GA influences the timing of the termination of the root apical meristem and changes in ground-tissue cytoarchitecture

Alice J. Paquette¹ and Philip N. Benfey²

¹New York University, New York, NY, ²Duke University, Durham, NC

Recent reports suggest that hormones such as auxin and ethylene regulate root growth by altering gibberellin (GA) signaling (Fu and Harberd, Nature 2003; Achard, et al., Plant Cell 2003). GA responses are repressed by the action of a set of putative transcription factors that make up the DELLA subfamily of GRAS proteins. Two other GRAS family members that regulate root growth are SCARECROW (SCR) and SHORT-ROOT (SHR). It has been reported that both of these proteins are required for the formation of two layers of ground tissue and for maintenance of the root apical meristem.

While SHR does, indeed, appear to be required for these two processes, SCR is not strictly essential for either. Analysis of new null alleles of scr indicate that, while mutant roots are agravitropic and elongate more slowly during the first week of growth; afterwards, scr-mutant roots attain almost wild-type growth rates and exhibit sporadic periclinal division of the ground tissue. This timing parallels a potential phase change in root growth, where the QC becomes two-tiered, and a third layer of ground tissue, the middle cortex, begins to form.

Levels of GA signaling influence the scr and shr mutant phenotypes. Increased GA signaling can enhance the phenotype of scr mutant roots to resemble that of shr mutants with respect to meristem termination and ground-tissue architecture. Conversely, reduced GA signaling appears to rescue the initial slow growth rate and ground-tissue defect of scr mutants. For shr, while increased GA signaling appears not to exacerbate the mutant phenotype, decreased GA signaling may rescue the meristem-maintenance defect. Reduction of GA signaling does not appear to enable periclinal division of the ground tissue, however, in the absence of SHR.