The characteristic that is always true for all members of the plant Kingdom is that they are autotrophic. This means they produce their own food through photosynthesis, using sunlight, water, and carbon dioxide.

Here's why this is a defining characteristic:

* Photosynthesis: Plants have chloroplasts, which contain chlorophyll, the pigment that absorbs sunlight energy to convert it into chemical energy in the form of sugars. This process is the foundation of the plant kingdom.

* Energy production: Plants use the sugars they produce to fuel their growth, development, and other life processes.

* Ecological importance: Plants are the base of most food chains, providing energy for all other organisms.

While plants may have other common features like having cell walls, being multicellular, and reproducing sexually, not all plants exhibit all of these traits. For example, some plants lack true roots, stems, or leaves. But autotrophy is a defining characteristic that sets plants apart from other kingdoms.