Alternative splicing (AS) functions as a key regulatory mechanism and increases transcriptome and proteome diversity. Recent genome-wide studies have substantially expanded our estimation of the frequency of AS in plants (Reddy et al., 2013; Staiger and Brown, 2013). However, the proportion of AS events that lead to increased proteome diversity in plants, rather than imperfect pre-mRNA processing, remains unsolved. Here, we report an analysis of AS events in presumably translated mRNAs, i.e., those associated with polysomes, in Arabidopsis thaliana. We found that 35% of AS events identified in total mRNAs occur in polysome-associated mRNAs, suggesting translation into proteins. Furthermore, most (81%) of these presumably translated alternative isoforms lead to diversified protein-coding capacity. By contrast, weakly translated, but not non-translated, alternative isoforms likely undergo nonsense-mediated mRNA decay (NMD). Sequence analyses identified structural features of transcripts and cis-elements that were associated with AS. The results suggest that AS in plants increases proteome complexity but shows clear differences from AS in animals.