*Article* **The Arabidopsis RLCK VI\_A2 Kinase Controls Seedling and Plant Growth in Parallel with Gibberellin**

**Ildikó Valkai <sup>1</sup> , Erzsébet Kénesi <sup>1</sup> , Ildikó Domonkos <sup>1</sup> , Ferhan Ayaydin <sup>1</sup> , Danuše Tarkowská 2 , Miroslav Strnad <sup>2</sup> , Anikó Faragó 3,4, László Bodai <sup>3</sup> and Attila Fehér 1,5,\***


Received: 3 September 2020; Accepted: 29 September 2020; Published: 1 October 2020

**Abstract:** The plant-specific receptor-like cytoplasmic kinases (RLCKs) form a large, poorly characterized family. Members of the RLCK VI\_A class of dicots have a unique characteristic: their activity is regulated by Rho-of-plants (ROP) GTPases. The biological function of one of these kinases was investigated using a T-DNA insertion mutant and RNA interference. Loss of RLCK VI\_A2 function resulted in restricted cell expansion and seedling growth. Although these phenotypes could be rescued by exogenous gibberellin, the mutant did not exhibit lower levels of active gibberellins nor decreased gibberellin sensitivity. Transcriptome analysis confirmed that gibberellin is not the direct target of the kinase; its absence rather affected the metabolism and signalling of other hormones such as auxin. It is hypothesized that gibberellins and the RLCK VI\_A2 kinase act in parallel to regulate cell expansion and plant growth. Gene expression studies also indicated that the kinase might have an overlapping role with the transcription factor circuit (PIF4-BZR1-ARF6) controlling skotomorphogenesis-related hypocotyl/cotyledon elongation. Furthermore, the transcriptomic changes revealed that the loss of *RLCK VI\_A2* function alters cellular processes that are associated with cell membranes, take place at the cell periphery or in the apoplast, and are related to cellular transport and/or cell wall reorganisation.

**Keywords:** *Arabidopsis thaliana*; cell expansion; gibberellins; hypocotyl growth; transcriptomic analysis; plant hormones; plant size; receptor-like cytoplasmic kinase; skotomorphogenesis
