The plasma membrane aquaporin NtAQP1 is a key component of the leaf unfolding mechanism in tobacco

Epinastic leaf movement of tobacco is based on differential growth of the upper and lower leaf surface and is distinct from the motor organ-driven mechanism of nyctinastic leaf movement of, for example, mimosa species. The epinastic leaf movement of tobacco is observed not only under diurnal light regimes but also in continuous light, indicating a control by light and the circadian clock. As the transport of water across membranes by aquaporins is an important component of rapid plant cell elongation, the role of the tobacco aquaporin Nt aquaporin (AQP)1 in the epinastic response was studied in detail. In planta NtAQP1-luciferase (LUC) activity studies, Northern and Western blot analyses demonstrated a diurnal and circadian oscillation in the expression of this plasma membrane intrinsic protein (PIP)1-type aquaporin in leaf petioles, exhibiting peaks of expression coinciding with leaf unfolding. Cellular water permeability of protoplasts isolated from leaf petioles was found to be high in the morning, i.e. during the unfolding reaction, and low in the evening. Moreover, diurnal epinastic leaf movement was shown to be reduced in transgenic tobacco lines with an impaired expression of NtAQP1. It is concluded that the cyclic expression of PIP1-aquaporin represents an important component of the leaf movement mechanism

The tobacco plasma membrane aquaporin NtAQP1.

This paper gives a summary of a project to characterize a tobacco aquaporin. The cDNA and gene, including the 5' upstream region, for the tobacco aquaporin NtAQP1 has been isolated and the encoded protein characterized. The significance of promoter regions for an abscisic acid- and gibberellic acid-induced gene expression could be restricted to a region between -1450 and -1112 upstream of the transcription start point by transient transformation of a bicistronic vector into tobacco protoplasts. NtAQP1 expression in tobacco plants was found to be elevated in flowers, stems and roots. In roots, the protein was detected close to xylem vessels in pitch-like structures. Studies with a NtAQP1-GFP fusion indicated a plasma membrane location. For a functional analysis, the cDNA was expressed in Xenopus oocytes. NtAQP1 was found to be a heavy metal-insensitive aquaporin with additional permeability for glycerol. Mutation of a threonine at position 233 to a cysteine transformed NtAQP1 into a heavy metal-sensitive aquaporin

The plasma membrane aquaporin NtAQP1 is a key component of the leaf unfolding mechanism in tobacco

Epinastic leaf movement of tobacco is based on differential growth of the upper and lower leaf surface and is distinct from the motor organ-driven mechanism of nyctinastic leaf movement of, for example, mimosa species. The epinastic leaf movement of tobacco is observed not only under diurnal light regimes but also in continuous light, indicating a control by light and the circadian clock. As the transport of water across membranes by aquaporins is an important component of rapid plant cell elongation, the role of the tobacco aquaporin Nt aquaporin (AQP)1 in the epinastic response was studied in detail. In planta NtAQP1-luciferase (LUC) activity studies, Northern and Western blot analyses demonstrated a diurnal and circadian oscillation in the expression of this plasma membrane intrinsic protein (PIP)1-type aquaporin in leaf petioles, exhibiting peaks of expression coinciding with leaf unfolding. Cellular water permeability of protoplasts isolated from leaf petioles was found to be high in the morning, i.e. during the unfolding reaction, and low in the evening. Moreover, diurnal epinastic leaf movement was shown to be reduced in transgenic tobacco lines with an impaired expression of NtAQP1. It is concluded that the cyclic expression of PIP1-aquaporin represents an important component of the leaf movement mechanism

Aquaporins and plant water balance.

The impact of aquaporin function on plant water balance is discussed. The significance of these proteins for root water uptake, water conductance in the xylem, including embolism refilling and the role of plant aquaporins in leaf physiology, is described. Emphasis is placed on certain aspects of water stress reactions and the correlation of aquaporins to abscisic acid as well as on the relation of water and CO2 permeability in leaves