Whole-mount RNA in situ hybridization technique in Torenia ovules

The expression pattern of an interested gene at a cellular level provides strong evidence for its functions. RNA in situ hybridization has been proved to be a powerful tool in detecting the spatial-temporal expression pattern of a gene in various organisms. However, classical RNA in situ hybridization (ISH) technique is time-consuming and requires sophisticated sectioning skills. Therefore, we developed a method for whole-mount in situ hybridization (WISH) on ovules of Torenia fournieri, which is a model species in the study of plant reproduction. T. fournieri possesses ovules with protruding embryo sacs, making it easy to be observed and imaged through simple manipulation. To determine the effect of classical ISH and our newly established WISH, we detected the expression of a D-class gene, TfSTK3, using both methods. The expression patterns of TfSTK3 are similar in classical ISH and WISH, confirming reliability of the WISH method. Compared with WISH, classical ISH always leads to distorted embryo sacs, hence difficult to distinguish signals within the female gametophyte. To understand whether our WISH protocol also works well in detecting genes expressed within embryo sacs, we further examined the expression of a synergid-enriched candidate, TfPMEI1, and clearly observed specific signals within two synergid cells. To summarize, our WISH technique allows to visualize gene expression patterns in ovules of T. fournieri within one week and will benefit the field of plant reproduction in the future.