Most bacteria have a peptidoglycan-rich cell wall. Peptidoglycans are polysaccharide chains (amino sugars) cross-linked by small peptides. Cell walls in prokaryotes (e.g., bacteria) define the cell's shape and protects them from osmotic rupture in watery environmnets. However their features do not help in multicellularity.
Cell walls of plants on the other hand are mainly made up of cellulose. Here a small number of distinct chemical polymers are tightly woven into a meshwork with specific 3D architectural organization, allowing the respective cell walls to perform various mechanical functions (like providing tensile strength to the plant body and formation of a physical barrier to harsh biotic and environmental insults) and biochemical functions (like reorganization of cell wall components and possibly signal transduction in response to pathogen attack, environmental stresses, and during different developmental stages). These characteristics of cell walls allow the plants to grow to remarkable heights, to avoid predation, to minimize water loss, and to function and reproduce successfully in very diverse habitats.
Thus we can see that though plants and bacteria both possess cell walls, the difference in their cell walls bring about different body designs in them.