Zinc deficiency up-regulates expression of high-affinity phosphate transporter genes in both phosphate-sufficient and -deficient barley roots

Abstract

Phosphate (P) is taken up by plants through high-affinity P transporter proteins embedded in the plasma membrane of certain cell types in plant roots. Expression of the genes that encode these transporters responds to the P status of the plants, and their transcription is normally tightly controlled. However, this tight control of P uptake is lost under Zn deficiency, leading to very high accumulation of P in plants. We examined the effect of plant Zn status on the expression of the genes encoding the HVPT1 and HVPT2 high-affinity P transporters in barley (Hordeum vulgare L. cv Weeah) roots. The results show that the expression of these genes is intimately linked to the Zn status of the plants. Zn deficiency induced the expression of genes encoding these P transporters in plants grown in either P-sufficient or -deficient conditions. Moreover, the role of Zn in the regulation of these genes is specific in that it cannot be replaced by manganese (a divalent cation similar to Zn). It appears that Zn plays a specific role in the signal transduction pathway responsible for the regulation of genes encoding high-affinity P transporters in plant roots. The significance of Zn involvement in the regulation of genes involved in P uptake is discussed.

Figure 1

Plant growth, Zn and P concentrations of shoots at two rates of Zn addition in a Zn-deficient soil. ses (n = 4) are shown as vertical bars. Zn 0 is nil Zn addition in the soil. Zn 0.36 is 0.36 mg Zn kg⁻¹ soil addition in the soil. Plants were harvested at D12, D16, and D23.

Figure 2

Accumulation of transcripts corresponding to genes encoding high-affinity P transporters in barley roots at two rates of Zn addition in a Zn-deficient soil. Zn treatments and harvest time are the same as those in Figure 1. A ³²P-labeled fragment of the HVPT1 cDNA that encodes a high-affinity P transporter expressed in barley roots was used as a hybridization probe. The panel labeled 2.0 kb is the hybridizing P transporter mRNA. The rRNA panel is the loading control, obtained by reprobing the stripped blot with [³²P]18S rDNA.

Figure 3

Plant growth, Mn and P concentrations of shoots at two rates of Mn, and three rates of P in a Mn-deficient soil. Plants were harvested after 28 d. ses (n = 4) are shown as vertical bars. Mn15P65 is plants grown at 15 mg Mn plus 65 mg P kg⁻¹ soil. Mn100P16 is plants grown at 100 mg Mn plus 16 mg P kg⁻¹ soil. Mn100P33 or Mn100P65 is plants grown at 100 mg Mn plus 33 mg P kg⁻¹ soil or 65 mg P kg⁻¹ soil.

Figure 4

Accumulation of transcripts corresponding to genes encoding high-affinity P transporters in barley roots grown at two rates of Mn and three rates of P addition in a Mn-deficient soil. Plants were harvested after 28 d. Mn and P treatments are the same as those in Figure 3. A ³²P-labeled fragment of the HVPT1 cDNA that encodes a high-affinity P transporter expressed in barley roots was used as a hybridization probe. The panel labeled 2.0 kb is the hybridizing P transporter mRNA. The rRNA panel is the loading control, obtained by reprobing the stripped blot with [³²P]18S rDNA.

Figure 5

Plant growth and Zn and P concentrations at three rates of ZnHEDTA in a chelate-buffered nutrient solution. ses (n = 4) are shown as vertical bars. Plants were harvested at D12 and D20. Nil ZnHEDTA, 1 μm ZnHEDTA, and 10 μm ZnHEDTA addition in the nutrient solution are shown as Zn 0, Zn 1, and Zn 10, respectively.

Figure 6

Accumulation of transcripts corresponding to genes encoding high-affinity P transporters in barley roots grown at three concentrations of ZnHEDTA in a chelate-buffered nutrient solution. Plants were harvested at D12 and D20. Zn treatments are the same as those in Figure 5. A, RNA gel-blot analysis. A ³²P-labeled fragment of the HVPT1 cDNA that encodes a high-affinity P transporter expressed in barley roots was used as a hybridization probe. The panel labeled 2.0 kb is the hybridizing P transporter mRNA. The rRNA panel is the loading control, obtained by reprobing the stripped blot with [³²P]18S rDNA. B, Relative signal density plot. The signal density of the high-affinity P transporters and 18S rRNA was obtained by using a phosphor imager, and relative density was given as percentage. ses (n = 3) are shown as vertical bars.