Plant growth-promoting bacteria and their effects on maize plants (Zea mays L.) under concurrent drought and heat stress

Abstract

Under field conditions, drought and heat stress are more prevalent in combination than as isolated phenomena. Studies that were aimed at characterizing the consequences of both heat and drought stresses on plants reported significantly enhanced negative effects on the growth and productivity of crops for the combination of these stresses compared to each stress applied individually. The combined effects of heat and drought stresses have unique effects on various plant physiological and molecular response parameters suggesting that plants have different response mechanisms to the combination of these stresses compared to the individual effects of these stresses. The use of plant growth-promoting rhizobacteria is thought to be key to the adaptation and survival of plants to abiotic stresses. As such, the study aimed to evaluate drought and heat tolerant PGPR isolates at mitigating the effects of dual drought and heat stress on the early growth of maize, as well as elucidate the expression of drought and heat stress response genes which actively participate in PGPR-induced tolerance. The study screened and identified drought and heat tolerant PGPR that are potential biofertilizer candidates with unique attributes of enhancing plant response and adaptation to climate change effects. The isolated PGPR could promote the growth of maize plants under dual drought and heat stress under in-vitro conditions and in a pot trial experiment. The potential PGPR were identified as relatives of Bacillus thuringiensis (11MN1), Bacillus pseudomyciodes (21MN1B), Lelliottia amnigena (33MP1), and Leclercia sp. (36MP8). Bacterial isolates that demonstrated the potential to induce tolerance to maize plants subjected to dual drought and heat stresses in vitro were evaluated for compatibility using the dual culturing technique. The isolates from the Bacillus group showed slight antagonism against L. amnigena and Leclecia sp. belonging to the Enterobacteriaceae group. However, there was no antagonism between the Bacillus spp. and likewise between the Enterobacteriaceae group. The pot-experiments showed that isolates L. amnigena 33MP1, Leclercia sp. 36MP8 and the co-inoculation (11MN1, 21MN1, 33MP1, 36MP8) showed the greatest potential for alleviating the detrimental effects of induced concurrent drought and heat stress. The relative quantitative real-time PCR conducted to understand the expression of selected stress response genes in PGPR-inoculated maize plants showed that induced tolerance was achieved by initiating “Induced Systemic Tolerance” and by modulating the CAT2 and DHN2 stress response genes. 6 This approach is an environmentally friendly option for commercial and smallholder farmers in South Africa with the potential to maximize increase in maize production, especially under adverse climate conditions. These PGPR isolates also have the potential to reduce the need for extensive irrigation of maize crops throughout their growth cycle.