## 3 Discussion
The most common PGPR-induced root phenotype is either the inhibition of the primary root growth when it is coupled with the proliferation of the lateral roots and root hairs [72], leading to an increased shoot biomass, or the increase in the primary root growth that is coupled with an increase in the plant biomass [73]. The PGP bacteria PsJN that were used in this study had positive effects on the primary, 1 st-, and 2 nd-order lateral roots, although we also found that the magnitude and direction of the growth stimulation varied depending on the root type and trait, timing, and temperature condition (Figures 2-4). Unlike obligate symbionts, PGPR can interact with numerous host plants, modifying the RSA to improve the plant's health and stress tolerance through a multitude of direct and indirect mechanisms [26; 27; 28]. These include the formation of biofilms; the production of phytohormones (e.g., jasmonic acid, salicylic acid, gibberell, indole-3-acetic acid (IAA)), exopolysaccharide, and 1-aminocyclopropane 1-carboxylic acid (ACC) deaminase [74; 75]; the production of cell-wall-degrading enzymes, antibiotics, hydrogen cyanide, siderophores and quorum quenching [24; 76]. These PGPR-induced modifications of the RSA allow for the enhanced functions of different root regions, which are responsible for soil exploration for nutrient acquisition [77]. The PsJN bacteria that were used in this study also stimulated the increase in the projected leaf area, and this, together with the root growth promotion, may have led to a higher shoot dry weight, the magnitude of which varied according to the temperature condition (Figure 6). The simultaneous capture of these time-specific events under the bacterial application in two temperature regimes on both the root and shoot growth dynamics are some of the new findings of this study which have not been previously reported. P. phytofilmans PsJN Induce Modifications of the Components of the Root System Architecture Contributing to Improved Plant Growth  
The magnitude of the bacteria-imparted growth promotion on the root architecture varied depending on the specific root type, which to our knowledge, has not been dissected before, and was influenced by the temperature and time. Significant effects due to the bacterial inoculation were observed in the root lengths, growth rates, and the number of the lateral roots (Figure 2a-g,i,j). These results support other studies showing that PGPR assist plants by facilitating the elongation of the root tissues and improving their growth rates for the optimized absorption of available nutrients [32; 78]. The increase in the depth of the roots indicates the prominent effect of the bacteria PsJN on the primary roots (Figure 2b,g). This can be attributed to the bacteria's involvement in the main processes governing cell division and cell elongation, which are mainly regulated by the phytohormones [79; 80], ACC deaminase, siderophores, and other secondary metabolites [39; 81], which have been proposed to act as signaling molecules during plant root-bacteria communication for efficient root colonization [82]. On the other hand, the increased formation and elongation of the lateral roots (Figure 2c,d,h,i) that underwent bacterial inoculation may also be due to the alteration of the endogenous pool of growth-regulating hormones such as IAA by the bacteria PsJN [1; 39]. This study also showed significant increases in the number of lateral roots that underwent bacterial inoculation (Figure 2e,j). This coincides with the outcome of in vitro potato plantlets that were inoculated with a PsJN strain showing a better performance in the plants' growth, likely due to the development of more secondary roots [83], although our study has further dissected the contribution of the 2 nd-order lateral roots (also known as tertiary roots).