Ectopic expression of TAPETUM DETERMINANT1 affects ovule development in Arabidopsis

Plants have evolved to extensively employ leucine-rich repeat receptor-like kinases (LRR-RLKs), the largest family of RLKs, to control growth, development, and defense. In Arabidopsis thaliana, the EXCESS MICROSPOROCYTES1 (EMS1) LRR-RLK and its potential small protein ligand TAPETUM DETERMINANT1 (TPD1) are specifically required for anther cell differentiation; however, TPD1 and EMS1 orthologs also control megaspore mother cell proliferation in rice and maize ovules. Here, the molecular function of TPD1 was demonstrated during ovule development in Arabidopsis using a gain-of-function approach. In ovules, the EMS1 gene was primarily expressed in nucellus epidermis and chalaza, whereas the expression of TPD1 was weakly restricted to the distal end of integuments. Ectopic expression of TPD1 caused pleiotropic defects in ovule and seed development. RNA sequencing analysis showed that ectopic expression of TPD1 altered expression of auxin signaling genes and core cell-cycle genes during ovule development. Moreover, ectopic expression of TPD1 not only affected auxin response but also enhanced expression of cyclin genes CYCD3;3 and CYCA2;3 in ovules. Thus, these results provide insight into the molecular mechanism by which TPD1-EMS1 signaling controls plant development possibly via regulation of auxin signaling and cell-cycle genes.