I used blast to identify the orthologous genes between plant and human, but the similarity is lower than 50%. My question is, is this reasonable and what is the general protein level similarity between plant and human orthologous genes?
Generally it is well established that if two sequences have > 20% similarity, their structures are often similar irrespective of species. Hence, their functions are implied to be similar in most cases. However, there are exceptions to this inference.
Following along from the above answers, a strategy like BLAST is really only going to find the most obvious orthologs when you're looking at such a large taxonomic distance. Even comparing human genes and yeast genes can be problematic. There have been a variety of approaches to defining orthologous genes (I tend to like eggNOG - http://eggnog.embl.de/version_3.0/ ) but it's still going to be very difficult to define a threshold at which protein sequence similarity defines true human vs. plant orthology.
So, the short answer to your question is still "it depends".
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