Nutrients Efficiency Of Brassica Napus Root Mutants And Its Relation To Endogenous Cytokinin

Phosphorus (P) is an essential mineral nutrient for plant growth and development. Oilseed rape (Brassica napus), an important oil crops in China as well as in the world, need more P than other crops. Plant root is a major organ to uptake nutrients and water from the soil, which could be regulated by endogenous Cytokinins. At low P, the primary root elongation is repressed and lateral root number increased significantly, and cytokinin could be one of the candidate substances that regulate the response of plant root to adapt to P stress. However, it remains unknown the role of cytokinins played in the root growth at low P. Using a quick and reliable method, we identified several root mutants of Brassica napus insensitive to exogenous-cytokinins (6-BA). Shi T et al (2011) results showed that Bnprl1 (primary root length mutant 1) and Bnlrn1(lateral roots number mutant 1) grew better than wild type (WT, Ningyou-7) whether at normal condition or at low P. In this research, hydroponics and paper culture system were used to study the difference in nutrients uptake and utilization efficiency, endogenous cytokinins concentrations, root morphology and architecture at low and high P, respectively. These results could provide some clues to reveal how cytokinins regulate root morphology and shoot growth at low P. The main results were as follows:1. Difference in root morphology between mutants and WT and construction of mapping population for the mutated traitsBnprl1 grew less root hair than WT 0.2μM 6-BA, whereas Bnlrn1 had more lateral roots. There was no significant different in root apex meristem between the mutants and WT. Moreover, spiral thick root was observed in WT, but not in mutants. Microstructure analysis showed that there was no difference in the meristem zone of root tip between mutants and WT, suggesting they may have difference in the elongation zone. Mapping populations for the mutated traits of Bnprl1 and Bnlrn1 were constructed. Primary root length of nine BC1F2 lines derived from Bnprl1 (2Mi01,2Mi06,2Mi07,2Mi08,2Mi10,2Mi11,2Mi12,2Mi13 and 2Mi14) and lateral root density of five BC1F2 lines derived from Bnlrnl (2Mil9,2Mi21 2Mi22, 2Mi23and 2Mi24) were investigated at ddH2O and 6-BA, respectively. The lines with target traits were screened and self-pollinated to produce BC1F3.2. Phosphorus utilization efficiency of mutants and WT Solution culture and paper culture system were used to study phosphorus use efficiency, cytokinins concentration and root morphology of mutants and WT at low and high P, respectively. Results showed that shoot dry matter of Bnprl1 and Bnlrn1 were higher than that of WT at both P levels. Comparing with WT, both of the two mutants had higher shoot P accumulation at high P and higher P uptake efficiency at low P. Bnlrn1 showed more sensitive to low P than WT, e.g. the ratios of these parameters at low P to that at high P were very lower than those of WT and Bnprl1, which showed that the mutation quicken the response of the mutants to low P.The significant difference in ZR (zeatin riboside) and IPA (isopentenyladenosine) concentration between Bnprl1, Bnlrn1 and WT only showed in the root at low P. IPA content of root of both mutants was lower than that of WT. ZR concentration of root of Bnlrn1 was higher than that of WT, whereas, that of Bnprl1 was lower than that of WT. At low P, IPA and ZR concentration of shoot of mutants and WT increased significantly, but there was no significant difference among them. At normal condition, mutants showed higher cytokinins content, higher P content and lower P use efficiency in root than WT. However, at low P, mutants showed higher IPA content, lower root dry weight than WT, suggesting high IPA inhibited root growth at low P.There was no significant difference in primary root growth between Bnprl1 and WT at both P levels. The primary root growth of Bnlrn1 was repressed seriously, however, total root length, root surface area and root volume of Bnlrn1 increased significant at low P, which indicated that the root growth of Bnlrn1 was more sensitive to low P than WT. There was no significant difference in total root length, root surface area and root volume between Bnlrn1 and WT at low P.3. Difference of nutrients uptake and use efficiency at low nitrogen, low potassium and low boron between mutants and WTSolution culture was used to evaluate the nutrients use efficiency of Bnprl1, Bnlrn1 and WT at low nitrogen (N), low potassium (K) and low boron (B). At normal condition, both of the two mutants had higher shoot dry weight and N accumulation than WT; whereas, at low N, they did not perform better than WT. Bnlrn1 grew better than WT at low K, however, at low B, the shoot growth of Bnlrn1 was repressed significantly, indicating that Bnlrn1 was a boron inefficient line.