Cells in the root apical meristem of plants proliferate to increase root length before differentiating. Recent studies have shown that TARGET OF RAPAMYCIN (TOR) kinase regulates cell proliferation and differentiation in meristems, but the underlying mechanisms are not fully understood. Now, Benoît Menand, Marie-Hélène Montané and colleagues identify new components of the TOR pathway through a pharmacogenetic screen in Arabidopsis mutants using the TOR inhibitor AZD-8055. In wild-type plants, AZD-8055 reduces primary root length. However, the authors discover three mutant alleles encoding the kinase YAK1 that are resistant to AZD-8055. Interestingly, YAK1 is conserved in eukaryotes and treating plants with inhibitors for the mammalian orthologue of YAK1 (DYRK1) phenocopies YAK1 loss of function. The researchers use reporter lines to reveal that YAK1 regulates growth in the root meristem downstream of TOR. They show that in the absence of TOR activity, YAK1 induces expression of SIAMESE-RELATED (SMR) genes, which repress cyclin-dependent kinases (CDKs) and promote differentiation. Conversely, TOR kinase promotes growth by inhibiting YAK1, allowing CDKs and cyclin to maintain proliferation. Taken together, these results demonstrate that the TOR-YAK1 axis plays a crucial role in regulating root meristem activity, and provide new leads to help understand TOR functions in other developmental programmes.
