Mechanism of Low Phosphorus Inducing the Main Root Lengthening of Rice

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.