Hox transcription factors play a conserved role in specifying positional identity during animal development, with posterior Hox genes typically repressing the expression of more anterior Hox genes. Here, we dissect the regulation of the posterior Hox genes nob-1 and php-3 in the nematode C. elegans. We show that nob-1 and php-3 are co-expressed in gastrulation-stage embryos in cells that previously expressed the anterior Hox gene ceh-13. This expression is controlled by several partially redundant transcriptional enhancers. These enhancers act in a ceh-13-dependant manner, providing a striking example of an anterior Hox gene positively regulating a posterior Hox gene. Several other regulators also act positively through nob-1/php-3 enhancers, including elt-1/GATA, ceh-20/ceh-40/Pbx, unc-62/Meis, pop-1/TCF, ceh-36/Otx, and unc-30/Pitx. We identified defects in both cell position and cell division patterns in ceh-13 and nob-1;php-3 mutants, suggesting that these factors regulate lineage identity in addition to positional identity. Together, our results highlight the complexity and flexibility of Hox gene regulation and function and the ability of developmental transcription factors to regulate different targets in different stages of development.

Hox transcription factors play a conserved role in specifying positional identity during animal development, with posterior Hox genes typically repressing the expression of more anterior Hox genes. Here, we dissect the regulation of the posterior Hox genes nob-1 and php-3 in the nematode C. elegans. We show that nob-1 and php-3 are co-expressed in gastrulation-stage embryos in cells that previously expressed the anterior Hox gene ceh-13. This expression is controlled by several partially redundant transcriptional enhancers. Surprisingly, these enhancers require ceh-13 for expression, providing an example of an anterior Hox gene positively regulating a posterior Hox gene. Several other regulators including elt-1/GATA, ceh-20/ceh-40/Pbx, unc-62/Meis, pop-1/TCF, ceh-36/Otx and unc-30/Pitx also act positively through nob-1/php-3 enhancers. We identified defects in both cell position and cell division patterns in ceh-13 and nob-1;php-3 mutants, suggesting that these factors regulate lineage identity in addition to positional identity. Taken together, our results highlight the complexity and unexpected flexibility of Hox gene regulation and function.

Plant growth is dependent on oriented cell divisions that employ the microtubular preprophase band (PPB) to position the cell plate. It has been intriguing how this transient cytoskeletal array imprints the spatial information to be read by the cytokinetic phragmoplast at later stages of mitotic cell division. InArabidopsis thaliana, we discovered that the PPB recruited the Myosin XI motor MYA1 to the cortical division site where it joined microtubule-associated proteins and motors to form a ring of prominent cytoskeletal assemblies which received the expanding phragmoplast. This regulatory function of MYA1 in phragmoplast guidance is dependent on intact actin filaments. The discovery of these assemblies revealed the mechanism underlying how two dynamic cytoskeletal networks govern PPB-dependent division plane orientation during vegetative growth in flowering plants.

No abstract