A simple model of quintessential inflation with the modified exponential potential exp(-alpha phi) [A + (phi-phi_0)^2] is analyzed in the braneworld context. Considering reheating via instant preheating, it is shown that the evolution of the scalar f
ield phi from inflation to the present epoch is consistent with the observational constraints in a wide region of the parameter space. The model exhibits transient acceleration at late times for 0.96 < A alpha^2 < 1.26 and 271 < phi_0 alpha < 273, while permanent acceleration is obtained for 2.3 10^{-8} < A alpha^2 < 0.98 and 255 < phi_0 alpha < 273. The steep parameter alpha is constrained to be in the range 5.3 < alpha < 10.8.
