The role of aquaporins in the nitrogen induced hydraulic response of maize roots

Abstract

Water and nitrogen are essential for plant growth and development. In many plants, the soil nitrogen availability, often in the form of nitrate, influences the water uptake rate of the root system through changes to root hydraulic conductivity. This increase may be an evolutionary mechanism to increase nitrate transport to the roots via the flow of water through the soil in a process known as mass transport. A subset of the membrane intrinsic proteins, the aquaporins, are permeable to water and changes in their activity can lead to changes in hydraulic conductance and root water uptake. In maize, nitrate stimulates root water uptake through changes to root hydraulic conductivity in several inbred lines and varieties. However, the hydraulic response to nitrate of B73, the maize reference genome, is not known. Maize aquaporin research is largely based on the set of aquaporins described by Chaumont et al. (2001) despite a reference maize genome being available since 2009. Here, a search of the maize (Zea mays) inbred line B73 genome revealed 43 aquaporins split among four subfamilies and included 14 ZmPIPs (Plasma membrane Intrinsic Proteins), 15 ZmTIPs (Tonoplast Intrinsic Proteins), 11 ZmNIPs (Nodulin-26 like Intrinsic Proteins), and 3 ZmSIPs (Small and Basic Intrinsic Proteins). This set of aquaporins shows diversity from sequenced clones in other studies, including nucleotide and amino acid changes within isoforms. The B73 genome does not contain ZmPIP2;7, but includes several isoforms not previously described, including ZmPIP2;9 and ZmPIP3;1. In addition, the upgraded APGv4 genome contains several differences to the previous genome version, AGPv3. Aquaporin function has been often assessed through heterologous expression in Xenopus laevis oocytes. Many maize studies, along with plants in general, have examined the role and regulatory means of influencing water transport through the PIP subfamily of aquaporins. Here, cloning confirmed several sequence variations found in the B73 genome, and showed water conductance not only for ZmPIPs, but also high conductance through ZmTIP1;2b and ZmTIP2;1. Originally, this research sought to characterise the hydraulic response to both nitrate starvation and resupply across maize inbred lines, including in B73. Conductance measurements made on individual maize root segments of B73 and F44 did not reveal a standard hydraulic response to nitrate across multiple growing conditions (hydroponics and aeroponics). Large variation in hydraulic conductance was observed that was examined as a function of changing light conditions, N nutrition, or switching from hydroponic to aeroponic growth. It was concluded that much of the variation occurred between individual root segments. This variation was observed in where along the root axis xylem vessels become conducting, seen as the proportion of root with high axial hydraulic conductance. Variations in root hydraulic conductivity was still present, and may be due to the activity of aquaporins. The expression of the whole set of maize aquaporins was monitored at once under nitrate starvation along with genes for nitrogen uptake and assimilation using a real-time PCR gene expression system (OpenArray). This revealed the expression profile of root and shoot aquaporins, in the root, not previously studied in ZmTIPs, ZmNIPs or ZmSIPs. Under three day nitrogen starvation, both B73 and F44 had changes in gene expression, and each had root and shoot aquaporins significantly down regulated. The aim of this research was to examine the mechanism used in maize to change root hydraulic conductance in response to nitrogen. The research presented in this thesis solidifies the genetic basis of water transport through aquaporins, provides evidence that some are water permeable, and that gene expression of aquaporins is responsive to nitrate conditions. This suggests the change in maize root hydraulics is in part mediated by the activity of aquaporins.