LeNRT1.1 Improves Nitrate Uptake in Grafted Tomato Plants under High Nitrogen Demand

Abstract

Grafting has become a common practice among tomato growers to obtain vigorous plants. These plants present a substantial increase in nitrogen (N) uptake from the root zone. However, the mechanisms involved in this higher uptake capacity have not been investigated. To elucidate whether the increase in N uptake in grafted tomato plants under high N demand conditions is related to the functioning of low- (high capacity) or high-affinity (low capacity) root plasma membrane transporters, a series of experiments were conducted. Plants grafted onto a vigorous rootstock, as well as ungrafted and homograft plants, were exposed to two radiation levels (400 and 800 µmol m^-2 s^-1). We assessed root plasma membrane nitrate transporters (LeNRT1.1, LeNRT1.2, LeNRT2.1, LeNRT2.2 and LeNRT2.3) expression, Michaelis‒Menten kinetics parameters (V_max and K_m), root and leaf nitrate reductase activity, and root respiration rates. The majority of nitrate uptake is mediated by LeNRT1.1 and LeNRT1.2 in grafted and ungrafted plants. Under high N demand conditions, vigorous rootstocks show similar levels of expression for LeNRT1.1 and LeNRT1.2, whereas ungrafted plants present a higher expression of LeNRT1.2. No differences in the uptake capacity (evaluated as V_max), root respiration rates, or root nitrate assimilation capacity were found among treatments.

Keywords: growth rate; nitrate transporters expression; root membrane transporters; root respiration.

Figure 1

Shoot (top) and root (bottom) relative growth rate (RGR) in each treatment under medium (400 µmol m⁻² s⁻¹) or high (800 µmol m⁻² s⁻¹) light levels during a 30-day growth period. Bars represent mean ± SE of eight replicates. p-Values for treatment (T), light (L) and the interaction (TxL) effects are included in each case. Different letters denote significant differences (p < 0.05) among treatments within each light level. Asterisks indicate the significance level (*: p < 0.05; **: p < 0.01; ***: p < 0.0001) for the differences in the same treatment across light levels. AT: Non-grafted treatment; AT-AT: Homograft; and, AT-KA: Graft treatment.

Figure 2

Total plant N uptake in each treatment under medium (400 µmol m⁻² s⁻¹) or high (800 µmol m⁻² s⁻¹) radiation during a 30-day period. Bars represent mean ± SE of eight replicates. p-Values for treatment (T), light (L) and the interaction (TxL) effects are included. Asterisks indicate the significance level (**: p < 0.01) for the differences in the same treatment across light levels. AT: Non-grafted treatment; AT-AT: Homograft; and, AT-KA: Graft treatment.

Figure 3

Relative expression of LeNRT1.1 (A), LeNRT1.2 (B), LeNRT2.1 (C), LeNRT2.2 (D), and LeNRT2.3 (E) in roots of plants from each grafting combination exposed to medium (400 µmol PAR m⁻² s⁻¹) or high light level (800 µmol PAR m⁻² s⁻¹). The reference gene corresponds to α-tubulin. Bars represent mean ± SE of four replicates. p-Values for treatment (T), light (L) and the interaction (TxL) effects are included in each case. Different letters denote significant differences (p < 0.05) among treatments within each light level. Asterisks indicate the significance level (*: p < 0.05; **: p < 0.01) for the differences in the same treatment across light levels. AT: Non-grafted treatment; AT-AT: Homograft; and, AT-KA: Graft treatment.

Figure 4

Nitrate Reductase Activity (NRA) in leaves from plants exposed to medium (400 µmol PAR m⁻² s⁻¹) or high (800 µmol PAR m⁻² s⁻¹) light level. Bars represent mean ± SE of four replicates. p-Values for treatment (T), light (L) and the interaction (TxL) effects are included. Asterisks indicate the significance level (**: p < 0.01) for the differences in the same treatment across light levels. AT: Non-grafted treatment; AT-AT: Homograft; and, AT-KA: Graft treatment.

Figure 5

Daily time-course of root respiration in plants from each treatment growing under medium (A) or high (B) light intensity. Measurements started one hour before the lights turned on until one hour after the lights turned off. Symbols represent mean ± SE of eight replicates. AT: Non-grafted treatment; AT-AT: Homograft; and, AT-KA: Graft treatment.

Figure 5

Daily time-course of root respiration in plants from each treatment growing under medium (A) or high (B) light intensity. Measurements started one hour before the lights turned on until one hour after the lights turned off. Symbols represent mean ± SE of eight replicates. AT: Non-grafted treatment; AT-AT: Homograft; and, AT-KA: Graft treatment.

Figure 6

Diagram of the system used to measure root respiration rates.