Study On Developmental Anatomy And Transcriptomics Of Podocarpus Macrophyllus Root Nodules

Nodular plants are important nitrogen fixing resources in nature,which have great ecological and application value.In this paper,samples were collected from experimental forests of Guangxi Zhuang Autonomous Region Academy of Forestry Sciences.Using the methods of freehand sectioning and paraffin sectioning,the root nodules of the Podocarpus nagi,root nodules of Acacia mangium,Acacia aulacocarpa from the legume plant-Rhizobium symbiotic nitrogen fixing system,and the root nodules of Alnus formosana and Myrica rubra from the non-legume-Frankia symbiotic nitrogen-fixing system were used as reference comparisons to study the morphological structure of Podocarpus macrophyllus root nodules at different development stages.At the same time,three generations of full-length transcriptomes were measured on the root nodules of Podocarpus macrophyllus,and the target protein was searched and clustered,and the mechanism of oxygen shielding and protein genetic characteristics in nitrogen fixation of the root nodules of Podocarpus macrophyllus were discussed,and provides theoretical basis for the basic research of plant symbiosis system and the prediction of evolution trend.The study reached the following conclusions:(1)The way of root nodules growth and their external morphological characteristics: The root nodules of Podocarpus macrophyllus and Podocarpus nagi are spherical or nearly conical,with a diameter of 0.2-1mm.A few of them have root hairs and nodule hairs,and the color of the nodules gets darker with the growth of the nodule age;the nodules mainly distributed on the lateral root with a large amount of them,arranged in two rows along the growth direction of the root,which are usually solitary,occasionally clustered and easy to fall off.The root nodules of the other four plants are all scattered,small in number and irregular in shape,but can be bifurcated,the diameter of the root nodules of legume plants is about 0.5-8mm,which is easy to fall off,most of the root nodules of Alnus formosana and Myrica rubra are more than 1cm in diameter,which is not easy to peel off.(2)The internal tissue structure of root nodules: The overall structure of Podocarpus macrophyllus and Podocarpus nagi root nodules are composed of periderm,epidermis,nodule cortex,nodule endodermis and central vascular system from the outside to the inside,the infection area is located in the cortex of root nodule,which is divided into Rhizobium functional area in the middle cortex and Frankia functional area in the outer cortex,which has the unique cyst structure of legume root nodules and the unique vesicle structure of Frankia root nodules.The structure of the other four kinds of root nodule is similar to that of Podocarpus macrophyllus and Podocarpus nagi root nodules,but the arrangement of their parts is different.In the ultrastructural observation,it can be found that the change of microbe and content,the infection line,the migration path of bacteria and the change of the internal tissue of the root nodule.(3)The origin and "secondary growth" of root nodules: The new nodules on the young roots of Podocarpus macrophyllus and Podocarpus nagi originate from the pericycle of the young roots,which is homologous with the lateral roots;the new nodules on the old roots originate from the phelloderm;the new nodules on the old ones originate from the vascular bundle,and the "secondary growth" of the nodules is formed by the dedifferentiation of the pericycle of the mature nodules to form the meristematic tissue to division and differentiation,and the new nodules are superimposed on the old ones,which belong to a special case of determinate nodule and indeterminate nodule.The other four types of root nodules originated from the pericycle of young roots,and the "secondary growth" was caused by the meristem in the nodules,which belong to a typical indeterminate nodule.(4)Transcriptomics characteristics of Podocarpus macrophyllus root nodules: It was found that there were 1 kind of non symbiotic plant hemoglobin(ns Hb)and 33 kinds of similar genes of lectins in the protein of Podocarpus macrophyllus root nodules.shared by leguminous plants,and 28 kinds of nonleguminous plants lectins in the protein of Podocarpus macrophyllus root nodules.Neighbor joining method(NJ)was used to establish phylogenetic trees of two types of lectins in the root nodules of Podocarpus macrophyllus and those from legumes,Frankia and Rhizobium.The results indicated that the lectins of Podocarpus macrophyllus root nodules have close relationship with some lectins of legumes plants,non-legumes plants and Rhizobium from a genetic point of view.(5)To sum up,from the perspective of root nodule morphology,development,genetic characteristics and the order of origin of plant groups,it is highly possible that legume-Rhizobium nodules and non-legume-Frankia nodules are differentiated and evolution from gymnosperm Podocarpus macrophyllus nodules.