Objectives The aim of this study was to investigate the effects of auxin on the growth, development and potassium uptake of plant roots under low potassium stress,to provide a theoretical basis for increasing potassium levels in plants.

Methods Using indoor hydroponic method, model plant tobacco was used as the test material, and two potassium concentrations (5 mmol/L, 0.15 mmol/L) and five exogenous auxin concentrations were set (exogenous auxin was selected from 3-indole acetic acid, the concentration was set as 0, 5, 10, 20, 40 μmol/L), the physiological characteristics of plant roots, endogenous auxin concentration, potassium accumulation, potassium uptake kinetics and related potassium channel gene transcriptional expression were compared.

Results 1) Compared with the normal potassium level, the dry weight of the aboveground part of the plant decreased by 15.6% under low potassium stress, among the 8 indicators of root scan, except for the average root diameter, the other 7 indicators were significantly lower, the activity of ATPase decreased by 43.3%; the concentration of endogenous auxin in the main root tip, lateral root tip and leaf increased significantly, the Vmax and Km values of the parameters were reduced by 89.2% and 99.6% respectively, plant root and leaf potassium concentrations were reduced by 93.0% and 62.2% respectively, controlled potassium ion influx gene NtKC1 in roots was reduced by 56%. 2) After adding exogenous auxin, the root dry weight, root activity, main root tip and lateral root tip endogenous auxin concentration of the normal potassium level plant increased. The Vmax value and the expression of the relevant channel gene that regulated the influx of potassium ions were significantly increased. Under low potassium conditions, the plants showed similar regularity to normal potassium level. In addition, the root growth of low potassium plants was significantly improved. The ATPase activity and the aboveground and root potassium concentration increased significantly, while the expression level of the outflow-type potassium channel gene Ntork1 was significantly reduced. 3)When the concentration of auxin added was 10 μmol/L, compared with no auxin added, the dry weight of the aboveground part and root of the regular potassium level plants increased by 6.05% and 8.54%; the root volume and root overlap increased by 16.5% and 23.2%; the root activity increased by 298%, and the Vmax value increased by 118%. The dry weight of the aboveground part and root of the low potassium level plant was increased by 5.61% and 28.6% compared with no addition, and the root activity reached 113 μg/(g·h), FW, which was 3.3 times of that of no auxin. The root ATPase activity was increased by 87.5%. Root potassium concentration increased by 250% and potassium channel gene NKT2 in roots increased by 7.04 times and the expression of Ntork1 in roots and leaves was reduced by 49.5% and 72.5%.

Conclusions Low potassium stress affects plant root growth and plant uptake and accumulation of potassium. Adding appropriate concentration of exogenous auxin can improve the growth and development of plant roots, increase the expression of the influx potassium channel genes NKT2 and NtKC1, and reduce the expression of the outflow-type potassium channel gene Ntork. Moreover, the plant potassium kinetic parameter Vmax value is increased, which increases the absorption capacity and affinity of the plant for potassium ions, thereby increasing the potassium content of the plant.