Abstract
Directional root growth control is crucial for plant fitness. The degree of root growth deviation depends on several factors, whereby exogenous growth conditions have a profound impact. The perception of mechanical impedance by wild-type roots results in the modulation of root growth traits, and it is known that gravitropic stimulus influences distinct root movement patterns in concert with mechanoadaptation. Mutants with reduced shootward auxin transport are described as being numb towards mechanostimulus and gravistimulus, whereby different growth conditions on agar-supplemented medium have a profound effect on how much directional root growth and root movement patterns differ between wild types and mutants. To reduce the impact of unilateral mechanostimulus on roots grown along agar-supplemented medium, we compared the root movement of Col-0 and auxin resistant 1-7 in a root penetration assay to test how both lines adjust the growth patterns of evenly mechanostimulated roots. We combined the assay with the D-root system to reduce light-induced growth deviation. Moreover, the impact of sucrose supplementation in the growth medium was investigated because exogenous sugar enhances root growth deviation in the vertical direction. Overall, we observed a more regular growth pattern for Col-0 but evaluated a higher level of skewing of aux1-7 compared to the wild type than known from published data. Finally, the tracking of the growth rate of the gravistimulated roots revealed that Col-0 has a throttling elongation rate during the bending process, but aux1-7 does not.
Highlights
The perception of mechanical impedance by wild-type roots results in the modulation of root growth traits, and it is known that gravitropic stimulus influences distinct root movement patterns in concert with mechanoadaptation
Mutants with reduced shootward auxin transport are described as being numb towards mechanostimulus and gravistimulus, whereby different growth conditions on agar-supplemented medium have a profound effect on how much directional root growth and root movement patterns differ between wild types and mutants
Published studies have shown that wild-type roots reduce their growth rate when confronted with obstacles, whereas the aux1 mutant shows no reduction in its root growth rate under mechanical stress conditions [7]
Summary
Roots have evolved to grow in darkness and surrounded by soil along the gravity vector [1–4] They adapt their growth direction and rate to their ever-changing environment, which includes changes in soil density or nutrient availability [5,6]. In addition to continuous growth along the gravitropic vector and the modulation of root system architecture to ensure efficient nutrient uptake, roots change their growth pattern form circumnutation to strict penetration depending on soil compaction [5,11–13]. This requires appropriate mechanostimulus perception, followed by signal transmission and mechanoadaptation [5,6,14,15]. Because the root responds to mechanical stress by reducing its elongation rate and cell length, which likely allows for an increase in the root diameter to ensure better soil penetration, we speculated that AUX1 loss may negatively affect root velocity adaptation in response to mechanical impediment [37]
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