Root hair polar growth is endogenously controlled by auxin and sustained by oscillating levels of reactive oxygen species (ROS). These cells extend several hundred-fold their original size toward signals important for plant survival. Although their final cell size is of fundamental importance, the molecular mechanisms that control it remain largely unknown. Here we show that ROS production is controlled by the transcription factor RSL4, which in turn is transcriptionally regulated by auxin through several auxin response factors (ARFs). In this manner, auxin controls ROS-mediated polar growth by activating RSL4, which then up-regulates the expression of genes encoding NADPH oxidases (also known as RESPIRATORY BURST OXIDASE HOMOLOG proteins) and class III peroxidases, which catalyze ROS production. Chemical or genetic interference with ROS balance or peroxidase activity affects root hair final cell size. Overall, our findings establish a molecular link between auxin and ROS-mediated polar root hair growth.

Proline‐rich extensin‐like receptor kinases (PERKs) belong to the hydroxyproline‐rich glycoprotein (HRGP) superfamily known to be involved in many plant developmental processes. Here, we characterized two pollen‐expressed PERKs from Arabidopsis thaliana, PERK5 and PERK12. Pollen tube growth was impaired in single and double perk5‐1 perk12‐1 loss of function mutants, with an impact on seed production. When the segregation was analysed, a male gametophytic defect was found, indicating that perk5‐1 and perk12‐1 mutants carry deficient pollen transmission. Furthermore, perk5‐1 perk12‐1 displayed an excessive accumulation of pectins and cellulose at the cell wall of the pollen tubes. Our results indicate that PERK5 and PERK12 are necessary for proper pollen tube growth, highlighting their role in cell wall assembly and reactive oxygen species homeostasis.