WOX gene phylogeny in Poaceae: a comparative approach addressing leaf and embryo development

The phylogeny based on the homeodomain (HD)amino acid sequence of the WOX (WUSCHEL related homeobox)was established in the three major radiations of the Poaceae family: Pooideae (Brachypodium distachyon), Bambusoideae (Oryza sativa)and Panicoideae (Zea mays). The genomes of all three grasses contain an ancient duplication in the WOX3 branch and the cellular expression patterns in maize and rice indicate sub-functionalisation of paralogues during leaf development, which may relate to the architecture of the grass leaf and the encircling of the stem. The use of maize WOX gene family members as molecular markers in maize embryo development for the first time allowed us to visualize cellular decisions in the maize proembryo, including specification of the shoot/root axis at an oblique angle to the apical basal polarity of the zygote. All molecular marker data are compatible with the conclusion that the embryonic shoot/root axis comprises a discrete domain from early proembryo stages onwards. Novel cell fates of the shoot and the root are acquired within this distinct morphogenic axis domain, which elongates and thus separates the SAM and RAM anlagen in the maize embryo.