Root lengthening is regarded as a strategy for rice to adapt to the low phosphorus level. Namely, by increasing the length and surface area of root, rice has an opportunity to enhance its phosphorus adsorption efficiency under low phosphorus conditions. The relationship between main root lengthening of rice seedlings under low phosphorus conditions with the endogenous hormone contents and expression of expansin genes were examined by using methods of the hormone content determination and transcriptome sequencing. The experimental results suggested that after low phosphorus treatment for 15 days, the main root length of Tongjing981 (TJ981) and Zhendao99 (ZD99) was considerably longer than that of the control. However, the main root length of Zhenghan6 (ZH6) was shorter compared to the control. It reveals that the main root length of different rice varieties had different response modes to low phosphorus treatment. After low phosphorus treatment, the IAA content of TJ981 and ZD99 had dramatically increased IAA contents compared to the control, while the IAA content in the root system of ZH6 evidently decreased compared to the control. This finding indicates that the response mode of the IAA content in the roots of three rice varieties for the test to low phosphorus treatment was consistent with that of the main root length. Meanwhile, low-concentrated (5 mg L −1 ) exogenous IAA markedly elongated the main root of TJ981. Consequently, a higher IAA content in rice roots than that of the control may result in lengthening the rice main root induced by low phosphorus treatment. Low phosphorus treatment induced the expression of multiple expansin genes in TJ981 and ZD99 roots; meanwhile, exogenous IAA treatment at various doses also promoted the expression of expansin genes. Hence, the expression of expansin genes in rice roots induced by low phosphorus treatment was related to the increased IAA content caused by low phosphorus stress. Moreover, low phosphorus treatment promoted the length of cells in the elongation zone of TJ981 compared with the control.

中文翻译：

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低磷诱导水稻主根伸长的机制

根系延长被认为是水稻适应低磷水平的一种策略。即，通过增加根的长度和表面积，水稻有机会在低磷条件下提高其对磷的吸附效率。采用激素含量测定和转录组测序方法研究了低磷条件下水稻幼苗主根延长与内源激素含量和扩展蛋白基因表达的关系。试验结果表明，低磷处理15 d后，桐井981（TJ981）和镇道99（ZD99）的主根长度明显长于对照。然而，与对照相比，Zhenghan6 (ZH6) 的主根长度较短。说明不同水稻品种的主根长对低磷处理有不同的响应模式。低磷处理后，TJ981和ZD99的IAA含量较对照显着增加，而ZH6根系IAA含量较对照明显降低。该结果表明，3个水稻品种的根系IAA含量对低磷处理的响应模式与主根长的响应模式一致。同时，低浓度（5 mg L -1 ）外源IAA显着拉长了TJ981的主根。因此，水稻根中IAA含量高于对照可能会导致低磷处理诱导的水稻主根延长。低磷处理诱导了TJ981和ZD99根中多种扩展蛋白基因的表达；同时，不同剂量的外源IAA处理也促进了扩展蛋白基因的表达。因此，低磷处理诱导水稻根系中扩展蛋白基因的表达与低磷胁迫引起的IAA含量增加有关。此外，与对照相比，低磷处理促进了TJ981伸长区细胞的长度。