Conjugated polyamines and reproductive development: Biochemical, molecular and physiological approaches

Abstract

Whole tobacco plants containing the root‐inducing, left‐hand transferred DNA (Ri TL‐DNA) display a transformed phenotype, that includes alterations in a number of developmental processes, such as floral induction, flowering and reproduction. We show that the entire Ri TL‐DNA is responsible for repression of ornithine and tyrosine decarboxylases while it exerts no effect on transferase and the methyl transferase activities. Evidence is provided that two genes from the Ri TL‐DNA, rolA and rolC, alter polyamine metabolism as well as floral induction and flowering. Thus, plants transformed by the rolC gene (under the control of the 35S promoter from cauliflower mosaic virus) were male‐sterile (non‐viable pollen) and female fertility was reduced by approximatively 80%. A constitutive overexpression of the rolC gene may directly or indirectly cause inhibition of the accumulation of water‐insoluble amine conjugates located in the anthers and all the methyl transferases, leading to increases of ornithine decarboxylase, phenylalanine ammonia lyase and putrescine caffeoyl‐CoA transferase. The results suggest that male sterility is associated with catabolic processes exerted at the level of water‐insoluble amine conjugates and support the view that diamine oxidase may be involved in the regulation of the amine concentration during sexual differentiation, a factor that should be considered when attempting to decipher the mechanisms of control of sexual differentiation. The rolC gene could be useful in determining the role of diamine oxidase in the physiology of flowering. These results suggest that elevated free polyamine and water‐soluble polyamine levels (located in the ovaries) contribute to abnormal floral development. The transformed phenotype due to P35S‐rolA(the rolA gene fused to the 35S promoter) consisted of inhibited or delaved flowering, and altered floral morphology in the form of flower abortion. The effects of P35S‐rolA on flowering and fertility are closely correlated with limitations in the accumulation of the water‐soluble and ‐insoluble amine conjugates and increase in accumulation of free amines, indicating that amine conjugates (via transferases) have important functions in floral induction, floral evocation and reproduction. Spermidine availability as well as tyramine availability (in conjugated forms) could be limiting factor(s) in sexual development in tobacco.