The Molecular Mechanism Of Small Molecule Polypeptide IDL6 In Regulation Of Plant Immunity And Development Of Arabidopsis Thaliana

Plant endogenous small molecular peptides are gradually known as a new type of phytohormone, and play essential roles in regulation of plant development and environmental stress response. To find out the functional small molecular peptides in plant immunity and development, we analyzed the gene chip treated with PAMP flg22, and identified several flg22 upregulated precursor genes encoding small molecular peptides. Among them, the function unkown gene IDL6 (At5g05300) is good candidate. Homologous comparison analysis shows that the IDL6 is one of the 6 members of IDA family. All IDAs have an N-terminal signal peptide and an approximately 18 conservative amino acid sequence in the C-terminal. The sequence between signal peptide and conservative area is variable region.Here, we identified IDA-like 6 (IDL6), a gene which transcription could be upregulated by flg22 and the bacterial Pseudomonas syingae pv. tomato (Pst) DC3000 as well, indicting that this is a putative functional gene in plant innate immunity. To find out the physiological roles of it, the expression profile of it was investigated with promoter-GUS strategy, and the result turned out that the IDL6 was prominently expressed in Arabidopsis leaves.IDL6 overexpression and knock-down lines were also generated and employed to test if this gene was involved in plant immunity, and found these two lines were with decreased and increased resistance to Pst DC3000 infection, indicated that IDL6 may facilitate the plant susceptibility to plant pathogen. Moreover, IDL6 also promoted the transcription of ADPG2, a polygalacturonase (PG) encoding gene, and increased PG activity in leaves, which in turn reduced leaf pectin content and leaf robustness. On the contrary, ADPG2 overexpression restrained the plant while its loss-of-function mutants increased the resistance to Pst DC3000 infection. Further data showed that the Pst DC3000 infection could also elevate the ADPG2 expression, and this effect was partially through downstream receptors, HAE and HSL2. Taken together, our results suggested that the IDL6-HAE/HSL2 pathway facilitate the ingress of Pst DC3000 by promoting pectin degradation of Arabidopsis leaves, and thus to enhance the plant susceptibility to pathogen infection.Peptide hormones have been suggested to play crucial roles in plant development besides plant immunity. By using root growth assay, we found that the both IDL6 overexpression lines were with significantly reduced meristem size than wild type and IDL6 loss-of-function lines. In addition, in the IDL6 overexpression lines, the expression of auxin marker DR5rev:GFP and the cell division marker pCYCB1;1:GUS were both decreased, while, the expression of SHY2, the cell differentiation essential gene, was increased. Taken together, our data indicated that the IDL6 could be a negative regulator in plant root elongation by suppressing the cell division and stimulating the cell differentiation in Arabidopsis root meristem.In conclusion, IDL6, the IDA-like small peptide, is involved in regulaiton of cell wall remodeling by influencing the pectin content, and which in turn leads to facilitate the pathogenic bacteria invasion. On the other hand,IDL6 is also invoked in auxin and cytokinin regulated cell division and differentiation during root tip development.