Regulatory roles of cytokinins and cytokinin signaling in response to potassium deficiency in Arabidopsis

Abstract

Potassium (K) is an important plant macronutrient that has various functions throughout the whole plant over its entire life span. Cytokinins (CKs) are known to regulate macronutrient homeostasis by controlling the expression of nitrate, phosphate and sulfate transporters. Although several studies have described how CKs signal deficiencies for some macronutrients, the roles of CKs in K signaling are poorly understood. CK content has been shown to decrease under K-starved conditions. Specifically, a CK-deficient mutant was more tolerant to low K than wild-type; however, a plant with an overaccumulation of CKs was more sensitive to low K. These results suggest that K deprivation alters CK metabolism, leading to a decrease in CK content. To investigate this phenomenon further, several Arabidopsis lines, including a CK-deficient mutant and CK receptor mutants, were analyzed in low K conditions using molecular, genetic and biochemical approaches. ROS accumulation and root hair growth in low K were also influenced by CKs. CK receptor mutants lost the responsiveness to K-deficient signaling, including ROS accumulation and root hair growth, but the CK-deficient mutant accumulated more ROS and exhibited up-regulated expression of HAK5, which is a high-affinity K uptake transporter gene that is rapidly induced by low K stress in ROS- and ethylene-dependent manner in response to low K. From these results, we conclude that a reduction in CK levels subsequently allows fast and effective stimulation of low K-induced ROS accumulation, root hair growth and HAK5 expression, leading to plant adaptation to low K conditions.

Figure 1. K deprivation reduces CK content.

Analysis of CK content in roots and shoots treated with K-sufficient (+K) or K-deficient (−K) conditions for one, three or seven days. (A) The content of tZ-type (tZ + tZR + tZRPs) CKs. (B) The content of iP-type (iP + iPR + iPRPs). White bar indicates CK content in K-sufficient grown plants and gray bar indicates CK content in K-deficient grown plants. Each error bar indicates standard error and * indicates the statistical difference between +K and −K (*P<0.05,**P<0.01; Student t-test) (n>6).

Figure 2. Root growth assay of Arabidopsis WT, IPT3-ox and ipt1,3,5,7 plants under +K and -K conditions.

Plants were grown under +K conditions for 4 days and then transferred and grown on +K or −K medium for 7 days. Length of primary root (A) and number of lateral roots (B) were analyzed (n>30). Different letters indicate significant differences from each other as determined using ANOVA(P<0.05) and significances were corrected post hoc using Tukey’s HSD comparisons.

Figure 3. Root growth assay of WT and ahk mutants under +K and −K conditions.

Plants were grown under +K conditions for 4 days and then transferred and grown on +K or −K medium for 7 days. Length of primary root (A) and number of lateral roots (B) were analyzed (n>30). Different letters indicate significant differences from each other as determined using ANOVA (P<0.05) and significances were corrected post hoc using Tukey’s HSD comparisons.

Figure 4. Low K-dependent ROS accumulation was obstructed in ahk2ahk3 mutant and enhanced in ipt1,3,5,7 mutant.

(A) Pseudo-colored ROS fluorescence signals were detected in WT, ahk2ahk3 and ipt1,3,5,7 roots after staining with 20 µM of DFFDA for 20 min. Bar indicates 0.5 mm. (B) Quantification of DFFDA fluorescence signal shown in Figure 4A. Pixel intensity of the roots was measured from the root hair differentiation zone to 0.5 mm. Different letters indicate significant differences from each other as determined using ANOVA (P<0.05) and significances were corrected post hoc using Tukey’s HSD comparisons. (n>20).

Figure 5. Root hair growth analysis in WT, ahk2ahk3 and ipt1,3,5,7 plants.

Root hair length (A) and number of root hairs (B) in the seedlings were analyzed. The number of root hairs was counted in a 3 mm region from the starting point of the RHDZ. In order to measure root hair length, the longest root hairs (n = 8) per seedling (n>10) were chosen and measured. Different letters indicate significant differences from each other as determined using ANOVA (P<0.05) and significances were corrected post hoc using Tukey’s HSD comparisons.

Figure 6. A schematic model for the roles of CKs in low K signal pathway.

Low K conditions lead to reduced levels of endogenous CKs, which are negative regulators under these conditions. Lower CK levels result in ROS production, HAK5 expression, and then altered root growth. Previous studies showed that ethylene is a positive regulator in low K signaling that controls ROS production, HAK5 expression as well as root hair growth . ABA and auxin also are involved in controlling low K signaling pathway. Arrowhead line, positive regulation; bar-head lines, negative regulation; gray color, previously identified; black color, identified in this study.