Habituation to thaxtomin A increases resistance to common scab in ‘Russet Burbank’ potato

Abstract

Common scab is a potato disease characterized by the formation of scab-like lesions on the surface of potato tubers. The actinobacterium Streptomyces scabiei is the main causal agent of common scab. During infection, this bacterium synthesizes the phytotoxin thaxtomin A which is essential for the production of disease symptoms. While thaxtomin A can activate an atypical programmed cell death in plant cell suspensions, it is possible to gradually habituate plant cells to thaxtomin A to provide resistance to lethal phytotoxin concentrations. Potato ‘Russet Burbank’ calli were habituated to thaxtomin A to regenerate the somaclone RB9 that produced tubers more resistant to common scab than those obtained from the original cultivar. Compared to the Russet Burbank cultivar, somaclone RB9 generated up to 22% more marketable tubers with an infected tuber area below the 5% threshold. Enhanced resistance was maintained over at least two years of cultivation in the field. However, average size of tubers was significantly reduced in somaclone RB9 compared to the parent cultivar. Small RB9 tubers had a thicker phellem than Russet Burbank tubers, which may contribute to improving resistance to common scab. These results show that thaxtomin A-habituation in potato is efficient to produce somaclones with increased and durable resistance to common scab.

The Streptomyces scabiei Pathogenicity Factor Thaxtomin A Induces the Production of Phenolic Compounds in Potato Tubers

“…Treatment of potato tuber slices with TA induces a dark brown coloration of the parenchyma tissue that is commonly thought to be due to necrosis ( Figure 1 ) [ 14 , 17 , 18 , 21 , 38 ]. Since TA is the main pathogenicity factor of the common scab causative agent S. scabiei , this TA test has often been used to determine the level of sensitivity of different potato varieties to this disease [ 14 , 16 , 17 , 18 ]. However, our results indicated that the intensity of the browning response induced by TA did not correlate with the level of susceptibility to common scab resistance of the RB and YG potato varieties.…”

“…Phenolic compounds synthesised in response to TA can be incorporated in the biopolymer suberin, which is a central component of the potato tuber periderm that forms the main barrier against pathogens [ 42 , 43 ]. Thickening of the tuber periderm and lenticel suberization have been reported in response to S. scabiei and TA, and in common scab resistant somaclones regenerated from TA-selected cells, suggesting that accumulation of phenolics may increase resistance to the disease [ 16 , 28 ]. Phenol accumulation was more important in the RB variety than in YG, suggesting that the strong induction of the phenylpropanoid pathway in RB tubers may contribute to producing a thicker suberin-enriched layer for tuber protection against S. scabiei [ 32 , 44 , 45 ].…”

“…Extraction and purification of TA produced from S. scabiei EF-35 [ 54 ] were performed as described previously [ 16 ]. Extracted TA was diluted in MeOH at a final concentration of 1 mM or 3.5 mM.…”

“…TA has often been used for the preliminary screening of potato germplasm for resistance to common scab [ 14 , 15 , 16 ]. TA treatment of tuber slices induces tuber parenchyma browning, with an intensity that was proposed to correlate with sensitivity to common scab [ 14 , 17 , 18 ].…”

The Role of Thaxtomin A and Necrosis Protein in Virulence of Streptomyces scabies that Cause Potato Common Scab Disease

“…Fourteen days after inoculation, mini tubers were rinsed in water, left to dry, and rated for disease severity. The percentage of surface area covered by necrotic lesions was utilized to evaluate the scab index as follows: 0 = no scab; 0.5 = less than 1%; 1 = 1 to 5%; 2 = 6 to 10%; 3 = 11 to 25%; 4 = 26 to 50%; 5 = 51 to 75%; and 6 = 76 to 100% tuber surface coverage (Beaudoin et al, 2021). The percentage of scab severity index was calculated for each tested isolate as follows:…”