Morphological Structure Of Tracheary Elements In Interfamilial Grafts

Grafting plays an important role in agricultural production and horticulture research,through grafting can improve the yield and quality of crops.In recent years,grafting technology is only widely used the same species or the different species with close relatives.In the cross-species grafting with distant relatives,the problem of graft incompatibility usually occurs due to the different genetic background and morphological structure between the rootstock and scion,which leads to the growth stagnation of the graft and eventually leads to grafting failure.Therefore,how to successfully construct distant grafts is an important problem that needs to be solved in the development of grafting technology.In the process of graft healing,the reconnection of vascular bundle is the key to graft success.In previous literature,numerous scholars have made a lot of elaboration on the mechanism of grafting healing,but there are relatively few studies on the basic unit of vascular bundle,tubular element(TE).Therefore,the reconnection of TE in grafts deserves further exploration.In this study,Portulaca oleracea was used as rootstock to construct distant grafts between families and above with scions from different species,respectively.Using cell staining and scanning electron microscopy techniques to observe the microstructure of tubular molecules in the xylem between different grafting scions and rootstocks,and infer the reason for distant grafting incompatibility from the perspective of anatomical structure.And the problem of graft incompatibility was improved by changing the morphology and structure of tubular element.The main research findings are as follows:1.Through the observation of phenotypic changes in the growth process of three distant grafts of Arabidopsis thaliana/Portulaca oleracea(At/Po),Nicotiana benthamiana/Portulaca oleracea(Nb/Po)and Brassica napus/Portulaca oleracea(Bn/Po),it was found that the three distant grafts would appear the phenomenon of enlarged grafting interface,yellowing leaves,adventitious root and growth stagnation.Through phenotype analysis,it can be concluded that At/Po grafting is not compatible in the later stage,Nb/Po grafting is highly incompatible,and Bn/Po grafting is completely incompatible.2.In this study,we found that an emergent structure,or the spiraling TEbundles consisting of TE masses occurring at the graft interface,was extensively present in the highly incompatible interfamilial graft of Bn/Po and Nb/Po.This special structure mostly appeared in the local area near the grafting union,and the frequency and quantity of the spiraling tracheary element bundles were much higher in the scion than in the rootstock.Nevertheless,only a small portion of At/Po interfamilial grafts showed a less spiraled TEs at the grafting union(usually a circular TE),which is consistent with its growth performance.3.Arabidopsis At-mi R165 a overexpressed plants were constructed,which inhibited the development of TE,resulting in changes in their morphological structure compared with that of wild-type Arabidopsis thaliana,which was more similar to the TE morphological structure of Purslane.Compared with wild-type Arabidopsis thaliana/Portulaca oleracea(Col/Po).The leaf yellowing rate and adventitious root generation rate of At-mi R165a/ Portulaca oleracea(At-mi R165a/Po)interfamilial grafts decreased significantly,both lower than 30%,while Col/Po interfamilial grafts leaf yellowing rate and adventitious root generation rate were both higher than 50%.At the same time,the spiraling structure production rate of At-mi R165a/Po interfamilial grafts decreased to less than 10% compared with 16.3% in Col/Po interfamilial grafts.The grafts of At-mi R165a/Po were superior to those of Col/Po both in terms of the growth of grafts and TE reconnections between scion and rootstock.Therefore,by changing the morphology and structure of TE,the connection of TE between rootstocks and scion can be improved,thereby enhancing grafting compatibility.This study consolidated that spiraling TE bundles were an important indicator for graft incompatibility.The possible reason for the formation of spiraling TE bundles in interfamilial grafts was discussed and explored improved on this basis.It provides more theoretical basis for further solving the problem of distant graft incompatibility.