Overexpression of X Intrinsic Protein 1;1 in and Arabidopsis reduces boron allocation to shoot sink tissues.

Major Intrinsic Proteins (MIP) are a family of channels facilitating the diffusion of water and/or small solutes across cellular membranes. X Intrinsic Proteins (XIP) form the least characterized MIP subfamily in vascular plants. XIPs are mostly impermeable to water but facilitate the diffusion of hydrogen peroxide, urea and boric acid when expressed in heterologous expression systems. However, their transport capabilities in planta and their impact on plant physiology are still unknown. Here, we demonstrated that overexpression of NtXIP1;1 in by the En2pPMA4 or the 35S CaMV promoter and in Arabidopsis, which does not contain any gene, by the 35S CaMV promoter, resulted in boron (B)-deficiency symptoms such as death of the shoot apical meristem, infertile flowers, and puckered leaves. Leaf B concentrations in symptomatic tissues and B xylem sap concentrations were lower in the overexpressors than in control plants. Importantly, expression of under the control of the promoter complemented the B deficiency phenotype of the knockout mutant, defining its ability to act as a boric acid channel . Protein quantification analysis revealed that NtXIP1;1 was predominantly expressed in young B-demanding tissues and induced under B-deficient conditions. Our results strongly suggest that NtXIP1;1 plays a role in B homeostasis and its tissue-specific expression critically contributes to the distribution of B within tobacco.