Postembryonic Seedling Lethality in the Sterol-Deficient Arabidopsiscyp51A2Mutant Is Partially Mediated by the Composite Action of Ethylene and Reactive Oxygen Species

Abstract

Seedling-lethal phenotypes of Arabidopsis (Arabidopsis thaliana) mutants that are defective in early steps in the sterol biosynthetic pathway are not rescued by the exogenous application of brassinosteroids. The detailed molecular and physiological mechanisms of seedling lethality have yet to be understood. Thus, to elucidate the underlying mechanism of lethality, we analyzed transcriptome and proteome profiles of the cyp51A2 mutant that is defective in sterol 14a-demethylation. Results revealed that the expression levels of genes involved in ethylene biosynthesis/signaling and detoxification of reactive oxygen species (ROS) increased in the mutant compared with the wild type and, thereby, that the endogenous ethylene level also increased in the mutant. Consistently, the seedling-lethal phenotype of the cyp51A2 mutant was partly attenuated by the inhibition of ethylene biosynthesis or signaling. However, photosynthesis-related genes including Rubisco large subunit, chlorophyll a/b-binding protein, and components of photosystems were transcriptionally and/or translationally downregulated in the mutant, accompanied by the transformation of chloroplasts into gerontoplasts and a reduction in both chlorophyll contents and photosynthetic activity. These characteristics observed in the cyp51A2 mutant resemble those of leaf senescence. Nitroblue tetrazolium staining data revealed that the mutant was under oxidative stress due to the accumulation of ROS, a key factor controlling both programmed cell death and ethylene production. Our results suggest that changes in membrane sterol contents and composition in the cyp51A2 mutant trigger the generation of ROS and ethylene and eventually induce premature seedling senescence.

Sterol Methyl Oxidases Affect Embryo Development via Auxin-Associated Mechanisms

“…Polar auxin transport (PAT) or auxin response was hindered in sterol biosynthetic mutants (Willemsen et al, 2003;Men et al, 2008;Carland et al, 2010;Pullen et al, 2010). Inhibition of ethylene biosynthesis or signaling partially rescued fk, hyd1, and cyp51A2 mutants (Kim et al, 2010;Pullen et al, 2010). Sterols also can regulate the generation of reactive oxygen species, which play important roles in plant growth and cell death (Posé et al, 2009;Kim et al, 2010).…”

“…Inhibition of ethylene biosynthesis or signaling partially rescued fk, hyd1, and cyp51A2 mutants (Kim et al, 2010;Pullen et al, 2010). Sterols also can regulate the generation of reactive oxygen species, which play important roles in plant growth and cell death (Posé et al, 2009;Kim et al, 2010). A specific plant sterol biosynthetic intermediate (SBI) was speculated to have signaling functions.…”

Suppressing Farnesyl Diphosphate Synthase Alters Chloroplast Development and Triggers Sterol-Dependent Induction of Jasmonate- and Fe-Related Responses

“…This, together with the marked reduction of bulk membrane sterols detected in these plants (Fig. 3, A and B) and the suggested possible connection between impaired sterol biosynthesis and altered chloroplast development (Babiychuk et al, 2008;Kim et al, 2010), prompted us to investigate whether sterol depletion is responsible for the transcriptional response to FPS silencing. To this end, we compared the mRNA levels of 39 genes representative of the main physiological responses observed in FPSsilenced plants (Table I) with those in wild-type plants treated with terbinafine (Tb), a specific inhibitor of squalene epoxidase (SQE; Ryder, 1992), and cvp1/smt3 mutant plants (Carland et al, 2010).…”

“…Sterols are integral components of plant cell membranes that are found predominantly in the plasma membrane and in a much lower amount in the tonoplast, ER, mitochondria (Hartmann, 1998;Horvath and Daum, 2013), and the outer membrane of chloroplasts (Moeller and Mudd, 1982;Hartmann-Bouillon and Benveniste, 1987;Lenucci et al, 2012). In addition to this key structural role, sterols also play pivotal roles in embryonic, vascular, and stomatal patterning (Jang et al, 2000;Carland et al, 2002;Qian et al, 2013), cell division, expansion, and polarity (He et al, 2003;Men et al, 2008), hormonal regulation (Souter et al, 2002;Kim et al, 2010), vacuole trafficking (Li et al, 2015), and cell wall formation (Schrick et al, 2012). Some recent reports also point toward a role for sterols in proper plastid development (Babiychuk et al, 2008;Kim et al, 2010;Gas-Pascual et al, 2015).…”

“…In addition to this key structural role, sterols also play pivotal roles in embryonic, vascular, and stomatal patterning (Jang et al, 2000;Carland et al, 2002;Qian et al, 2013), cell division, expansion, and polarity (He et al, 2003;Men et al, 2008), hormonal regulation (Souter et al, 2002;Kim et al, 2010), vacuole trafficking (Li et al, 2015), and cell wall formation (Schrick et al, 2012). Some recent reports also point toward a role for sterols in proper plastid development (Babiychuk et al, 2008;Kim et al, 2010;Gas-Pascual et al, 2015). As key components of cell membranes, sterols are dynamic modulators of their biophysical properties, so that changes in the composition of sterols affect membrane fluidity and permeability (Roche et al, 2008;Grosjean et al, 2015) and, therefore, modulate the activity of membranebound proteins (Carruthers and Melchoir, 1986;Cooke and Burden, 1990;Grandmougin-Ferjani et al, 1997) and the plant adaptive responses to different types of abiotic and biotic stress, including tolerance to thermal stress (Hugly et al, 1990;Beck et al, 2007;Senthil-Kumar et al, 2013), drought (Posé et al, 2009;Kumar et al, 2015), metal ions (Urbany et al, 2013;Wagatsuma et al, 2015), and hydrogen peroxide (Wang et al, 2012a), and to bacterial and fungal pathogens (Griebel and Zeier, 2010;Wang et al, 2012b;Kopischke et al, 2013).…”