Arabidopsis expressing a cyanobacterial nitric oxide synthase increased yield even under uv-b exposure

Abstract

The enzyme nitric oxide synthase (NOS) catalyzes the biosynthesis of nitric oxide (NO) and citrulline from the substrate arginine. The NOS from the cyanobacteria Synechococcuss PCC 7335 (SyNOS) possesses the oxygenase and reductase domains like animal NOS, but it has an extra domain encoding a globin at the N terminus. In vitro assays demonstrated that the globin domain of SyNOS acts as a NO dioxygenase, oxidizing the NO to nitrate. As a result, SyNOS is able to produce NO from arginine and also to oxidize it to nitrate with a release rate 75:25 (nitrate:NO). Thus, our hypothesis is that SyNOS expression in plants may allow a greater remobilization of internal nitrogen (N), improving growth and yield. Furthermore, both nitrate and NO produced by SyNOS may positively affect the signaling of various growth processes and/or responses to stresses. Recently, we showed that heterologous expression of SyNOS in Arabidopsis improves N use efficiency, N-deficiency tolerance and yield. In this work, we evaluate the response of SyNOS-expressing Arabidopsis plants to UV-B exposure. Our results show that UV induces morphological changes in UV acclimated plants (exposed to UV-B 1.1 W.m-2, 2 h per day during 14 days) which include decreased rosette diameter, decreased inflorescence height, increased numbers of flowering stems and decreased numbers of secondary branches. These UV-induced morphological changes were observed in both transgenic SyNOS and Rdr-6 control plants. However, the transgenic lines presented increased shoot branching and seed production compared to Rdr-6 plants in both conditions (with and without UV-B exposure). Chronic UV-B radiation did not affect flavonoid pigment levels, cell damage or ROS production, indicating that UV-acclimated plants were not stressed. Additionally, the effect of prolonged UV-B exposure (irradiated with UV-B 0.5 W.m-2 during 6 days) was analyzed in seedlings grown in nutrient agar medium with high (+N, 9 mM NO3-) and low N (-N, 0.5 mM NO3-) conditions. UV-B irradiation as well as -N condition inhibited hypocotyl elongation in all lines. In -N condition without UV-B SyNOS lines elongated more the hypocotyl compared to control plants. Under UV-B exposure, increased hypocotyl elongation of transgenic lines was observed only in +N condition. Further investigation is necessary to understand the UV-B acclimation response of transgenic SyNOS lines. Acclimation to a state of stress combination has been shown to involve integrating responses to each of the individual stresses that simultaneously impact the plant (e.g., low N or UV-B stress), as well as the induction of a new type of response, sometimes involving thousands of transcripts, that is unique to the state of stress combination. In summary our results are encouraging towards obtaining crops with better yield under combined stressful conditions.