Analysis of codon usage bias of WRKY transcription factors in Helianthus annuus

The phenomenon of codon usage bias is known to exist in many genomes and is mainly determined by mutation and selection. Codon usage bias analysis is a suitable strategy for identifying the principal evolutionary driving forces in different organisms. Sunflower (Helianthus annuus L.) is an annual crop that is cultivated worldwide as ornamentals, food plants and for their valuable oil. The WRKY family genes in plants play a central role in diverse regulation and multiple stress responses. Evolutionary analysis of WRKY family genes of H. annuus can provide rich genetic information for developing hybridization resources of the genus Helianthus.

Bases composition analysis showed the average GC content of WRKY genes of H. annuus was 43.42%, and the average GC3 content was 39.60%, suggesting that WRKY gene family prefers A/T(U) ending codons. There were 29 codons with relative synonymous codon usage (RSCU) greater than 1 and 22 codons ending with A and U base. The effective number of codons (ENC) and codon adaptation index (CAI) in WRKY genes ranged from 43.47-61.00 and 0.14-0.26, suggesting that the codon bias was weak and WRKY genes expression level was low. Neutrality analysis found a significant correlation between GC12 and GC3. ENC-plot showed most genes on or close to the expected curve, suggesting that mutational bias played a major role in shaping codon usage. The Parity Rule 2 plot (PR2) analysis showed that the usage of AT and GC was disproportionate. A total of three codons were identified as the optimal codons.

Apart from natural selection effects, most of the genetic evolution in the H. annuus WRKY genome might be driven by mutation pressure. Our results provide a theoretical foundation for elaborating the genetic architecture and mechanisms of H. annuus and contributing to enrich H. annuus genetic resources.