Abstract
Phototropins are plasma membrane-associated photoreceptors of blue light and UV-A/B radiation. The Arabidopsis thaliana genome encodes two phototropins, PHOT1 and PHOT2, that mediate phototropism, chloroplast positioning, and stomatal opening. They are well characterized in terms of photomorphogenetic processes, but so far, little was known about their involvement in photosynthesis, oxidative stress responses, and cell death. By analyzing phot1, phot2 single, and phot1phot2 double mutants, we demonstrated that both phototropins influence the photochemical and non-photochemical reactions, photosynthetic pigments composition, stomata conductance, and water-use efficiency. After oxidative stress caused by UV-C treatment, phot1 and phot2 single and double mutants showed a significantly reduced accumulation of H2O2 and more efficient photosynthetic electron transport compared to the wild type. However, all phot mutants exhibited higher levels of cell death four days after UV-C treatment, as well as deregulated gene expression. Taken together, our results reveal that on the one hand, both phot1 and phot2 contribute to the inhibition of UV-C-induced foliar cell death, but on the other hand, they also contribute to the maintenance of foliar H2O2 levels and optimal intensity of photochemical reactions and non-photochemical quenching after an exposure to UV-C stress. Our data indicate a novel role for phototropins in the condition-dependent optimization of photosynthesis, growth, and water-use efficiency as well as oxidative stress and cell death response after UV-C exposure.
Highlights
The optimization of light absorption to variable natural conditions is essential to balance photochemistry and photosynthesis reaction rates, and for acclimatory and defense responses in plants
The phot2 and phot1phot2 mutants demonstrated a significantly higher water-use efficiency (WUE) parameter (Figure 1E), which was measured as dry weight per water used. These results indicate that phot2 together with phot1 has a positive impact on plant biomass production and stomatal density, and influences WUE
ascorbate peroxidase (APX) activity dropped severely in the phot2 mutant 48 h after UV-C irradiation, reaching, together with the phot1phot2 double mutant, a lower level than in the wild type. All these results indicate that phototropins affect foliar H2 O2 content as well as the fine-tuning of the superoxide dismutase (SOD), CAT, and APX activities after oxidative stress
Summary
The optimization of light absorption to variable natural conditions is essential to balance photochemistry and photosynthesis reaction rates, and for acclimatory and defense responses in plants. Plants have evolved several avoidance and dissipation mechanisms that protect the photosynthetic apparatus against EEE and optimize photosynthetic reactions [4,5,6,7] These mechanisms include the chloroplast photorelocation movement, photochemical (qP) and non-photochemical quenching (NPQ), and state transitions [4,5,6,8,9,10]. Arabidopsis possesses two phototropins, phot and phot, that share functional redundancy in processes such as chloroplast accumulation and phototropism control [14], stomata opening [15,16], leaf expansion [17], and leaf positioning [18]. Phot is involved in chloroplast avoidance responses [13,22] and light-dependent nuclear positioning [23]
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