Overexpression of Artemisia annua sterol C-4 methyl oxidase gene, AaSMO1, enhances total sterols and improves tolerance to dehydration stress in tobacco

Biosynthesis of sterols is a multistep process in higher plants where the precursor cycloartenol gets converted into functional phytosterols after removal of two methyl groups at C-4 by an enzyme complex involving a sterol C-4 methyl oxidase (SMO). We identified and cloned a cDNA from Artemisia annua designated as AaSMO1 showing similarity to SMO. The cDNA predicted to encode a polytopic protein with characteristic histidine-rich motifs and an ER retrieval signal. GFP-AaSMO1 fusion protein was localized in endoplasmic reticulum of transformed protoplast and onion epidermal cells. AaSMO1 expression was drastically induced upon osmotic/dehydration stress and its promoter showed the presence of abscisic acid responsive element. Transgenic tobacco plants ectopically overexpressing AaSMO1 were raised, and various biochemical and physiological analyses of transgenics revealed increased total sterol, better germination and growth in subsequent generations. They also exhibited reduced sensitivity towards osmotic/dehydration stress which may be attributed to enhanced SMO1 activity. Our studies demonstrated that apart from acting as phytohormones, plant sterols also participate in providing capability to plants for improved growth and adaptation during stress conditions. AaSMO1 can be used as an excellent candidate for generating dehydration/drought tolerant plants.