined [16][17][18]. The genome of a German isolate of P. belbahrii was sequenced and described [19].…”