We found that PME and endo-PG activity is significant in F. oxysporum isolated from Beni Abbes and Ghardaia, but this activity diverges between strains. By way of comparison, Foa strains from Morocco showed high PME, low PG, and moderate PGTE activity (El Moudafar and El Boustani 2000), and F. oxysporum, F. oxysporum f.sp. ciceri, and Foa strains from Algerian soil showed different PME and PG activities (Karkachi et al. 2014). The significant activity suggests that PME and endo-PG are involved in cell wall pectin hydrolysis. However, previous findings revealed this significance only in single strains: exo-PG in F. oxysporum f.sp. cubense (Zhangyong and Zhenzhong 2015), PME and PG gene expression in F. oxysporum (Wojtasik et al. 2016), and cooperative activity of PG1 and PGx6 in F. oxysporum f.sp. lycopersici (Ruiz et al. 2016). It is also important to note that in vitro growth conditions decrease the activity; for example, F. oxysporum f.sp. vasinfectum PG1 gene expression is 15.4 fold after infection of cotton but 4.3 fold in pectin (Liu et al. 2017a). Therefore, the activity of the studied strains may be higher in vivo.
PG hydrolyzes polygalacturonic acids to galacturonic acids identified in F. oxysporum f.sp. cubense PG activity (Zhangyong and Zhenzhong 2015), and it increases simple sugars (Wojtasik et al. 2016). The Vmax of F. oxysporum f.sp. vasinfectum PG1 is 16 to 20 units/mg (Liu et al. 2017a), and the optimal activity of F. oxysporum f.sp. cubense PGc3 is at pH 4.5 and 50°C (Zhangyong and Zhenzhong 2015), and F. oxysporum f.sp. vasinfectum PG1 is at pH 9 (Liu et al. 2017a). Optimal conditions could increase endo-PG activity in the studied strains. PME, on the other hand, hydrolyzes methyl-polygalacturonic acid ester bonds. F. oxysporum f.sp. cubense PGc3 molecular weight is 45 KDa (Zhangyong and Zhenzhong 2015), and F. oxysporum f.sp. vasinfectum PG1 is 37.18 kDa (Liu et al. 2017a). The N-terminal sequence “TSSRNSALPKRPHVE’’ (Zhangyong and Zhenzhong 2015) and the domains NXD, DD, GHGXSIG, RIK, and Y (Ruiz et al. 2016) exist in F. oxysporum f.sp. cubense PGc3 and F. oxysporum f.sp. lycopersici polygalacturonases (PGs), respectively. The sequences of transcription factors binding upstream promoters in some of the latter enzymes are “TYATTGGTGGAA’’ and “CCCTGA’’ (Ruiz et al. 2016). Most F. oxysporum forms have the “G/CYGGGG’’ motif, which is related to carbon catabolite repression, in their PG promoters (Ruiz et al. 2016). The absence of this mechanism suggests that some or all studied strains do not have these motifs.
We also found that the change between glucose and pectin growth conditions did not cause a significant change in endo-PG activity. However, in other studies, there was inhibition in glucose- and pectin-inducing conditions (Ruiz et al. 2016) and only in glucose (Liu et al. 2017a). Glucose triggers carbon catabolite repression (Ruiz et al. 2016), but this effect was not observed in this study; comparison of endo-PG activity in glucose and pectin revealed that changes in carbohydrates do not inhibit the activity. Simple sugars are produced by PG (Wojtasik et al. 2016) or cellulase but are not directly involved in endo-PG inhibition according to our statistical analysis. This is probably another mechanism that regulates endo-PG repression in the presence of glucose.
PME and endo-PG likely hydrolyze cell wall pectin in vivo. In contrast, inhibition of F. oxysporum f.sp. lycopersici PG1 and PGx6 decreases virulence (Ruiz et al. 2016). Additionally, F. oxysporum f.sp. cubense PGs cause maceration of banana and necrosis in stem vascular tissues (Zhangyong and Zhenzhong 2015), F. oxysporum f.sp. vasinfectum PGs decrease cell wall width and cause cotton leaf necrosis (Liu et al. 2017b), and inoculation of F. oxysporum in flax seedlings decreases pectin synthesis gene expression (Wojtasik et al. 2016). In addition, phenol, lignin, and cafeoylshikimic acid in resistant date palms protect cell walls against pectinase and cellulase (El Moudafar et al. 2000; El Moudafar and El Boustani 2001). Other defense mechanisms exist: F. oxysporum f.sp. vasinfectum PG-inhibiting protein (GhPGI1) increases after inoculation and protects against PGs (Liu et al. 2017b), infection of flax seedlings by F. oxysporum triggers rearrangement of cell wall contents to increase resistance (Wojtasik et al. 2016)d oxysporum f.sp. vasinfectum PG1 increases defense enzyme and gene activity (Liu et al. 2017a). Endo-pectinases produce pectin gels that block sap circulation inside the xylem (Semal 1989). Therefore, ascendant sap in P. dactylifera seems to be gradually blocked, and then the rachis is gradually macerated while F. oxysporum endo-PG is active and cell wall pectin is being hydrolyzed.