- The growth of a plant’s root system is essential for its survival. Preliminary observations suggest that a protein complex called the exocyst may play a role in the development of the root system by facilitating the growth of roots and root hairs. Previous work discovered a protein that potentially interacts with the exocyst to affect root growth. The protein was named a NERD (New Enhancer of Root Dwarfism). The identity of the gene encoding this NERD is a mystery. The purposes of this project were to characterize the nerd mutant, to test the hypothesis that the NERD interacts with the exocyst and to determine if the nerd mutation might be identical to one of a number of known mutations affecting root growth.
Measurement of root growth rates demonstrates that when the nerd mutation is combined with a mutation in the exocyst component SEC8, there is a synergistic effect on root growth rate, supporting the hypothesis of a NERD-EXOCYST interaction. Lugol staining of starch granules in the root cap indicates that the nerd mutation by itself has little effect on the cell division leading to the formation of the columella cell layers. However, the nerd mutation in combination with an exocyst mutation sometimes results in a disorganized structure of the columella. A quantitative analysis of root hair morphology indicates that the double nerd sec8-6 mutant shows a more severe defect than either of the single mutations alone. Observations of root hair defects, and root growth on media containing different sugar concentrations, suggest that the nerd mutation is most similar to the gin2-1 mutant. This work lays the foundation for additional studies that will determine the identity of the NERD, illuminate the role of the exocyst in plant roots, and improve our understanding of the cellular mechanisms that control root and root hair growth.