Adequate phenylalanine synthesis mediated by G protein is critical for protection from UV radiation damage in young etiolated Arabidopsis thaliana seedlings

Abstract

Etiolated Arabidopsis thaliana seedlings, lacking a functional prephenate dehydratase1 gene (PD1), also lack the ability to synthesize phenylalanine (Phe) and, as a consequence, phenylpropanoid pigments. We find that low doses of ultraviolet (UV)-C (254 nm) are lethal and low doses of UV-B cause severe damage to etiolated pd1 mutants, but not to wild-type (wt) seedlings. Furthermore, exposure to UV-C is lethal to etiolated gcr1 (encoding a putative G proteincoupled receptor in Arabidopsis) mutants and gpa1 (encoding the sole G protein a subunit in Arabidopsis) mutants. Addition of Phe to growth media restores wt levels of UV resistance to pd1 mutants. The data indicate that the Arabidopsis G protein-signalling pathway is critical to providing protection from UV, and does so via the activation of PD1, resulting in the synthesis of Phe. Cotyledons of etiolated pd1 mutants have proplastids (compared with etioplasts in wt), less cuticular wax and fewer long-chain fatty acids. Phederived pigments do not collect in the epidermal cells of pd1 mutants when seedlings are treated with UV, particularly at the cotyledon tip. Addition of Phe to the growth media restores a wt phenotype to pd1 mutants.

The dehydratase ADT3 affects ROS homeostasis and cotyledon development

“…An additional transgenic line harboring the ADT3::ADT3-GFP fusion in the adt3-1 mutant background recapitulated the ADT3::ADT3-CFP expression pattern and showed complementation of all adt3 defects reported herein (Supplemental Fig. S1; see below) and elsewhere (Warpeha et al, 2008). A similar expression pattern of ADT3::ADT3-GFP was observed in the wildtype background, where a more intense GFP signal was observed compared with the mutant background.…”

“…Part of the acute sensitivity of adt3 seedlings to UV-C irradiation doses that are tolerated by the wild type (Warpeha et al, 2008) also could be explained by the observation that other enzymes involved in ROS removal are downregulated in adt3 (Table I; Supplemental Table S1). This could lead to a chronic imbalance between the production of ROS upon high-energy UV light treatment and the ROS-buffering capacity of adt3 seedlings.…”

“…The adt3 mutation has been shown previously to make etiolated seedlings vulnerable to UV irradiation, and the defect was proposed to be due to a decrease in the production of phenylpropanoids upon exposure to both blue light and UV light (Warpeha et al, 2006(Warpeha et al, , 2008. To elucidate the physiological basis of UV light sensitivity in relation to phenylpropanoids, we examined the accumulation of ROS in the epidermis of adt3-1 cotyledons before and after UV light treatment, as these compounds are produced in response to shortwavelength light (Wituszy nska and Karpi nski, 2013; Consentino et al, 2015).…”

“…Activation of ADT3 leads to an increase in Phe content and in the production of phenylpropanoids (Warpeha et al, 2006). Accordingly, loss of ADT3 results in an enhanced sensitivity to UV irradiation in etiolated seedlings due to the reduced synthesis of photoprotective compounds and UV light-scattering epicuticular waxes (Warpeha et al, 2008). However, the physiological and molecular bases of this phenotype and the function of ADT3 in the seed-to-seedling transition remain to be elucidated.…”

“…After emerging from the seed coat, the embryo pushes through the soil to reach the surface; at this time, it is more vulnerable to biotic and abiotic stresses (Raven et al, 2005), and underlying actors of this transition are relatively unstudied (Warpeha and Montgomery, 2016). Phe-derived compounds, the phenylpropanoids, play an important role in the first line of defense by contributing to the reinforcement of the external cuticle layer and by conferring UV light protection properties to epicuticular waxes (Steyn et al, 2002;Pollard et al, 2008); in addition, phenylpropanoids influence wax production in response to UV light exposure (Rozema et al, 2002;Pollard et al, 2008;Warpeha et al, 2008). The activity of the phenylpropanoid pathway provides an additional line of defense, as phenolic compounds take part in a nonenzymatic mechanism to efficiently scavenge reactive oxygen species (ROS), whose levels increase as a result of metabolic reactions and when plants initiate a stress response (Sharma et al, 2012;Agati et al, 2013).…”