Importance of KUP8 for K+ uptake in rooted plantlets of Elaeis guineensis under K+ sufficient conditions

Potassium is a major nutrient essential for plant growth and development. Acquisition of this vital element and maintenance of K+ homeostasis are complex processes, facilitated by an array of membrane transporters including carriers and channels. Key mediators of K+ uptake are the KT/KUP/HAK family of transporters. The oil palm (Elaeis guineensis), is an agriculturally important crop, but the molecular mechanisms of nutrient acquisition in this plant are poorly understood. Here we report the full sequences of three KUP transporters, EgKUP3, EgKUP8 and EgKUP11, from oil palm, obtained by a combination of database searching, PCR and 5′ and 3′ RACE techniques. Gene expression analysis of these three transporters was conducted on oil palm rooted plantlets at 7, 14, and 21 days after treatment with a range of KNO3 concentrations: 0.2 mM, 10 mM and 20 mM of in root and leaf tissues. The results indicated EgKUP8 expression is significantly upregulated in root tissues under K+ depleted conditions (0.2 mM) at 14 and 21 days. In contrast, the expression of EgKUP3 and EgKUP11 was not sensitive to changes in external K+ concentration. Functional complementation using Escherichia coli knockout strain defective in K+ uptake systems revealed that all EgKUPs complemented growth at 50 mM K+ concentration while EgKUP8 was also able to complement growth at 5 mM K+ when tested at pH 7.5. In contrast, none of the EgKUPs were able to complement growth at pH 5.5 under all external K+ concentrations tested. These observations may suggest that although EgKUP8 is less efficient at <5 mM K+ and low pH condition (pH 5.5), at least, when expressed heterologously in E. coli, the escalated gene expression observed in-planta under similar environmental condition may be indicative of a more complex cellular role of EgKUP8 in K+ uptake in plants. This piece of work provides the first insights into the molecular mechanisms of mineral uptake in the oil palm and provides a number of tools for further research in this area.