| In higher plants,there is a hydrophobic structure in root endodermis,called Casparian strip.Casparian strip is an impregnation of endodermal cell wall,forming an apoplastic diffusion barrier which forcing the symplastic and selective transport of nutrients across endodermis.The extracellular structure protecting the internal tissues from damage of environmental stress,is thought to provide the protection of vascular tissues.Tomato,one of the most major commercial horticultural crops,has a great demand in the market at home and abroad and plays an important role in the supply of vegetable and fruit market.Based on the previous studies and results of Casparian strip in Arabidopsis,the phylogenetic trees of Casparian strip related genes were constructed.Moreover,the study revealed a spatiotemporal conservation of the framework regulating Casparian strip in tomato.Together with the phylogenetic conservation of essential regulators of Casparian strip among all plant species,further functional analyses implied that there might exist a conserved mechanism prompting Casparian strip formation during the terrestrialization of land plants.The main results were as follows:1.Phylogenetic analysis of the regulatory factors involved in lignin deposition.The phylogenetic trees,containing the upstream regulators,SHR and MYB36,and the downstream genes CIFs,PER64,SGN1,SGN2,SGN3,SGN4,CASP1 and ESB1,showed the high homology and conservation of most genes among different species ranging from dicotyledon and monocotyledon to gymnosperm and fern.Based on phylogenetic analysis,the homologous genes of Casparian strip regulators in tomato were identified.These genes are highly and specifically expressed in the root through the transcriptome analysis of various tissues in tomato from tomato digital expression data.2.The expression patterns of Casparian strip homologous genes in tomato.The histological chemistry analysis showed tomato genes SlMYB36,SISGN3-1,SlSGN3-2 and SIPER64 are expressed in tomato root endodermis,which is similar to that of Arabidopsis thaliana.SISHR and SlCIF,showing the expression in stele,is consistent with Arabidopsis thaliana.The results indicated that the expression patterns of Casparian strip genes in tomato and Arabidopsis are conservative.3.Functional analysis of tomato gene SlSHR.The overexpressed SISHR roots produced more cortex cell layers,compared to the normal hair roots.Only one cell layer that is in direct contact with stele formed lignified Casparian strip band,which is similar to the observation in Arabidopsis thaliana.In slshr,the cell-layer numbers of hair roots are less than the normal level,suggesting SISHR may affect the normal formation of Casparian strip.4.Functional analysis of tomato gene SlMYB36-1.In over-expressed SlMYB36-1 lines,lignin was ectopically deposited in cortex and formed discontinuous Casparian strip-like structure in the cortex cell walls,similar to that of inducible AtMYB36 in Arabidopsis.For the mutant slmyb36-1,continuous Casparian strip was finally formed in endodermis.It is not clear whether the formation of functional Casparian strip is delayed in the mutant.5.Functional analysis of tomato gene SISGN3-2.In the mutant slsgn3-2,Casparian strip was discontinuous in endodermis,similar to the mutant sgn3 in Arabidopsis,indicating that SGN3 plays a conserved role in the Casparian strip development.6.Promoter analysis.AtCASP1 promoter in Arabidopsis can drive similar expression in tomato and soybean.Furthermore,the promoters of other Casparian strip regulating genes(AtSGN3,AtMYB36)driving endodermis-specific expression in Arabidopsis showed the same expression pattern in tomato.The results implied that the transcriptional activation elements within these promoters may be conserved across species.The analysis of the promoter sequences showed some cis-acting regulatory elements are common and their relative position are conserved in different promoter regions,indicateing that there are conserved motifs which could account for the similar expression pattern in different species. |
