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Background  The  INNER NO OUTER  ( INO ) gene is expressed in the outermost cell layer of the outer integument of bitegmic ovules and is essential for this organ’s growth. The role and cross-species functional conservation of  INO  orthologs were examined in members of the Solanaceae, which have unitegmic ovules. Unitegmy has evolved several times in disparate angiosperm lineages.  INO  expression has been observed in the outermost cell layers of all examined unitegmic ovules, but the functional role of  INO  in unitegmic ovules has not previously been evaluated.    Results   INO  orthologs were unambiguously identified in tobacco and tomato by sequence homology. Expression of the tomato  INO  gene was limited to the outer cell layer of the single integument indicating that this single integument has properties of the outer integument. Expression occurred only after integument initiation, later than observed in ovules of other examined angiosperms. Virus-induced knock-down of expression of the  INO  ortholog in tobacco inhibited growth of the outer cell layer of the integument leading to a decrease in both integument extension and curvature of the ovule. The altered ovules closely resemble those of the  aberrant testa shape  ( ats )  ino  mutant combination in Arabidopsis where we see the effect of the  ino  mutation on a single fused integument produced by the  ats  mutation. Despite significant sequence identity and similar expression patterns, the tomato  INO  coding region was not able to complement the Arabidopsis  ino  mutant.    Conclusions  The similarity of effects of  ino  mutations on the unitegmic ovules of tobacco and the fused integuments of the Arabidopsis  ats  mutant show that: 1)  INO  orthologs play the same role in promoting integument growth in ovules of tobacco and Arabidopsis; and 2) the unitegmic ovules of tobacco (and hence other solanaceous species) are most likely the result of a congenital fusion of two ancestral integuments. Our results further indicate that INO has a conserved role in growth of the outermost cell layer of integuments. The curvature of solanaceous ovules is driven by unequal growth of the outer layers of the single integument that likely correspond to an ancestral outer integument.    Electronic supplementary material  The online version of this article (doi:10.1186/s12870-016-0835-z) contains supplementary material, which is available to authorized users.

Conservation of the role of INNER NO OUTER in development of unitegmic ovules of the Solanaceae despite a divergence in protein function