Transgenic tobacco plants that express an antisense construct derived froma Medicago sativa cDNA encoding a rac-related small GTP-binding protein fail to develop necrotic lesions upon elicitor infiltration

Using an RT-PCR approach a vac-related cDNA clone, designated Ms-rac1, was isolated from Medicago sativa (alfalfa). Ms-rac1 encodes a putative protein of 197 amino acids, which is closely related to known Rac-related GTP-bind ing proteins from Pisum sativum and Arabidopsis thaliana. RT-PCR analysis d emonstrated that Ms-rac1 is ubiquitously expressed in various tissues in al falfa. Expression of Ms-rac1 in suspension cultures occurred independently of treatment with elicitor, indicating that it is constitutively expressed. Heterologous expression of an antisense Ms-rac1 cDNA construct in transgen ic tobacco plants was associated with poor growth and retarded flowering. F ollowing infiltration with yeast elicitor, transgenic tobacco plants transf ormed with either the empty vector or Ms-rac1 in sense orientation develope d brown necrotic lesions and subsequently cell death was observed within th e infiltrated tissues. In contrast, the majority of the antisense transform ants neither formed necrotic lesions nor showed any other visible defence r eactions, demonstrating that Rac-related GTPases play an important role in the establishment of plant defence reactions.