The increase or decrease in soil water potential caused by changes in elevation is called the Gravitational Water Potential component of the soil water potential. In general water moves from regions of higher to lower water potential not just gravitational potential. If the gravitational potential component is all that changes from place to place then water actually does run downhill. However, if other components of the water potential change appropriately water can, in fact, run uphill. For example, when a dry sponge is placed vertically in a dish of water, water will move uphill into the sponge.
Water moving out of the soil into the plant is also an example of water moving uphill.
An elevation is chosen arbitrarily where the gravitational potential is defined to be zero. The sign of the gravitational potential can be negative or positive. Water located below the specified reference elevation will have a negative gravitational potential. Although the choice of reference elevation is arbitrary, it must be kept constant during any set of calculations. The difference in gravitational potential from place to place in the soil-plant system is what is important rather than the absolute value of gravitational potential.