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 en-
zyme catalyzes phosphorylation of L-Ara to L-arabinose-1-phosphate, which is then converted into UDP-L-arabinopyranose
(UDP-L-Arap) by UDP- sugar pyrophosphorylase (USP) followed by conversion into UDP-L-arabinofuranose (UDP-L-Araf )
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-Arap can lead to toxicity. To investigate this, we gen-
erated Arabidopsis thaliana 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-Arap levels compared with nonfeeding conditions. Interestingly, wildtype
seedlings showed no visible phenotype regardless of available L-Ara and despite the increase in UDP-L-Arap, 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 demon-
strate that modest increases in UDP-L-Arap 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