Multiple High-Affinity K+ Transporters and ABC Transporters Involved in K+ Uptake/Transport in the Potassium-Hyperaccumulator Plant Phytolacca acinosa Roxb

Abstract

Potassium is an important essential element for plant growth and development. Long-term potassium deprivation can lead to a severe deficiency phenotype in plants. Interestingly, Phytolacca acinosa is a plant with an unusually high potassium content and can grow well and complete its lifecycle even in severely potassium deficient soil. In this study, we found that its stems and leaves were the main tissues for high potassium accumulation, and P. acinosa showed a strong ability of K⁺ absorption in roots and a large capability of potassium accumulation in shoots. Analysis of plant growth and physiological characteristics indicated that P. acinosa had an adaptability in a wide range of external potassium levels. To reveal the mechanism of K⁺ uptake and transport in the potassium-hyperaccumulator plant P. acinosa, K⁺ uptake-/transport-related genes were screened by transcriptome sequencing, and their expression profiles were compared between K⁺ starved plants and normal cultured plants. Eighteen members of HAK/KT/KUPs, ten members of AKTs, and one member of HKT were identified in P. acinosa. Among them, six HAKs, and two AKTs and PaHKT1 showed significantly different expression. These transporters might be coordinatively involved in K⁺ uptake/transport in P. acinosa and lead to high potassium accumulation in plant tissues. In addition, significantly changed expression of some ABC transporters indicated that ABC transporters might be important for K⁺ uptake and transport in P. acinosa under low K⁺ concentrations.

Keywords: ABC transporters; Phytolacca acinosa; high-affinity K+ transporters; potassium transport.

Figure 1

(A) The area percentage of available potassium (> 100 mg/ kg) in fields in the main regions of China; (B) Sample collection sites in Hunan Province; (C) Content ranges of potassium in different tissues of Phytolacca acinosa collected from fields; (D–F) Potassium accumulation in different tissues of P. acinosa under different K⁺ treatments. Data shown as means ± SD of three biological replicates (n = 30). Asterisks indicate a significant difference based on a Dunnett’s test. * significant difference at 5% level (P < 0.05); ** significant difference at 1% level (P < 0.01).

Figure 2

(A) The growth status of P. acinosa under different potassium treatments at vegetative growth stage. (B) Plant height of P. acinosa under different potassium treatments at vegetative growth stage. (C) Leaf area at vegetative growth stage under different potassium treatments. (D) Number of infructescences under different potassium treatments. (E) The growth status of P. acinosa under different potassium treatments at reproductive growth stage. (F) Growth status of infructescences under different potassium treatments. (G) Plant height of P. acinosa under different potassium treatments at reproductive growth stage. (H) Leaf area at reproductive growth stage under different potassium treatments. (I) Length of infructescences under different potassium treatments. (J) Chloroplast content under different potassium treatments. (K) Effect of potassium treatments on photosynthesis of P. acinosa. (L) The 1000-seed weight of P. acinosa under different potassium treatments. Data shown as means ± SD of three biological replicates (n = 30). Asterisks indicate a significant difference based on a Dunnett’s test. * significant difference at 5% level (P < 0.05); ** significant difference at 1% level (P < 0.01).

Figure 3

Global transcriptome analysis. (A) The density distribution map of gene expression. (B) The sequence length distribution of unigenes in P. acinosa under 0 and 3.0 mmol/L K⁺ treatment. (C) Classification of the genes in response to the K⁺ starvation treatment compared with the 3.0 mmol/L K⁺ treatment via Gene Ontology (GO) analysis.

Figure 4

Expression profiles of transporter families. The green means upregulation and the red means downregulation of gene expression at K⁺ deficiency treatment. The value of 0 represents no change in gene expression between K⁺ starvation treatment (TP02) and normal K⁺ treatment (TP01). The value represents fragments per kilobase million (FPKM).

Figure 5

Phylogenetic analysis and nomenclature of target gene. (A) Phylogenetic analysis of ABC transporter family. (B) Phylogenetic analysis of HKT family. (C) Phylogenetic analysis of AKT family. (D) Phylogenetic analysis of HAK/KT/KUP family.

Figure 6

Expression level of high-affinity K⁺ transporter genes in different tissues. The expression of a known gene PaHAK1 in root was normalized as value 1. Three biological replicates were performed in qRT-PCR (n = 3).

Figure 7

(A) Potassium starvation significantly regulated the expressions of ABC transporter genes. The values represent fragments per kilobase million (FPKM). The sample under 3.0 mmol/L K⁺ treatment (average K⁺ treatment) was represented by TP01, and the sample under K⁺ starvation was represented by TP02. (B) Low potassium induced the expression of high-affinity K⁺ transporter genes in different tissues. The value represents the ratio of gene expression in K⁺ deficiency vs. normal K⁺ level.

Figure 8

Model of K⁺ uptake/transport mediated by high-affinity K⁺ transporters.