A mutation in THREONINE SYNTHASE 1 uncouples proliferation and transition domains of the root apical meristem: experimental evidence and in silico proposed mechanism.

A continuum from stem to transit-amplifying to a differentiated cell state is a common theme in multicellular organisms. In the plant root apical meristem (RAM), transit-amplifying cells are organized into two domains: cells from the proliferation domain (PD) are displaced to the transition domain (TD), suggesting that both domains are necessarily coupled. Here, we show that in the Arabidopsis thaliana mto2-2 mutant, in which threonine (Thr) synthesis is affected, the RAM lacks the PD. Through a combination of cell length profile analysis, mathematical modeling and molecular markers, we establish that the PD and TD can be uncoupled. Remarkably, although the RAM of mto2-2 is represented solely by the TD, the known factors of RAM maintenance and auxin signaling are expressed in the mutant. Mathematical modeling predicts that the stem cell niche depends on Thr metabolism and that, when disturbed, the normal continuum of cell states becomes aborted.

Arabidopsis thaliana PrimPol is a primase and lesion bypass DNA polymerase with the biochemical characteristics to cope with DNA damage in the nucleus, mitochondria, and chloroplast.

PrimPol is a novel Primase-Polymerase that synthesizes RNA and DNA primers de novo and extents from these primers as a DNA polymerase. Animal PrimPol is involved in nuclear and mitochondrial DNA replication by virtue of its translesion DNA synthesis (TLS) and repriming activities. Here we report that the plant model Arabidopsis thaliana encodes a functional PrimPol (AtPrimPol). AtPrimPol is a low fidelity and a TLS polymerase capable to bypass DNA lesions, like thymine glycol and abasic sites, by incorporating directly across these lesions or by skipping them. AtPrimPol is also an efficient primase that preferentially recognizes the single-stranded 3'-GTCG-5' DNA sequence, where the 3'-G is cryptic. AtPrimPol is the first DNA polymerase that localizes in three cellular compartments: nucleus, mitochondria, and chloroplast. In vitro, AtPrimPol synthesizes primers that are extended by the plant organellar DNA polymerases and this reaction is regulated by organellar single-stranded binding proteins. Given the constant exposure of plants to endogenous and exogenous DNA-damaging agents and the enzymatic capabilities of lesion bypass and re-priming of AtPrimPol, we postulate a predominant role of this enzyme in avoiding replication fork collapse in all three plant genomes, both as a primase and as a TLS polymerase.