Arabidopsis thaliana RPL13aC affects root system architecture through shoot potassium accumulation

Plant root system architecture shows complex patterns adapting to different nutritional conditions. In Arabidopsis thaliana, root slanting is a behaviour that is observed when plants are grown on a solid agar plate vertically. However, the regulatory mechanisms of root slanting in response to nutrient conditions are not fully understood. In this study, we found that mutants of A. thaliana ribosome protein RPL13aC, which is expressed in root tips and leaves, exhibit a decreased root-slanting phenotype. Ionomic analysis revealed that rpl13ac mutants have a reduced K content in shoots but not in roots. Because K⁺ availability has been suggested to affect root coiling, we hypothesized that the decreased root slanting of rpl13ac mutants is caused by the decrease in K content in their shoots. Decapitating shoots or limiting K supply dramatically decreased root slanting in wild-type (WT) plants. We found that the expression of HIGH-AFFINITY K⁺ TRANSPORTER 5 (HAK5) significantly decreased in the roots of rpl13ac mutants. Mutants of hak5 showed decreased shoot K contents and decreased root slanting, supporting that the decreased shoot K⁺ accumulation results in less root slanting. K⁺ replenishment to the shoots of rpl13ac, hak5 mutants and K-starved WT plants recovered their root slanting significantly. These results indicate that plants adjust root slanting in response to K⁺ accumulation in shoots. Further analysis showed that rpl13ac mutants have abnormal thigmotropic responses, which may be responsible for their defects in root slanting. Altogether, these results revealed K⁺ -dependent mechanisms that affect root system architecture.