Auxin and Root Gravitropism: Addressing Basic Cellular Processes by Exploiting a Defined Growth Response

Abstract

Root architecture and growth are decisive for crop performance and yield, and thus a highly topical research field in plant sciences. The root system of the model plant Arabidopsis thaliana is the ideal system to obtain insights into fundamental key parameters and molecular players involved in underlying regulatory circuits of root growth, particularly in responses to environmental stimuli. Root gravitropism, directional growth along the gravity, in particular represents a highly sensitive readout, suitable to study adjustments in polar auxin transport and to identify molecular determinants involved. This review strives to summarize and give an overview into the function of PIN-FORMED auxin transport proteins, emphasizing on their sorting and polarity control. As there already is an abundance of information, the focus lies in integrating this wealth of information on mechanisms and pathways. This overview of a highly dynamic and complex field highlights recent developments in understanding the role of auxin in higher plants. Specifically, it exemplifies, how analysis of a single, defined growth response contributes to our understanding of basic cellular processes in general.

Genome-Wide Identification and Characterization of PIN-FORMED (PIN) Gene Family Reveals Role in Developmental and Various Stress Conditions in Triticum aestivum L.

“…Auxin forms gradients and concentration maxima in tissues and organs to stimulate diverse biological processes including gravitropism [ 5 , 6 ], organ initiation [ 7 ], leaf venation [ 8 ], apical dominance [ 9 ], embryo axis formation [ 10 ], root architecture [ 11 ], leaf vascular development [ 12 ], tropisms [ 13 , 14 ], fruit ripening [ 15 ], phototropism [ 16 ], phyllotactic patterning [ 17 ], lateral root emergence [ 18 ], root hair growth [ 19 ], apical hook development and root patterning [ 20 ], and sporophyte and male gametophyte development [ 21 , 22 ]. Several researchers have demonstrated that metabolic changes and transport of auxin play a key role in tissue differentiation, embryogenesis, organogenesis, differential growth, and tropic responses [ 2 , 3 , 4 , 23 , 24 , 25 , 26 ]. Auxin is synthesized in various plant tissues by several different pathways [ 27 , 28 ] and subjected to long- and short-range transport mediated by influx and efflux auxin transporters [ 25 , 29 ].…”

The AtCRK5 Protein Kinase Is Required to Maintain the ROS NO Balance Affecting the PIN2-Mediated Root Gravitropic Response in Arabidopsis

“…Redistribution of auxin plays an important role in plant gravitropism [ 1 , 11 , 57 , 58 , 59 , 60 , 61 ]. We followed the establishment of the auxin gradient by the auxin induced DR5::GFP construct [ 59 ] in vertically placed and rotated roots with and without PQ treatment.…”

“…It is well accepted that the bending of the root towards the gravity vector is due to the asymmetry in the auxin distribution between the lower and upper tissues of the horizontally oriented root tip [ 1 , 57 , 58 , 59 , 61 , 64 ]. The asymmetry in auxin distribution is due to the gravity-induced differential regulation of the turnover of PM-bound auxin transporter proteins (including AUX1, PIN3, and PIN2) at the two sides of the root.…”

“…This phosphorylation is required to stabilize the PIN2 protein in the apical membrane of root cortex cells [ 35 ] that is needed for a correct gravitropic response [ 60 ]. In this way, sufficient auxin is transported to the meristem that gets redistributed asymmetrically upon gravistimulation [ 58 , 59 , 61 ]. The asymmetric auxin induces asymmetric ROS as well as NO accumulation at the lower epidermis of the root [ 30 , 32 ].…”

“…transported to the meristem that gets redistributed asymmetrically upon gravistimulation [58,59,61]. The asymmetric auxin induces asymmetric ROS as well as NO accumulation at the lower epidermis of the root [30,32].…”