Functional Analysis Of OsTUA2 For Development Of Lateral Root Primordia In Rice

Plants roots are required for acquisition of nutrients and water,and sense of biotic and abiotic stresses from environment.Lateral roots(LR) are branches out from primary roots(PR) and adventitious roots(AR).They are important components of root system. We isolated three allelic mutants with defect in lateral root development from an EMS-generated rice mutant library(Kasalath,indica).The previous work has cloned the gene for OsTUA2 on chromosome 11 by using map-based cloning strategy.In this study, we focus on the function of OsTUA2 at cellular level.The present results are summarized as follows:The molecular tests confirm that the overexpression of OsTUA2 can rescue the phenotype of Ostua2 mutant but doesn't result in other phenotypic changes compared with the wild type(WT) plants.The expression of the GUS report gene driven by promoter of OsTUA2 indicates that OsTUA2 expresses in pollens,lemma,palea,leaves and stalks.While in roots,the expression is higher in root tip and later root primordia.The expression also exists in exodermis(two layers under the epidermal cell) and vascular system.The transient expression of sGFP:OsTUA2 in onion epidermal cells showed a co-localization with RFP:MAP4 indicating that OsTUA2 participates in the polymerization of microtubule.The cell elongation in primary root is repressed in Ostua2 mutant.It may be due to the aberrant and uneven arrangement of cortical microtubules in meristematic cells and elongating cells.The cell division in the meristem zone of primary root is not affected in the mutant.While the aberrant cell division is found in lateral root primordia of Ostua2 mutant.The size of cells is enlarged and irregular.With incomplete cell wall or wall stubs,the multinuclear in one cell are formed.This may be due to the irregular cell division and cell expansion.The new cell wall can not be normally formed during telophase of cell cycle in the mutant. The ectopic accumulation of lignin was also found in lateral root primordia of Ostua2 mutant.This may be due to the inhibition of cell growth and entering the cell differentiation pathway earlier than wild-type cells.The aberrant vascular cell differentiation affects the site of lignin deposition,resulting in ectopic lignin accumulation.The GUS report gene expression promoted by DR5 was reduced in primary root tips of Ostua2 mutant.The expression in the tip of lateral root primordia in the mutant was also decreased.These results indicate that auxin transportation from vascular to lateral root tip is impaired.The decrease in the content of IAA in the root tip of the mutant also supports this point.The root elongation of Ostua2 mutant is much more induced by phosphate starvation compared with WT.The expression of phosphate starvation induced(PSI) genes were down regulated in Ostua2 mutant under low phosphate stress.The over expression of OsTUA2 down regulates the induced root length in the mutant under phosphate starvation.The content of cytokinin in the mutant is lower than that in the wide type,while increased in the mutant under phosphate starvation.These results suggest that OsTUA2 may be involved in phosphate signal system mediated by cytokinin.In conclusion,OsTUA2 is important for cell division in lateral root primordial and root elongation in root system.OsTUA2 is also involved in the phosphate signal system. This study provides us new insight into the molecular mechanism of lateral root development in cereal.