Among Five Tomato Cultivars, the Responses to Low Iron (Fe) of the Main Fe-Uptake Proteins and Total Protein in Roots Correlate with Effects on Biomass and Chlorophyll

Nutrient-uptake proteins (NUPs) have the potential to serve as biomarkers for crop improvement, with most studies to date focusing on NUP gene sequence or mRNA levels. In this paper, we examined if protein levels of the main iron (Fe)-uptake proteins in roots scaled with sensitivity to Fe deficiency among five tomato (Solanum lycopersicum) cultivars grown at low or medium Fe (0.5 or 50 μM). Across cultivars, biomass, shoot Fe concentration, and leaf chlorophyll concentration ([chl]) were lower at low vs. medium Fe, but the concentrations of the main Fe-uptake proteins in roots [iron-regulated transporter (IRT), plasma-membrane H+-ATPase (H+-ATPase), and ferric reduction oxidase (FRO)] were generally higher at low vs. medium Fe (both per unit total root protein and per g). Among cultivars, the % change between controls and low Fe in [chl], a common biomarker for Fe deficiency, and fresh biomass were positively correlated (r = 0.78, p = 0.12). In contrast, the % decrease in plant mass at low (compared to medium) Fe was inversely correlated with the % increase per unit total protein in levels of IRT (r = −0.93, p = 0.02) and, to a lesser extent, FRO (r = −0.82, p = 0.09), but was not correlated with % change in levels of H+-ATPase (p = 0.34). However, sensitive cultivars exhibited decreases in root protein concentration per g root at low vs. medium Fe, while tolerant cultivars had higher root protein levels at low Fe, which resulted in similar levels of IRT per g root among cultivars. Thus, among tomato cultivars, while low Fe generally induced increases in the main Fe-uptake proteins in roots, these increases were not higher in cultivars tolerant of low Fe; instead, tolerance to low Fe among cultivars was related to the ability to increase the concentration of total protein in roots.