Abstract
This study investigates the possible involvement of maize aquaporins which are regulated by arbuscular mycorrhizae (AM) in the transport in planta of ammonium and/or urea under well-watered and drought stress conditions. The study also aims to better understand the implication of the AM symbiosis in the uptake of urea and ammonium and its effect on plant physiology and performance under drought stress conditions. AM and non-AM maize plants were cultivated under three levels of urea or ammonium fertilization (0, 3 µM or 10 mM) and subjected or not to drought stress. Plant aquaporins and physiological responses to these treatments were analyzed. AM increased plant biomass in absence of N fertilization or under low urea/ ammonium fertilization, but no effect of the AM symbiosis was observed under high N supply. This effect was associated with reduced oxidative damage to lipids and increased N accumulation in plant tissues. High N fertilization with either ammonium or urea enhanced net photosynthesis (AN) and stomatal conductance (gs) in plants maintained under well-watered conditions, but 14 days after drought stress imposition these parameters declined in AM plants fertilized with high N doses. The aquaporin ZmTIP1;1 was up-regulated by both urea and ammonium and could be transporting these two N forms in planta. The differential regulation of ZmTIP4;1 and ZmPIP2;4 with urea fertilization and of ZmPIP2;4 with NH4+ supply suggests that these two aquaporins may also play a role in N mobilization in planta. At the same time, these aquaporins were also differentially regulated by the AM symbiosis, suggesting a possible role in the AM-mediated plant N homeostasis that deserves future studies.
Highlights
Nitrogen (N) is one of the most important macronutrients for plants, being required in significant amounts as is a constituent of nucleic acids, amino acids, proteins, lipids, co-enzymes, chlorophyll, phytohormones, and secondary metabolites [1]
Given the fact that the arbuscular mycorrhizae (AM) symbiosis regulates a wide number of plant aquaporins under drought stress [22] and that besides water, some of these aquaporins can transport ammonium or urea, it is plausible that these AM-regulated aquaporins may be involved in the plant mobilization of N forms, contributing in this way to the enhanced drought tolerance of AM plants
The experiment consisted of a randomized block design with (1) two inoculation treatments: non-inoculated plants (Non-AM) and plants inoculated with an arbuscular mycorrhizal fungus (AM plants); (2) two water regimes: WW plants and drought stress (DS) plants; and (3) two sources of nitrogen: urea and ammonium applied at three different concentrations: No N added, low concentration (3 μM)
Summary
Nitrogen (N) is one of the most important macronutrients for plants, being required in significant amounts as is a constituent of nucleic acids, amino acids, proteins, lipids, co-enzymes, chlorophyll, phytohormones, and secondary metabolites [1]. The availability of this nutrient is very variable in soils, and losses in chemical fixation and leaching lead to deficiency in many agricultural lands, which at the end limit plant growth. Plants absorb N from soils mainly as nitrate (NO3 − ). Nitrate is predominant in aerobic soils, while ammonium dominates in Plants 2020, 9, 148; doi:10.3390/plants9020148 www.mdpi.com/journal/plants. Plants 2020, 9, 148 acidic soils or flooded wetlands [2]. The augmentation of NH4 + deposition can be the consequence of human activity, such as agriculture, and the increased levels of this nutrient can be toxic to plants [3]
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