The magnesium-specific uptake and translocation transporters ZmMGT10 and MGR6 are upregulated not only by magnesium deficiency but also by high potassium concentrations in maize

The antagonism between potassium (K⁺) and magnesium (Mg²⁺) is the primary cause of Mg²⁺ deficiency worldwide. Recent studies have demonstrated that the suppressive effect of K⁺ on Mg²⁺ uptake is significantly reduced as the K⁺/Mg²⁺ ratio increases, as both cations share non-specific cation channels. Concomitantly, the relative root/shoot translocation of Mg²⁺ increased. In contrast, there are indications that elevated tissue [K⁺] impedes the primary physiological functions of Mg²⁺. In this study on Zea mays L., the involvement of the only known specific Mg²⁺ uptake transporter ZmMGT10 and that of the translocation transporter MGR6 was examined. In a hydroponic setup, young maize plants were subjected to eight distinct K⁺/Mg²⁺ ratios. Relative RNA expression of the two transporters was examined. In a second experiment, the effect of elevated leaf [K⁺] on the physiological functions of Mg²⁺ was investigated, while uptake antagonism was avoided. The maize plants here were subjected to a sufficient Mg²⁺ supply and absolute deficiency under conditions of adequate and excess [K⁺]. The analysis included chlorophyll values, starch, and nutrient concentration. While ZmMGT10;1 was higher expressed due to K⁺-induced lower root [Mg⁺²], ZmMGT10;2 showed a higher expression at high K⁺ exposure, although this response was independent of root [Mg⁺²]. A similar response was also observed for MGR6. It was found that the physiological functions of Mg²⁺ were not affected by increased [K⁺] in the tissue. In conclusion, the higher uptake and the elevated expression of translocation transporters were identified as an adaptation strategy of maize plants to K⁺-induced Mg²⁺ deficiency.