Humic Acid Enhances the Growth of Tomato Promoted by Endophytic Bacterial Strains Through the Activation of Hormone-, Growth-, and Transcription-Related Processes

Abstract

of growth-related processes stimulated by endophytic bacterial strains in tomato plants and reports the optimized dosages, complementation properties and gene markers for the further development of efficient PGPB-and HA-based biostimulants.

Beneficial Insects Deliver Plant Growth-Promoting Bacterial Endophytes between Tomato Plants

“…Two bacterial strains were used, namely, Paraburkholderia phytofirmans PsJN (PsJN), isolated from surface-sterilised onion roots [35], and Enterobacter sp. 32A (32A), isolated from a grapevine endosphere [40], which were able to promote tomato growth [39]. Bacterial strains were long-term stored in 80% glycerol at -80 • C. To obtain the inoculum, they were grown in 5 mL nutrient broth (NB) in sterile 15 mL tubes at 25 • C for 24 h under orbital shaking at 220 rpm.…”

“…Bacterial cells were collected by centrifugation at 3500× g for 10 min and washed twice with sterile 10 mM MgSO 4 . Bacterial cells were then suspended in sterile 10 mM MgSO 4 and the bacterial suspension was adjusted to 1.0 × 10 7 colony forming units (CFU) per unit of volume (CFU mL −1 ) based on an optical density conversion at 600 nm (OD 600 ) optimised for each strain (OD 600 0.1 corresponded to 2.34 × 10 8 CFU mL −1 for PsJN and to 6.90 × 10 7 CFU mL −1 for 32A) [39].…”

“…Seeds of S. lycopersicum L. cv. Moneymaker (Justseed, Wrexham, UK) were disinfected with 70% ethanol for 1 min and 2% sodium hypochlorite plus 0.02% Tween 20 for 5 min in a 50 mL tube with vigorous shaking and finally washed three times (3 min each) with sterile distilled water (Day 1; Figure 1) [39]. Surface-disinfected seeds (120 seeds) were transferred to Petri dishes (100 mm diameter, 20 seeds for each dish) containing 1% water agar (Thermo Fisher Scientific, Waltham, MA, USA) and then incubated at 21 ± 1 • C in a growth chamber (Bertagnin, Bologna, Italy) to allow seed germination.…”

“…These beneficial endophytes can positively influence plant growth through various mechanisms, such as the production of hormones, improvement of nutrient uptake and protection against biotic or abiotic stresses [34]. For example, the beneficial endophyte Paraburkholderia phytofirmans PsJN (PsJN), previously isolated from surface-sterilised onion roots [35] and classified as Pseudomonas and then Burkholderia [35,36], was able to promote plant growth [37][38][39] and upregulate the genes related to protein metabolism, transcription, transport, defence pathways, signal transduction and hormone metabolism in tomato [39]. Similarly, Enterobacter sp.…”

“…Similarly, Enterobacter sp. 32A (32A) was isolated from the grapevine endosphere [40] and promoted plant growth [39], activating a complex transcriptional reprogramming in tomato [39] and defence pathways in grapevine [41] plants. Moreover, 32A inhibited the growth of plant pathogens (e.g., Botrytis cinerea, Botryosphaeria dothidea and B. obtusa) [40] suggesting the use of PGPB to improve plant growth and health [39].…”