Arabidopsis Thaliana Root System Growth Behaviors Under Stress Gradient

Favorable soil environment is the premise and basis for the survival and growth of plants, which not only can support plants abundant water and nutrients, but also will exert suitable magnitude of mechanical stimulation on plant root system, further promote the growth and development of the root system and the overground part of plants. Mechanical sitmulation of high soil compactness directly influences the root system architecture, mainly including the inhibition of primary root elongation growth, the promotion of the radial growth of root system, and the increase of the formation of lateral roots. Meanwhile, the growth of the overground part will also be affected, mostly the decrease of leaf area, the inhibition of cell division, the lower of stomatal conductance and so on. Plant root tip can sense the variation of soil mechanical environment ahead, and transfer these mechanical signals, which will affect root system architecture by the integration with specific genes expression and the regulation of hormonal level.At present, large scale of mechanized farming and long term drought give rise to the compacted surface soil, and unreasonable grazing and soil weathering lead to the softed surface soil, these deterioration of soil conditions seriously affect plant growth and development and the yield and crops. Thus, it’s very important to establish relevant mechanical models for growing root to simulate the soil environment of the two conditions to study the growth of root and the overground part. Recently, some studies have reported to establish the soft to hard model to study the root system growth, but confined to the penetration capability and the development of helix behavior of primary roots. Therefore, in the present study, we estabished models which could apply changing mechanical stimuli on growing roots, including the soft to hard model and the hard to soft model, we employed the wild type Arabidopsis thaliana as material to study the effect of media mechanical strength variation on the root system and the leaf growth behaviors.The fisrt part of this study was to establish models to load dynamic mechanical stimuli on growing root system of Arabidopsis, the soft to hard model and the hard to soft model were selected. The growth force arising from cell turgor pushed roots growing downward, the root tip would be subjected to the squeeze of the media ahead while roots elongating, at the same time, there existed the friction between root hairs and the media. Therefore, meida with different mechanical strength would exert different mechanical forces on growing root system, which were capable for the model.secondly, we should measure the mechanical property of model materials and determine the range of the applying force on growing roots. We regulated the concentration of Phytagel to get culture media of various menchanical strength, and put these media on INSTRON(E1000) testing machine to test the breaking load and the Young’s modulus, which could define the magnitude of menchanical stimuli. Results showed that there was a very fine linear relationship between the media concentration and the breaking load. In order to determine the range of mechanical sitmuli, we studied the root system phenotypes in single uniform media of different mechanical strength, mainly including the average length of primary root, the density of lateral root, and the curvature ratio of primary root. Taken all results together, the range of media mechanical strength was 4.70 N-19.99 N.Finally, under these established models, we studied the penetration growth of Arabidopsis primary root, the primary root growth rates before and after penetration, leaf area and leaf electrolyte leakage rate. In the hard to soft model, the high mechanical strength of up-media suppressed the elongation growth of primary roots, the penetration ability of primary roots, and the leaf growth. After the primary roots penetrating the sub-media with lower mechanical strength, root tips could timely sense the decrease of mechanical stimuli, the primary roots and leaf growth were promoted. Results demonstrated that when the mechanical strength of sub-media was fixed to 4.70 N, and the difference of mechanical strength between the up- and sub-media was 2.77 N, the penetration growth of primary roots and leaf growth were significantly promoted. In the soft to hard model, when the up-media was fixed to 4.70 N, and the mechanical strength of the sub-media was equal to or higher than 14.70 N, the primary root could not penetrate the sub-media, leaf area decreased significantly and the electrolyte leakage rate increased remarkably. Results showed that when the difference of the mechanical strength of the two-layer media was too large, the penetration ability of primary roots would be seriously affected, while the penetration ability of primary roots would be significantly boosted when setting the buffer layer, where the mechanical strength was a little lower than the sub-media, what’s more, the lower the mechanical gradient of the three-layer media, the more capable for primary to penetrate, and the better of the root system and leaf growth.Plant root system can adapt to the gradually variation of mechanical stimulation, appropriate magnitude of mechanical stimulation will be beneficial for root system and leaf growth. Our study has certain academic value and potential application value in plant stress physiologh and selecting optimal crop cultivars in resisting dversity stress.