A total of 27,443 unigenes were classified into 24 COG functional categories (Fig. 4). And the largest category was “general function prediction only” (6177, 22.5 %), followed by “signal transduction mechanisms” (2220, 8.1 %) and “replication, recombination and repair” (2196, 8.0 %). Unigenes annotated as the “signal transduction mechanisms” in our study may allow for the identification of novel genes involved in signal transduction pathways. Approximately 16.9 % of the unigenes were associated with biochemical synthesis and metabolism, such as “carbohydrate transport and metabolism” (1242, 4.5 %), “amino acid transport and metabolism” (1184, 4.3 %) and “secondary metabolites biosynthesis, transport and catabolism” (544, 2.0 %). A total of 1225 unigenes (4.46 %) was “function unknown”.Fig. 4COG annotations of putative proteins. All putative proteins were aligned to the COG database and were functionally classified into at least 24 molecular families. The capital letters in x-axis indicates the COG categories as listed on the right of the histogram and the y-axis indicates the number of unigenes in the corresponding COG category