The molecular link between auxin and ROS-Mediated polar root hair growth

Abstract

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.auxin | root hair growth | RSL4 | ROS | peroxidases

Class III peroxidases PRX01, PRX44, and PRX73 potentially target extensins during root hair growth inArabidopsis thaliana

“…These 151 results together highlight partially redundant but not identical roles of PRX1, PRX44 and PRX73 as 152 positive regulators of polar growth. This is in agreement with the negative effect of SHAM, which 153 blocks the peroxidase activities of most peroxidases, on root hair growth (Mangano et al 2017). 154 Therefore, we measured in vitro total peroxidase activity using a guaiacol oxidation-based assay and 155 found that prx01,44,73 roots showed reduced peroxidase activity (close to 50% reduction) ( Figure 156 1C) while PRXs OE lines showed enhanced activity, with a 40-50% increase in PRX73 OE and 157 approximately 20% increase in PRX44 OE (Figure 2E).…”

Conflicting Auxin-Phosphate Signals Impact on RSL2 Expression and ROS-Homeostasis Linked to Root Hair Growth in Arabidopsis

“…This confirms that drastic changes in ROS-homeostasis controlled mostly by RBOHC inhibit auxin growth promoting effect. It was previously shown that the over-expression of either RSL4 or Auxin Response Factor 5 (ARF5) under the control of the root hair EXPANSIN 7 promoter (E7) led to high-levels of ROS (Mangano et al 2017). In addition, E7:RSL4 and E7:ARF5 were more insensitive to high Pi levels that Wt Col-0 and they developed almost regular extended root hairs ( Figure 1C ).…”

“…This result indicates that high expression of RSL4 or constitutive activated auxin signaling, both are able to partially repress the high levels of Pi-growth effect. To corroborate the ROS measurements performed with the H 2 DCF-DA probe as well as to determine if the main ROS molecule involved was hydrogen peroxide (H 2 O 2 ), we measured cytoplasmic H 2 O 2 ( cyt H 2 O 2 ) levels using a genetically encoded YFP-based H 2 O 2 sensor, HyPer (Mangano et al 2017). Similar results were obtained for high-levels of Pi that first repressed cytoplasmic H 2 O 2 while exogenous auxin was able to trigger an up-regulation of the ROS-signal in the presence of high-Pi in the root hairs ( Figure 1D ).…”

“…Although, in this process, other TFs might act in a coordinative manner under specific signals such as Pi-starvation and ethylene as this is the case for EIN3 (ETHYLENE INSENSITIVE 3)/EIL1 (ETHYLENE INSENSITIVE 3-LIKE 1) and LRL3 (LOTUS JAPONICA ROOTHAIRLESS-LIKE1) (Yi et al 2010; Song et al 2016, Feng et al 2017). Recently, it was found that auxin, by releasing several ARFs (e.g., ARF5, ARF7, ARF8 and ARF19) from Aux/IAA proteins, directly activates RSL4 expression and controls root hair growth linked to ROS-homeostasis involving three RESPIRATORY BURST OXIDASE HOMOLOG proteins (e.g., RBOHC,H,J) and four type-III secreted peroxidases (e.g., PER1,44,73) (Mangano et al 2017). In addition, RSL2 was also involved in the auxin-mediated growth response although it was unclear its mode of action at the molecular level (Mangano et al 2017).…”

“…Recently, it was found that auxin, by releasing several ARFs (e.g., ARF5, ARF7, ARF8 and ARF19) from Aux/IAA proteins, directly activates RSL4 expression and controls root hair growth linked to ROS-homeostasis involving three RESPIRATORY BURST OXIDASE HOMOLOG proteins (e.g., RBOHC,H,J) and four type-III secreted peroxidases (e.g., PER1,44,73) (Mangano et al 2017). In addition, RSL2 was also involved in the auxin-mediated growth response although it was unclear its mode of action at the molecular level (Mangano et al 2017). Conversely, high levels of inorganic phosphates (Pi) in the soil (or in the media) are able to strongly repress RSL4 expression linked to polar growth by an unknown mechanism while Pi-starvation results in an extensive outgrowth of root hairs.…”