Dissecting The Molecular Basis For The Variation Of Seedling Lateral Root Number And Root Hair Length In Diploid And Allopolyploid Wheats

Polyploidization has long been recognized as a prominent force shaping the evolution of flowering plants.Common wheat(BBAADD)is a typical allohexaploid species that originated from the hybridization and polyploidization of the diploid Aegilops tauschii(DD)with the allotetraploid Triticum turgidum(BBAA).As compared to diploid and tetraploid progenitors,hexaploid wheat has prominent performance in morphology,physiology and grain yield.However,the understanding of wheat root traits is relatively limited.Based on the previous findings in our laboratory,this study focused on dissecting the molecular basis of the variations in seedling lateral root number and root hair length during allopolyploid wheat evolution.The main results are summarized as follows:1.Lateral root number:First,the lateral root number of three sets of synthetic allohexaploid wheat were measured and compared.Significant differences for lateral root number at 8 day after germination(DAG)were observed,in the following order:diploid parent Aegilops tauschii>synthetic allohexaploid wheat>allotetraploid wheat(i.e.,the lateral root number of each synthetic allohexaploid wheat displayed a mid-parent advantage).Second,the lateral root number of 77 natural wheat accessions with varying ploidy levels at 8 DAG was measured and compared.At the diploid level,the average lateral root number of the DD group was much higher than that of SS and A A groups.Across different ploidy levels,the BBAADD group displayed increased lateral root number compared with the BBAA,AA and SS groups,but significantly lower than that of the DD group.In addition,the expression of the wheat gene TaLBD16 was positively correlated with the seedling lateral root number in diploid and allopolyploid wheats(P<0.01).Notably,increased lateral root number by overexpression of the TaLBD16 in wheat led to enhanced shoot fresh biomass under nutrient-poor conditions,suggesting that increased seedling lateral root number in wheat provides advantages for aerial growth.Taken together,we proposed that the D genome played a crucial role in the increased lateral root number of allohexaploid wheat compared with their allotetraploid progenitors,and TaLBD16 was an important gene involved in the variations of seedling lateral root number in diploid and alloployploid wheats.2.Root hair length:To investigate the molecular mechanism for the formation of the root hair length advantage in allopolyploid wheat,comparative transcriptome analysis between synthetic allotetraploid wheat and its parents was conducted.Of 7836 nonadditive expression genes,20 genes were involved in the ethylene biosynthesis pathway,among which 14 genes displayed a high-parent expression pattern and 4 genes showed a transgressive-upregulation expression pattern.The above results were consist with the high-parent pattern of endogenous ethylene in the seedling roots of synthetic allotetraploid wheat.In addition,exogenous ethylene precursor 1-aminocyclopropane-1-carboxylic acid(ACC)and ethylene synthesis inhibitor amino-ethoxyvinylglycine(AVG)could promote and reduce the root hair length of synthetic allotetraploid wheat and its parents,respectively.Moreover,the sensitivity of synthetic allotetraploid wheat to ACC and AVG treatment was significantly different from that of its parents.Besides,ethylene could positively regulate the expression of TaRSL4 gene,and the sensitivity of TaRSL4 expression to ACC and AVG treatment displayed significant differences between TaRSL4 transgenic lines and wild type.Conclusively,we proposed a simple model for ethylene-mediated formation of root hair length advantage in synthetic allotetraploid wheat.