  We present the analysis of the fundamental plane (FP) for a sample of 19
massive red-sequence galaxies ($M_{\star} >4\times10^{10} M_{\odot}$) in 3
known overdensities at $1.39<z<1.61$ from the KMOS Cluster Survey, a guaranteed
time program with spectroscopy from the K-band Multi-Object Spectrograph (KMOS)
at the VLT and imaging from the Hubble Space Telescope. As expected, we find
that the FP zero-point in $B$ band evolves with redshift, from the value 0.443
of Coma to $-0.10\pm0.09$, $-0.19\pm0.05$, $-0.29\pm0.12$ for our clusters at
$z=1.39$, $z=1.46$, and $z=1.61$, respectively. For the most massive galaxies
($\log M_{\star}/M_{\odot}>11$) in our sample, we translate the FP zero-point
evolution into a mass-to-light-ratio $M/L$ evolution finding $\Delta \log
M/L_{B}=(-0.46\pm0.10)z$, $\Delta \log M/L_{B}=(-0.52\pm0.07)z$, to $\Delta
\log M/L_{B}=(-0.55\pm0.10)z$, respectively. We assess the potential
contribution of the galaxies structural and stellar velocity dispersion
evolution to the evolution of the FP zero-point and find it to be $\sim$6-35 %
of the FP zero-point evolution. The rate of $M/L$ evolution is consistent with
galaxies evolving passively. By using single stellar population models, we find
an average age of $2.33^{+0.86}_{-0.51}$ Gyr for the $\log
M_{\star}/M_{\odot}>11$ galaxies in our massive and virialized cluster at
$z=1.39$, $1.59^{+1.40}_{-0.62}$ Gyr in a massive but not virialized cluster at
$z=1.46$, and $1.20^{+1.03}_{-0.47}$ Gyr in a protocluster at $z=1.61$. After
accounting for the difference in the age of the Universe between redshifts, the
ages of the galaxies in the three overdensities are consistent within the
errors, with possibly a weak suggestion that galaxies in the most evolved
structure are older.
