3.5 The expression of pectate lyase genes was higher in D. solani than in D. dianthicola
The capacity of D. solani to initiate rotting with a low bacterial load suggested a differential expression of pectate lyase genes between the two species. Expression levels of the pelA ,pelD and pelE genes were quantified by RT-qPCR in the fiveD. solani and five D. dianthicola strains grown separately in a rich medium (in the absence of pectin) and in rotted potato stems and tubers. The expression of two other virulence genes, cbsE andacsD, which are not involved in the plant cell wall degradation, but in iron-uptake, were also compared. Four RT-qPCR measurements were performed per condition and gene. For each gene, Kruskal-Wallis tests revealed differences between the six treatments (rich medium, stems and tubers for the two species, considering each strain as a replicate) for the different genes: pelA (k=17.1; DF=5 ; p=4x10-3), pelD (k=25.1; DF=5 ; p=1x10-4), pelE (k=24.9; DF=5; p=1x10-4), cbsE (k=20.3; DF=5 ; p=1x10-3) and acsD (k=21.3; DF=5; p=7x10-4).
Pairwise post-hoc Tukey tests were used to compare gene expression between D. solani and D. dianthicola . In the absence of pectin (Figure 5 ), the pelA (F=864.3; p< 10-11), pelD (F=864.3; p< 10-11) and pelE (F=2.9; p= 4 x 10-3) genes were expressed at a higher level inD. solani than in D. dianthicola . The expression levels of the cbsE (F=0.377; p=0.99) and acsD (F=-1.0; p=0.99) genes were not significantly different between species (Figure S5 ). In the absence of pectin, a higher expression of pectate lyase genes, especially pelE , could be considered as an advantage for initiating degradation of the plant cell wall polymers.
In symptomatic potato tubers and stems, gene expression was more variable among strains (Figures 5 and S5 ). The expression levels of pelD and pelE were higher in D. solanithan in D. dianthicola (pelD : F=864.3; p< 10-11 in tubers and F=4.5; p= 10-4in stems; pelE : F=4.5; p= 1.1 x 10-4 in tubers and F=8.1; p < 10-11 in stems). ThepelA (F=864.3; p< 10-11) andcbsE (F=3.4; p= 0.01) genes were expressed at higher levels inD. solani than in D. dianthicola in stem tissues, but not in tubers (pelA : F=-0.09; p=1 and cbsE : F=0.67; p=0.99). The acsD expression level did not differed between the two species in diseased tubers (F=-1.5; p=0.9) and stems (F=0.5 p=0.9). Overall, in plant symptoms, D. solani maintained a higher expression of the virulence genes pelD and pelE compared to D. dianthicola .