ABA-Dependent and -Independent Regulation of Tocopherol (Vitamin E) Biosynthesis in Response to Abiotic Stress in Arabidopsis

Abstract

Abiotic stresses in plants lead to elevated levels of reactive oxygen species that can have detrimental effects on photosystems and thylakoid lipids. To limit damage, plants increase the production of antioxidants such as tocopherol (vitamin E). While tocopherol biosynthesis has been investigated in detail, only little is known about the regulation of this pathway. Previously, it has been suggested that the phytohormone abscisic acid (ABA), which is involved in abiotic stress responses, may be a key regulator of tocopherol biosynthesis.<br /> To decipher the regulatory network between ABA and tocopherol biosynthesis in Arabidopsis, several ABA-deficient (<em>aba</em>) and ABA-insensitive (<em>abi</em>) mutants were exposed to (i) osmotic stress simulated by PEG8000 containing medium, (ii) drought stress on soil, and (iii) nitrogen deprived medium. Under all stress conditions the tocopherol content in the mutants increased similarly as in the wild type, except for <em>abi4-1</em>, <em>abi4-102</em> and <em>abi2-1</em> under osmotic stress. However, tocopherol synthesis was still upregulated in several <em>aba</em> or <em>abi</em> mutants. Therefore, Arabidopsis wild type plants were exposed to two additional abiotic stresses, i.e. (iv) ABA treatment and (v) high light exposure. Under all five stresses, tocopherol was increased. Transcript analyses via qPCR supported these findings showing an up-regulation of tocopherol biosynthesis genes under all conditions. LC-MS measurements revealed that ABA content increased in response to ABA treatment, drought stress and osmotic stress, but not during high light and nitrogen deprivation. This indicates that an ABA-independent pathway for regulating tocopherol biosynthesis must exist. Treatment of <em>abi</em> mutants with exogenous ABA showed a less pronounced increase in tocopherol content for the <em>abi4-102</em> and <em>pyl1pyr2pyr4pyr5pyr8</em> mutants. The expression of tocopherol biosynthesis genes is also not altered in <em>pyl1pyr2pyr4pyr5pyr8</em> and especially in <em>abi4-102</em>. These results support the scenario that tocopherol synthesis is regulated via an ABA-dependent and an ABA-independent pathway, and that the ABA receptor proteins PYL1PYR2PYR4PYR5PYR8 and the AP2 domain transcription factor ABI4 play important roles during ABA dependent tocopherol synthesis.