The future optical transport networks will be affected by limited bandwidth of information infrastructure, high power consumption, and heterogeneity of network segments. As a solution to all these problems, the multidimensional signaling has been proposed recently. In this invited paper, we follow a different strategy. Instead of conventional binary and 2mary signaling (m≥1) we propose to use the nonbinary pm-ary signaling, where p is a prime larger than 2. With pm-ary signaling we can improve the spectral of conventional 2m-ary schemes by log2p times for the same bandwidth occupancy. At the same time the energy efficiency of pm-ary signaling scheme is much better than that of 2m-ary signaling scheme based on binary representation of data. We further study the energy-efficient coded modulation for pm-ary signaling. The energy-efficient signal constellation design for pm-ary signaling is discussed as well. We will demonstrate that with the proposed pm-ary signaling in combination with energy-efficient signal constellation design, spectral-multiplexing, and polarization-division multiplexing, we can achieve beyond 1 Pb/s serial optical transport without a need for introduction of spatial-division multiplexing.