Which of the following statements is false? A SHR movement out of the stele requires SCR. B The SCR:SHR phenotype requires SCR activity. C SHR acts in a non-cell-autonomous manner. D SHR is sufficient to specify endodermal identity.
The correct answer and explanation is:
Correct Answer: D) SHR is sufficient to specify endodermal identity.
Explanation:
The SHORT ROOT (SHR) and SCARECROW (SCR) genes are critical regulators of root patterning and endodermal specification in Arabidopsis thaliana. These genes are involved in cell signaling and the establishment of distinct tissue layers in the root.
Option A is true. SHR is a transcription factor that is produced in the stele and moves outward to the adjacent cell layer, specifically the endodermis. This movement is dependent on the presence of SCR. Without SCR, SHR movement is disrupted. SCR helps in retaining SHR in the nucleus of endodermal cells, enabling its proper function in those cells.
Option B is true. The double mutant phenotype (scr:shr) exhibits a severe loss of normal ground tissue patterning, indicating that both genes are required. SCR activity is essential for the proper functioning of SHR. Without SCR, SHR cannot activate downstream targets that define the endodermal cell layer. This supports the idea that SCR is necessary for the SHR:SCR phenotype.
Option C is true. SHR acts in a non-cell-autonomous manner, which means that it is expressed in one cell type (the stele) but exerts its effects in another (the endodermis). After being synthesized in the stele, SHR moves into adjacent endodermal cells where it partners with SCR to activate transcription of target genes required for endodermal differentiation.
Option D is false. While SHR is necessary for endodermal identity, it is not sufficient. SHR needs to interact with SCR and other factors to fully induce endodermal specification. Experimental evidence shows that expressing SHR alone in other tissues does not always lead to endodermal cell identity. Therefore, SHR requires additional co-factors such as SCR to activate the full genetic program needed for endodermal differentiation.
In summary, although SHR plays a critical role in defining endodermal cells, it is not sufficient on its own, making option D the false statement.