## 6 Future Prospects  
Although there has been progress in cloning PGIP genes and investigating PGIP structure, function, expression, and regulation, future work is required to determine how to effectively integrate the heterologous PGIP gene into a plant genome for stable expression to improve plant disease resistance. In addition, more studies are required to characterize differences in the mechanism of action, expression conditions, expression effects, and antimicrobial effects for different PGIPs. The sequence similarity of PGIP genes in homologous plants is relatively high, and there is almost no difference in the protein sequence of coding products. Therefore, the study of PGIP gene sequence difference is not enough to explain the expression difference among PGIP gene family members, nor to reveal the difference of PGIP activity among varieties. More and more scholars believe that the most important thing is the difference in gene expression regulation. In addition, the same PGIP may have different activities to inhibit PG. Changing a single amino acid of PGIP can change the specificity of the PG-PGIP interaction. Therefore, the overexpression of PGIP genes can improve plant disease resistance; for example, the overexpression of active PGIPs genes or changing the amino acid residues of inhibition recognition sites will make PGIP produce stronger specific recognition and inhibition intensity [27; 31; 32]. This increases plant resistance [68]. Traditional disease resistance genetic breeding is not enough to solve the problem of plant disease resistance, but with the continuous improvement of genetic engineering, molecular biology, and transformation system, transgenic breeding can be used to transform a wide range of resistance genes into plants, so as to effectively improve plant disease resistance.  
Conceptualization, X.L. and C.Y.; methodology, S.C., R.L. and L.L.; software, S.C., R.L. and L.L.; validation, S.C., R.L. and L.L.; formal analysis, S.C., R.L. and L.L.; investigation, H.S.; resources, H.S.; data curation, H.S.; writing--original draft preparation, S.C., R.L. and L.L.; writing--review and editing, S.C., R.L. and L.L.; visualization, H.S.; supervision, X.L. and C.Y.; project administration, X.L. and C.Y.; funding acquisition, X.L. and C.Y. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the National Key Research and Development Program of China (2018 YED1000800 and 2019 YED1000300), the National Natural Science Foundation of China (Grant Nos. 31872105, 31972221, 32002048, and 31801862), the Zhejiang Province Public Welfare Technology Application Research Project (LGN18C020005), and the Zhejiang Natural Science Foundation of China (Grant Nos. LY19C150008, LY19C140004, LY21C150002).  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Gene** & **System for Expression** & **Enhanced Resistance** \\ \hline \multirow{4}{*}{Chinese cabbage (_Brussica rapa_ ssp. _pckinensis_)} & _BrPGIP1_ & \multirow{4}{*}{Cabbage} & _Pectobacterium carorooverum_ ssp. _Carotoxorum_ (Pcc) [45] \\  & _BrPGIP2_ & & _Pectobacterium carorooverum_ ssp. _Carotoxorum_ (Pcc) [45] \\  & _BrPGIP4_ & & _Pectobacterium carorooverum_ ssp. _Carotoxorum_ (Pcc) [45] \\  & _BrPGIP5_ & & _Pectobacterium carorooverum_ ssp. _Carotoxorum_ (Pcc) [45] \\  & _BrPGIP5_ & & _Pectobacterium carorooverum_ ssp. _Carotoxorum_ (Pcc) [45] \\  & \multirow{2}{*}{Pear (_Pyrus bretschneideri_)} & _BrPGIP_ & \multirow{2}{*}{Grape} & _Xyella fastidiosa_[84] \\  & & & \\ Tomato (_Lycopersicon esculentum_) & _PGIP_ & Tomato stem & _Ralstonia solanacearum_[83] \\  & \multirow{2}{*}{_OsPGIP1_} & \multirow{2}{*}{Rice} & Bacterial leaf streak [85] \\  & & & \\  & & & \\  & & & \\  & & & \\  & & \\  & & & \\  & & & \\  & & \\  & & & \\  & & & \\  & & \\  & & & \\ \end{tabular}
\end{table}
Table 3: PGIP genes studied in various systems against bacterial stresses.