  The soft gluon limit of the longitudinal part of the quark-gluon vertex is
studied by resorting to non-perturbative approaches to Quantum Chromodynamics
(QCD). Based on a Slavnov-Taylor identity (STI), the longitudinal form factors
is expressed in terms of the quark-ghost kernel, the quark self energy and the
quark wave function. An exact relation between the non-vanishing longitudinal
form factors is derived for the soft gluon limit and explored to understand the
behaviour of the vertex. Within a Ball-Chiu vertex, the form factor $\lambda_1$
was analysed using recent lattice simulations for full QCD for the soft gluon
limit. The lattice data shows that the gluon propagator resumes the momentum
dependence of such component of the vertex. This connection is understood via a
fully dressed one-loop Bethe-Salpeter equation. The behaviour of the remaining
longitudinal form factors $\lambda_2(p^2)$ and $\lambda_3(p^2)$ is investigated
combining both the information of lattice simulations and the derived relations
based on the STI.
