Regulatory Roles of Cytokinins and Cytokinin Signaling in Response to Potassium Deficiency in Arabidopsis

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.钾（K）是一类关键的植物大量营养元素，在植物整个生命周期的植株各组织中发挥多种生理功能。细胞分裂素（Cytokinins, CKs）可通过调控硝酸盐、磷酸盐及硫酸盐转运蛋白的表达，维持植物大量营养元素的稳态。尽管已有多项研究阐明了细胞分裂素如何介导部分大量营养元素缺乏的信号通路，但细胞分裂素在钾信号转导中的作用仍未被充分解析。研究证实，在缺钾条件下，植物体内的细胞分裂素含量会显著降低。具体而言，细胞分裂素缺失突变体对低钾环境的耐受性显著高于野生型植株；而细胞分裂素过量积累的植株对低钾环境则更为敏感。上述结果表明，钾匮乏会改变细胞分裂素的代谢进程，进而导致其含量下降。为进一步探究这一现象，本研究采用分子、遗传与生化实验手段，对包括细胞分裂素缺失突变体与细胞分裂素受体突变体在内的多个拟南芥品系在低钾条件下的表型与分子特征展开了分析。此外，本研究还考察了细胞分裂素对低钾条件下活性氧（Reactive Oxygen Species, ROS）积累与根毛生长的调控作用。结果显示，细胞分裂素受体突变体丧失了对缺钾信号的响应能力，包括ROS积累与根毛生长过程；而细胞分裂素缺失突变体则积累了更多的ROS，且HAK5基因的表达量显著上调——HAK5是一种高亲和性钾吸收转运蛋白编码基因，在低钾胁迫下，该基因会以依赖于活性氧与乙烯的方式被快速诱导表达。综合上述实验结果，本研究得出结论：细胞分裂素水平的降低可随后快速且有效地激活低钾诱导的ROS积累、根毛生长及HAK5基因表达，最终帮助植物适应低钾环境。