## 3 Discussion  
Our studies utilizing both the conventional closed-plate and a new open-top systems demonstrated the growth stimulatory effects of the _P. phytofirmans_ PsJN bacteria on the roots and shoots of the Arabidopsis plants at the ambient and high-temperature conditions. The morphological responses and growth dynamics of the roots and shoots were resolved by monitoring their development through non-destructive phenotyping which was combined with destructive harvesting. Here, we discuss the changes in the root system architecture of Arabidopsis in light of the known regulatory events by intrinsic factors such as hormones, as well as external biotic and abiotic environmental stimuli. Then, we discuss the unique characteristics of the bacterial strain PsJN that stimulated the plant root architectural changes, thus influencing the plants' overall development and adaptation to the high-temperature stress. We also discuss the advantages of the GrowScreen-Agar II platform over the conventional plant phenotyping pipelines in terms of a strategically designed plant cultivation system, the high-resolution imaging capacity, and the streamlined root and shoot trait quantification. Changes in the Root System Architecture Are Modulated by Intrinsic Factors and Different Root-Environment Interactions  
The interaction of Arabidopsis with the bacteria PsJN and the high temperature altered its RSA. This follows the response of the plant roots to the environment involving the adjustments to the organization of the structural components of the roots from a cellular to a whole-plant level [60]. Arabidopsis has a primary root and several orders of lateral roots (the 1 st- and 2 nd-order lateral roots, which have been investigated here (Figures 2 and 4)). The RSA exhibits plasticity under a changing environment, demonstrating the morphological responses in this study. In agreement with [PERSON] et al. 2020 [18], the high-temperature stress induces several morphological responses in the roots including changes such as the decrease in the primary root length; the reduction in the number, length, and emergence angle of the lateral roots; the increase in the diameter and number of the second and 3 rd-order laterals (Figures 2 and 3); the increase in the density of the root hairs, which were not investigated here. Plant roots need an optimal temperature range for them to achieve a proper growth rate and functioning [5; 61]. The response of the RSA to increasing temperature can be species-specific since the different plants have different optimum temperature requirements for their growth [62]. An increase in the temperature is associated with the changes in the plant metabolism and nutrient uptake [63]. This uptake is closely associated with the size, morphology, and functioning of the root system [64]. The modifications of the RSA and changes to the root growth which are induced by high temperatures can have undesirable effects on the plant's ability to capture resources due to the reduced volume of root access [61]. These heat stress-induced alterations to the root growth and structure were found to be due to a reduced rate of cell division [65], as well as the reduced elongation and cell production rate [66]. Changes in the RSA that occurred due to the high temperature also reduced the root-to-shoot ratio [67], linking the belowground to the aboveground biomass allocation patterns and the resource acquisition in plants [68]. This consequence of a modified RSA due to the high temperature was shown in our study for the shoots' development (Figure 6). It may be that the root trait response to the elevated temperature influenced the carbon fixation and nutrient acquisition, which may have caused the decrease in the shoot trait response [69; 70]. PGPR have been described to affect the post-embryonic root development by altering cell division and differentiation within the primary root as well as affecting root hair formation and lateral root development [71].