Dissecting Hierarchies between Light, Sugar and Auxin Action Underpinning Root and Root Hair Growth

Abstract

Plant roots are very plastic and can adjust their tissue organization and cell appearance during abiotic stress responses. Previous studies showed that direct root illumination and sugar supplementation mask root growth phenotypes and traits. Sugar and light signaling where further connected to changes in auxin biosynthesis and distribution along the root. Auxin signaling underpins almost all processes involved in the establishment of root traits, including total root length, gravitropic growth, root hair initiation and elongation. Root hair plasticity allows maximized nutrient uptake and therefore plant productivity, and root hair priming and elongation require proper auxin availability. In the presence of sucrose in the growth medium, root hair emergence is partially rescued, but the full potential of root hair elongation is lost. With our work we describe a combinatory study showing to which extent light and sucrose are antagonistically influencing root length, but additively affecting root hair emergence and elongation. Furthermore, we investigated the impact of the loss of PIN-FORMED2, an auxin efflux carrier mediating shootward auxin transporter, on the establishment of root traits in combination with all growth conditions.

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].…”

“…Environmental conditions, on the one hand, shape the overall architecture of the plant, but also determine its ability to cope with additive stress stimuli at the level of individual organs down to the cells [ 3 , 9 , 23 , 26 , 27 , 28 , 29 , 30 ]. Roots have evolved to anchor the plant in the soil and also to absorb water and minerals from it [ 3 , 4 ] Furthermore, roots developed to direct the growth direction along the gravity vector and to avoid direct illumination [ 9 , 26 , 30 ].…”

Continuous tracking of gravistimulated roots in a chambered coverslip by confocal microscopy allows first glimpse on mechanoadaptation of cell files during curvature initiation

“…The copyright holder for this preprint this version posted September 6, 2021. ; https://doi.org/10.1101/2021.09.05.459030 doi: bioRxiv preprint of media supplemented with sucrose or glucose, which is enhancing root growth, but also interferes with root trait establishment and can further mask phenotypes or cross-talk with stress adaptation [15,16,[18][19][20][21] .…”

“…Sucrose and glucose, especially in combination with direct root illumination result in enhanced deviation of root growth from vertical [19,21] . Therefore, in this study I studied the initiation of gravitropic bending at cellular level of roots, which were grown on 1% agar supplemented medium, but to avoid unnecessary stimuli, exogenous carbon source supplementation was excluded and the so-called D-root device to shade the root from direct illumination was used [16,19] .…”