ORIGINAL ARTICLE Gene dosage effects and signatures of purifying selection in lateral organ boundaries domain (LBD) genes LBD1 and LBD18 Roba Bdeir 1 • Victor Busov 1 • Yordan Yordanov 1,2 • Oliver Gailing 1 Received: 9 April 2015 / Accepted: 11 December 2015 Ó Springer-Verlag Wien 2016 Abstract Wood formation is an economically and envi- ronmentally important process and has played a significant role in the evolution of terrestrial plants. Despite its signif- icance, 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. Addi- tionally, structural modeling demonstrates that these regions have a significant impact on tertiary protein structure and thus are likely of significant functional importance. Keywords Ray cells Á Secondary growth Á Vascular cambium Á Woody growth Introduction In woody plants, perennial secondary growth produces lignified secondary xylem (wood) and secondary phloem (bark), contributing significantly to the total terrestrial biomass (Mellerowicz et al. 2001). Wood is an important, natural renewable resource that can be used for generation of biofuels, structural wood and paper. In addition, wood extracts are used in production of adhesives and food additives (Kumar et al. 2002; Sedjo 2001). Wood is critical to tree survival because it provides mechanical support to their large stems. It also has important functions in trans- port and storage of water, carbohydrates and other mole- cules (Lough and Lucas 2006; Poorter et al. 2010). Wood structure and anatomy have significant adaptive signifi- cance. For example, the proportion, size and types of dif- ferent cells determine water conductance and resistance to drought and freezing temperatures (Cavender-Bares 2005). Secondary growth is initiated in the vascular cambium, which consists of unspecialized meristem cells that initiate a single bifacial (i.e., bidirectional) division to differentiate into xylem, phloem and ray mother cells (Mellerowicz et al. Handling editor: Gu ¨nter Theißen. Electronic supplementary material The online version of this article (doi:10.1007/s00606-015-1272-4) contains supplementary material, which is available to authorized users. & Oliver Gailing ogailing@mtu.edu 1 School of Forest Resources and Environmental Science, Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931, USA 2 Department of Biological Sciences, Eastern Illinois University, Charleston, IL 61920, USA 123 Plant Syst Evol DOI 10.1007/s00606-015-1272-4