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Root hairs (RH) have emerged as an excellent model system for studying cell size regulation since they can elongate several hundred-fold their original dimensions. The rate at which cells grow is determined both by cell-intrinsic factors as well as by external environment signals. RHs represent an important proportion of the surface root area, crucial for nutrient uptake and water absorption.

Peroxidase activity on EXT cell

Here, researchers explore the molecular basis of this RH growth response by using a Genome Wide Association Study (GWAS) approach using Arabidopsis thaliana natural accessions. They identify the poorly characterized PEROXIDASE 62 (PRX62) and a related protein PRX69 as key proteins under moderate low temperature stress. Strikingly, a cell wall protein extensin (EXT) reporter reveals the effect of peroxidase activity on EXT cell wall association at 10 °C in the RH apical zone. Collectively, their results indicate that PRX62, and to a lesser extent PRX69, are key apoplastic PRXs that modulate ROS-homeostasis and cell wall EXT-insolubilization linked to RH elongation at low temperature.

Although there is a fairly well-known mechanistic view of how RH cell expands, the environmental signals that trigger the cell elongation process remain currently unknown. Due to its important role in root physiology, it has been anticipated that RH would be highly susceptible to environmental stresses such as heat or moderate temperature increase, which trigger extensive DNA methylation, transcriptomic and proteomic changes. Although RH development during cold acclimation remains largely unexplored, it has been observed that many RH-related genes respond to cold in the whole plant or seedlings. It is known that plants may perceive cold by a putative receptor at the cell membrane and initiate a signal to activate the cold-responsive genes and transcription factors for mediating stress tolerance.

The expression of both PRX encoding genes could be under the regulation of RSL4, which has a direct binding to PRX69 promoter-specific regions. Transcriptomic analyses revealed that upon PRX62 and PRX69 knockout, several other PRXs and cell wall EXTs encoding genes were differentially expressed, hinting at a compensatory mechanism. So PRX62 and PRX69 emerged as positive regulators of RH growth at low temperatures.