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.

Root-specific CLE3 expression is required for WRKY33 activation in Arabidopsis shoots.

KEY MESSAGE: This study focused on the role of CLE1-7 peptides as defense mediators, and showed that root-expressed CLE3 functions as a systemic signal to regulate defense-related gene expression in shoots. In the natural environment, plants employ diverse signaling molecules including peptides to defend themselves against various pathogen attacks. In this study, we investigated whether CLAVATA3/EMBRYO SURROUNDING REGION-RELATED (CLE) genes (CLE1-7) respond to biotic stimuli. CLE3 showed significant up-regulation upon treatment with flg22, Pep2, and salicylic acid (SA). Quantitative real-time PCR (qRT-PCR) analysis revealed that CLE3 expression is regulated by the NON-EXPRESSOR OF PR GENES1 (NPR1)-dependent SA signaling and flg22-FLAGELLIN-SENSITIVE 2 (FLS2) signaling pathways. We demonstrated that SA-induced up-regulation of CLE3 in roots was required for activation of WRKY33, a gene involved in the regulation of systemic acquired resistance (SAR), in shoots, suggesting that CLE3 functions as a root-derived signal that regulates the expression of defense-related genes in shoots. Microarray analysis of transgenic Arabidopsis lines overexpressing CLE3 under the control of a ß-estradiol-inducible promoter revealed that root-confined CLE3 overexpression affected gene expression in both roots and shoots. Comparison of CLE2- and CLE3-induced genes indicated that CLE2 and CLE3 peptides target a few common but largely distinct downstream genes. These results suggest that root-derived CLE3 is involved in the regulation of systemic rather than local immune responses. Our study also sheds light on the potential role of CLE peptides in long-distance regulation of plant immunity.

CLE2 regulates light-dependent carbohydrate metabolism in Arabidopsis shoots.

KEY MESSAGE: This study focused on the role of CLE1-CLE7 peptides as environmental mediators and indicated that root-induced CLE2 functions systemically in light-dependent carbohydrate metabolism in shoots. Plants sense environmental stimuli and convert them into cellular signals, which are transmitted to distinct cells and tissues to induce adequate responses. Plant hormones and small secretory peptides often function as environmental stress mediators. In this study, we investigated whether CLAVATA3/EMBRYO SURROUNDING REGION-RELATED proteins, CLE1-CLE7, which share closely related CLE domains, mediate environmental stimuli in Arabidopsis thaliana. Expression analysis of CLE1-CLE7 revealed that these genes respond to different environmental stimuli, such as nitrogen deprivation, nitrogen replenishment, cold, salt, dark, and sugar starvation, in a sophisticated manner. To further investigate the function of CLE2, we generated transgenic Arabidopsis lines expressing the ß-glucuronidase gene under the control of the CLE2 promoter or expressing the CLE2 gene under the control of an estradiol-inducible promoter. We also generated cle2-1 and cle2-2 mutants using the CRISPR/Cas9 technology. In these transgenic lines, dark induced the expression of CLE2 in the root vasculature. Additionally, induction of CLE2 in roots induced the expression of various genes not only in roots but also in shoots, and genes related to light-dependent carbohydrate metabolism were particularly induced in shoots. In addition, cle2 mutant plants showed chlorosis when subjected to a shade treatment. These results suggest that root-induced CLE2 functions systemically in light-dependent carbohydrate metabolism in shoots.