STOP1 Regulates LKS1 Transcription and Coordinates K+/NH4+ Balance in Arabidopsis Response to Low-K+ Stress

Abstract

Potassium and nitrogen are essential mineral elements for plant growth and development. The protein kinase LKS1/CIPK23 is involved in both K+ and NH4+ uptake in Arabidopsis root. The transcripts of LKS1 can be induced by low K+ (0.1 mM) and high NH4+ (30 mM); however, the molecular mechanism is still unknown. In this study, we isolated the transcription factor STOP1 that positively regulates LKS1 transcription in Arabidopsis responses to both low-K+ and high-NH4+ stresses. STOP1 proteins can directly bind to the LKS1 promoter, promoting its transcription. The stop1 mutants displayed a leaf chlorosis phenotype similar to lks1 mutant when grown on low-K+ and high-NH4+ medium. On the other hand, STOP1 overexpressing plants exhibited a similar tolerant phenotype to LKS1 overexpressing plants. The transcript level of STOP1 was only upregulated by low K+ rather than high NH4+; however, the accumulation of STOP1 protein in the nucleus was required for the upregulation of LKS1 transcripts in both low-K+ and high-NH4+ responses. Our data demonstrate that STOP1 positively regulates LKS1 transcription under low-K+ and high-NH4+ conditions; therefore, LKS1 promotes K+ uptake and inhibits NH4+ uptake. The STOP1/LKS1 pathway plays crucial roles in K+ and NH4+ homeostasis, which coordinates potassium and nitrogen balance in plants in response to external fluctuating nutrient levels.

The transcription factors, STOP1 and TCP20, are required for root system architecture alterations in response to nitrate deficiency

“…Additionally, this pleiotropy for STOP1 regulation of gene networks can involve different transcription factor partners for STOP1, such as TCP20 for NRT1,1 under N deficiency and MED16 for ALMT1 expression under low P/high Fe ( 56 ). Moreover, STOP1/CIPK23 enhances root K + uptake by activating the high-affinity K + transporters, HAK and AKT1, to help plant acquire K + from low K soils ( 29 , 32 ). Thus, STOP1 may act as a hub to integrate various root nutrient and toxic metal stress responses, which allow plants to manage the appropriate root growth responses that modify RSA under complex environments.…”

“…Interestingly, STOP1 also may play important roles in the alteration of RSA for the better acquisition of P and K. Under P-deficient conditions in acid soils, where iron (Fe 3+ ) availability is often excessive, STOP1 turns on the expression of the AL-ACTIVATED-MALATE-TRANSPORTER1 ( ALMT1 ), first identified as a major Al tolerance gene, and the root released malate solubilizes Fe 3+ from the Arabidopsis root cell wall and facilitates its accumulation into cells of the root tip, where it is involved in inhibition of primary root growth under low P. This presumably alters the emphasis of the RSA to more efficient P uptake by the formation of more shallow LRs, where more of the phosphate resides ( 30 , 31 ). In addition, recent STOP1 studies have identified other pleiotropic roles for it in nutrient acquisition, including transcriptional regulation of CIPK23 , which in turn phosphorylates the high-affinity K + transporters, HIGH-AFFINITY-K + -TRANSPORTER5 (HAK5) and ARABIDOPSIS-K + -TRANSPORTER1 (AKT1), leading to enhancement of root potassium uptake ( 29 , 32 ). The STOP1/CIPK23 complex is also involved in ammonium uptake by activation of AMMONIUM-TRANSPORTER1 under phosphorus deficiency ( 33 ).…”

STOP1 and STOP1-like proteins, key transcription factors to cope with acid soil syndrome

“…STOP1 localizes to the nucleus and up-regulates the expression of many genes involved in low pH tolerance and Al resistance ( Sawaki et al., 2009 ). Recent studies revealed that STOP1 is essential for low-O 2 ( Enomoto et al., 2019 ), low-Pi ( Balzergue et al., 2017 ; Mora-Macias et al., 2017 ), low-K ( Wang et al., 2021 ), drought and salt tolerance ( Sadhukhan et al., 2019 ) in Arabidopsis. These findings suggest that STOP1 functions as a central factor in modulating the response to coexisting environmental stresses in acid soils.…”

“…AtSTOP1 modulates the transcription of AtHAK5 (High-affinity K + transporter 5) ( Sawaki et al., 2009 ; Nakano et al., 2020 ), AtSULTR3;5 (Sulfate transporter 3;5) ( Sawaki et al., 2009 ), H + -coupled high-affinity NO 3 - symporter gene AtNRT1.1 (Nitrate transporter 1.1) ( Fang et al., 2016 ; Ye et al., 2021 ) and AtCIPK23 (CBL-interacting protein kinase 23) ( Sawaki et al., 2009 ). AtCIPK23 additionally regulates the activity of AtHAK5 ( Ragel et al., 2015 ; Wang et al., 2021 ), AtAKT1 (Arabidopsis K + transporter 1) ( Li et al., 2006 ; Xu et al., 2006 ), AtNRT1.1 ( Liu and Tsay, 2003 ; Leran et al., 2015 ), and AtAMTs (Ammonium transporters) ( Straub et al., 2017 ; Wang et al., 2021 ) through phosphorylation to influence ion uptake, overcome rhizosphere acidification and establish a favorable cellular pH. Besides, AtSTOP1 promotes the expression of AtTDT (tonoplast dicarboxylate transporter) to increase the concentration of dicarboxylate and, hence, enhance the capacity to produce OH - to regulate the pH homeostasis in the cytosol ( Hurth et al., 2005 ).…”

“…AtSTOP1 contributes to K uptake by mediating the transcription of AtHAK5 , which encodes a high-affinity K + transporter ( Sawaki et al., 2009 ). In addition, the AtCIPK23, downstream of AtSTOP1, together with AtCBL1/9 (Calcineurin B-like), enhances K + uptake by phosphorylating the K + transporters AtHAK5 and AtAKT1 ( Li et al., 2006 ; Xu et al., 2006 ; Ragel et al., 2015 ; Wang et al., 2021 ). As a partner of AtCBL2/3, AtCIPK23 also regulates K homeostasis redundantly with AtCIPK3/9/26 through activating tonoplast AtTPK (Two-pore K + ) channels that promote K + remobilization, which plays a vital role in plant adaptation to K deficiency ( Tang et al., 2020 ).…”