Arabinokinase Limits the Flux of Arabinose Into Nucleotide Sugars to Prevent Toxicity.

Arabinokinase (ARA1) is a key player in the recycling pathway of the major cell wall component L-arabinose (L-Ara). The enzyme catalyzes phosphorylation of L-Ara to L-arabinose-1-phosphate, which is then converted into UDP-L-arabinopyranose (UDP-L-Ara) by UDP-sugar pyrophosphorylase (USP) followed by conversion into UDP-L-arabinofuranose (UDP-L-Ara) by UDP-arabinopyranose mutases (UAM) before it is incorporated into cell wall polymers. While this pathway is typically nonessential for plant development, a threefold accumulation of UDP-L-Ara can lead to toxicity. To investigate this, we generated lines overexpressing the kinase domain of ARA1 (ARAK1-OE) and examined their response to L-Ara feeding. ARAK1-OE seedlings revealed dose-dependent root growth retardation and cell death. The presence of 3 mM L-Ara resulted in an eightfold increase in UDP-L-Ara levels compared with nonfeeding conditions. Interestingly, wildtype seedlings showed no visible phenotype regardless of available L-Ara and despite the increase in UDP-L-Ara, suggesting a critical threshold for the observed phenotype. Cell walls of ARAK1-OE revealed a stronger attachment of arabinogalactan proteins (AGPs). Gene expression analysis from seedlings grown on 3 mM L-Ara implied that accumulation of UDP-L-Ara in ARAK1-OE triggers cell death resembling pathogen-induced hypersensitive responses. Overall, our findings demonstrate that modest increases in UDP-L-Ara levels can lead to significant phenotypic effects, including programmed cell death. This study highlights the role of arabinokinase in regulating L-Ara flux into nucleotide sugars, preventing arabinose-induced toxicity, and offers novel insights into the regulatory function of arabinokinase in cell wall biosynthesis and plant stress responses.