The TOR–Auxin Connection Upstream of Root Hair Growth

Abstract

Plant growth and productivity are orchestrated by a network of signaling cascades involved in balancing responses to perceived environmental changes with resource availability. Vascular plants are divided into the shoot, an aboveground organ where sugar is synthesized, and the underground located root. Continuous growth requires the generation of energy in the form of carbohydrates in the leaves upon photosynthesis and uptake of nutrients and water through root hairs. Root hair outgrowth depends on the overall condition of the plant and its energy level must be high enough to maintain root growth. TARGET OF RAPAMYCIN (TOR)-mediated signaling cascades serve as a hub to evaluate which resources are needed to respond to external stimuli and which are available to maintain proper plant adaptation. Root hair growth further requires appropriate distribution of the phytohormone auxin, which primes root hair cell fate and triggers root hair elongation. Auxin is transported in an active, directed manner by a plasma membrane located carrier. The auxin efflux carrier PIN-FORMED 2 is necessary to transport auxin to root hair cells, followed by subcellular rearrangements involved in root hair outgrowth. This review presents an overview of events upstream and downstream of PIN2 action, which are involved in root hair growth control.

Lessons Learned from the Studies of Roots Shaded from Direct Root Illumination

“…While PIN2:ubq turnover in LGR can be inhibited by eBL, leading to increased PIN2:ubq levels at the PM, wild-type PIN2 abundance at the PM increases significantly after eBL treatment only in DGR, but no longer in LGR, which is consistent with previous studies suggesting that root illumination leads to stabilization of PIN2 at the PM [33]. In addition, a recent study showed that direct root illumination, which leads to increased root hair outgrowth closer to the meristem in wild-type roots, is reduced in the PIN2 knockout mutant eir1-4, consistent with the importance of shoot-directed auxin transport for PIN2-mediated root hair outgrowth [4,29,73]. Regulated shootward auxin distribution has been well described to ensure proper root growth and thus plant nutrition [19,74,75].…”

“…Finally, differential root development under direct illumination results in altered shoot growth, as shoots with shaded roots accumulate less mass and anthocyanins, demonstrating differential distribution of resources throughout the plant and fitness depending on the experimental growth conditions chosen [26]. The regulation of root growth (e.g., the timing of lateral root and root hair emergence, directional root growth) is highly dependent on the finely tuned distribution of numerous signaling molecules, including sugars, ROS, phytohormones, and other small molecules whose availability per cell is strongly modulated by internal and external conditions [4,16,19,29,33,[35][36][37]. Therefore, the direct interplay of phytohormones, light, and sugar signaling pathways in roots exposed to direct illumination compared to shaded roots has been the target of several recent studies [4,21,23,24,[26][27][28]32,[37][38][39][40].…”

Understanding the Intricate Web of Phytohormone Signalling in Modulating Root System Architecture

“…The energy status of plants also affects RH development. In a recent study, glucose-activated TOR was found to phosphorylate and stabilize PIN2 at the plasma membrane, which results in the shootward auxin transport to the RH [ 159 , 160 ].…”

Auxin and Target of Rapamycin Spatiotemporally Regulate Root Organogenesis

“…Several studies have used genetic and molecular approaches to identify functions of auxin [ 41 , 42 ] and TOR [ 43 , 44 , 45 , 46 ] signaling in the growth and development of plant roots. In recent years, various studies have further confirmed that TOR crosstalk with auxin mediates plant growth and development [ 47 , 48 , 49 ]. This review focuses on the interactions between auxin and TOR to regulate the development of plant ERs, PRs, LRs, and ARs.…”