The roles of CEP-CEPR1 signalling in controlling root system architecture in Arabidopsis thaliana

Abstract

Plants utilise their roots systems to acquire nutrients and moisture, in order to sustain growth and yield. To coordinate root growth plants must integrate multiple local and systemic signals, including metabolite and peptide hormones. In this thesis, the C-TERMINALLY ENCODED PEPTIDE (CEP) peptide hormone family and their corresponding major receptor, CEP RECEPTOR 1, were studied in Arabidopsis thaliana. Lateral roots comprise the majority of the root system, however, the distinct mechanisms controlling their growth are poorly understood. Here, we identified novel roles for CEP-CEPR1 signalling in negatively controlling lateral root (LR) growth in response to photosynthetically-derived carbon. We demonstrated that cepr1 mutants had longer LRs in the presence of sucrose, which was due to a greater cell number in the LR meristem, and an increased length of mature LR cells. Transcriptomic analysis identified that sucrose upregulated CEP5-8 in two Arabidopsis ecotypes. In addition, genes with basally perturbed expression in cepr1-1 overlap with wild type sucrose-responsive genes significantly. We subsequently observed that exogenous CEP5 inhibited LR growth by reducing LR meristem size and mature cell length. This result is consistent with CEP-CEPR1 acting to curtail the extent of sucrose-dependent LR growth. Reciprocal grafting indicates LR growth inhibition requires CEPR1 activity in both local and systemic circuits. Our results reveal a new role for CEP-CEPR1 signalling in controlling LR growth in response to sucrose. To further observe CEPR1-dependent changes in LR development, we developed a soil-based rhizobox system. This allowed us to view RSA over time in mature plants, and demonstrated that CEPR1 controls root system width in soil-grown Arabidopsis. The cepr1 mutants had a narrower RSA, which results from a steeper LR gravitropic set-point angle (GSA). Consistently, the application of CEP3 increased GSA in wild type but not in the CEP receptor mutants. Grafting showed that CEPR1 control of LR GSA occurred via the shoot only, and was, therefore, at least partially separable from its control of LR growth. Auxin signalling is known to reduce LR GSA. Therefore, we examined if CEP-CEPR1 signalling controlled LR GSA via an intersection with auxin signalling. Both cepr1 mutants exhibited an increase in rootward auxin transport, suggesting that an alteration to auxin transport may be responsible for the steeper GSA in cepr1 mutants. Consistent with this hypothesis, the application of auxin to wild type shoots induced a steeper GSA and auxin transport inhibitors counteracted the steep GSA phenotype of the cepr1 mutant. Using a DII-VENUS auxin signalling reporter line, we showed that CEP3 treatment applied specifically to the shoots increased DII-VENUS fluorescence in roots within 2 hours. Cytokinin acts as an anti-gravitropic offset in the determination of LR GSA. To assess if cytokinin and CEP-CEPR1 signalling intersected, we treated mutants defective in cytokinin signalling with CEP3. Consistent with an intersection, we showed that specific cytokinin pathway mutants have similar LR GSA phenotypes to cepr1 mutants. Conversely, we also found that cepr1-3 is partially resistant to the effects of tZ on LR GSA, suggesting that cytokinin and CEP-CEPR1 signalling converge in the control of LR GSA. However, when grown in soil rhizoboxes cytokinin pathway mutants did not fully phenocopy cepr1-3, indicating cytokinin alone is not sufficient to cause the steep RSA of cepr1 mutants. A mutant defective in specific CEP-CEPR1 downstream targets, cepd1,2, had a wild type-like RSA in soil. These results suggest that other downstream targets contribute to the control of RSA by CEP-CEPR1 and cytokinin signalling. Collectively, these results indicate that CEP-CEPR1 signalling controls multiple facets of root growth and RSA in response to both nitrogen and carbon status. Show more