Gene dosage effects and signatures of purifying selection in lateral organ boundaries domain (LBD) genes LBD1 and LBD18
2016
Bdeir, Roba | Busov, Victor | Yordanov, Yordan | Gailing, Oliver
Wood formation is an economically and environmentally important process and has played a significant role in the evolution of terrestrial plants. Despite its significance, the molecular underpinnings of the process are still poorly understood. We have previously shown that four lateral organ boundaries (LBD) transcription factors have important roles in the regulation of secondary (woody) growth with two (LBD1 and LBD4) involved in secondary phloem and ray cell development and two (LBD15 and LBD18) in secondary xylem formation. We studied gene copy number and variation in DNA and amino acid sequences of the four LBDs in a wide range of woody and herbaceous plant taxa with fully sequenced and annotated genomes. LBD1 showed the highest gene copy number across species, and gene copy number was strongly and significantly correlated with tangential ray width. The climbing vines, cucumber and grape, with wide multiseriate rays (>10 cells wide) showed the highest gene copy number. Because the growth habit of woody lianas like grape requires significant twisting and bending, it was suggested that the unlignified ray parenchyma cells likely facilitate stem flexibility and maintenance of xylem conductivity. We further demonstrate conservation of amino acids in LBD18 protein sequences specific to woody taxa downstream of the LBD domain. Neutrality tests showed evidence for strong purifying selection on these regions across various orders, indicating important functional roles in woody taxa. Additionally, structural modeling demonstrates that these regions have a significant impact on tertiary protein structure and thus are likely of significant functional importance.
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