Omega-3 fatty acid desaturase (FAD3, FAD7, FAD8) gene expression and linolenic acid content in cowpea leaves submitted to drought and after rehydration

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English. Membrane polyunsaturated fatty acids (PUFA) and particularly linolenic acid (18:3, LA) are known to be implicated in plant tolerance to low temperature. Their role in resistance to drought is much less investigated. In this work, three full-length cDNAs corresponding to omega-3 fatty acid desaturases: fad3 (endoplasmic reticulum), fad7 and fad8 (chloroplastic) were isolated from Vigna unguiculata leaves. Two cowpea cultivars, one drought-tolerant, EPACE-1, and one drought-susceptible, 1183, were compared in terms of fad isoform gene expression and leaf LA contents in plants submitted to water stress followed by rehydration. In EPACE-1, LA content in the main leaf polar lipids increased in response to mild water deficit. Severe water deficits induced a decrease in MGDG LA content while those of PC and DGDG continued to increase. Variations in FAD gene expression, matched those in LA contents. In 1183, LA contents decreased in all lipid classes in response to water stress, as did FAD3 and FAD8 gene expression levels. Rehydration after a moderate water stress induced stimulation mostly in FAD3 gene expression in both cvs. LA contents were equivalent to control levels in EPACE-1. In 1183, they were back to control levels in PC shortly after rehydration but remained low in galactolipids. These results suggested that omega-3 FAD activities were involved in the increase in leaf membrane unsaturation, in the drought tolerant plants whereas the sensitive plants lost PUFAs in response to the treatment. The significance of this discrepancy between the two cvs. in terms of adaptation to drought is discussed.