## 4 Biological Function and Mechanism of Disease Resistance by PGIP
### Regulation of PGIP Expression  
PGIP gene expression is affected by pathogen infections, ethylene, injury, oligogalacturonides (OGs), and induction by jasmonic acid and salicylic acid, indicating the complex molecular regulation of PGIP [33; 34; 35]. Most PGIPs are expressed under the stimulation of pathogens [36; 37], and different PGIPs have specific stimulating conditions. In rape, insect and mechanic damage can stimulate the high expression of Bnpgip1, with little effects of fungi and cold induction. The expression of _BnPGIP2_ responds to fungi infection and injury but not insects, and jasmonic acid but not salicylic acid stimulates both genes [13]. In strawberry, there is high PGIP induction after inoculation with _B. cinerea_, indicating that PGIP plays a role in defense [38] (Table 1). Under the stimulation of fungi, the expression of OsPGPI7 decreases, but _OsPGPI4_ gradually increases and lasts a relatively long time [36]. [PERSON] et al. [39] cloned the promoter of PGIP-1 in soybean and found it is activated by injury but not chemicals or pathogen infection [39]. In _Arabidopsis thaliana_, _AtPGIP1_ is regulated by OGs, but not by ethylene, salicylic acid, or methyl jasmonate. _AtPGIP2_ is regulated by methyl jasmonate, COI1, and JARI gene product but not by OGs [11]. Treatment of strawberry (_Fragaria chiloensis_ L.) with chitosan and methyl jasmonate for more than 48 h effectively inhibited grey mold incidence and was accompanied by the up-regulation of _FcPGIP1_ and _FcPGIP2_[40]. Proteomics analysis revealed a high expression of _AcPGIP_ after infection of kiwifruit (_Actinidia deliciosa_ \"Hayward\") by _B. cinerea_[41]. After biological and abiotic stress treatment, the expression level of different _OsPGIPs_ in rice showed significant changes with an overall significant increase in expression level, suggesting rice can improve its stress resistance by regulating the expression of _OsPGIPs_ under stress conditions [26]. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Gene** & **System for Expression** & **Mediated Factor** & **Enhanced Resistance** \\ \hline _Arabidopsis thaliana_ & _AtPGIP1_ & _Arabidopsis thaliana_ & Oligogalacturonides (OGs) [11] & _Botrytis cinerea_ \\ _Arabidopsis thaliana_ & _AtPGIP2_ & & Jasmonate [11] & _Botrytis cinerea_ \\ Kidney bean & _PvPGIP1_ & & Wound [6] & _Aspegillus niger_[42] \\ (_Phaseolus vulgaris_ L.) & _PvPGIP3_ & _Nicotiana benthamian_ & Oligogalacturonides (OGs) [6] & \\ Rapeseed & _BnPGIP1_ & & Jasmonate, insect herbivory, & _Sclerotinia sclerootirium_ \\ (_Brassica napus_) & & & mechanical wounding [43] & \\  & _BnPGIP2_ & & Jasmonate, defense hormones, & \\  & _BnPGIP2_ & & and _Sclerotinia sclerootirium_ & \\ _Solanum toruum_ & _StPGIP_ & & _Verticillium dahliae_ [44] & \\ Wheat & _TaPGIP3_ & & IAA, MeJA, SA, and ABA & _Fusarium graminearum_[25] \\ (_Tritcium aestivum_ L.) & _TaPGIP2_ & & & \\ Chinese cabbage & _BrPGIP1_ & & JA [45] & _Pectobacterium cartoororum_ \\ (_Brassica napus_ ssp. & _BrPGIP2_ & Cabbage & JA [45] & _Pectobacterium cartoororum_ \\ _pekinensis_) & _BrPGIP5_ & & JA [45] & _Ssp. Carooverum_ (Pcc) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: PGIP regulatory factors tested in different systems against PGs. During pathogen infection, PGIPs can interact with pathogen PGs, eventually leading to the accumulation of OGs [29]. OGs function as a damage-associated molecular pattern (DAMP) that is recognized by the receptor wall-associated kinase 1 (WAK1) to induce host immunity [53]. This can activate the defense system of plants in the model of pathogen-related resolution-associated molecular patterns (RAMPs), such as the accumulation of anti-disease factors, deposition of callose, and increased NO/ROS [54]. [PERSON] et al. [29] detected a high level of OGs in the PGIP-PG transgene family by the negative ion exchange chromatography pulse method, with the resistance of transgene material toward various types of fungi.