ARBUSCULAR MYCORRHIZAL FUNGI (AMF) FOR DROUGHT TOLERANCE IMPROVEMENT IN TOMATO (SOLANUM LYCOPERSICUM)

Abstract

Drought stress is the single most important abiotic factor in tomato (Solanum lycopersicum) production. It affects quantity and quality yield depending on the growth stage at which the plant is subjected to the stress. The use of rhizosphere microbes, such as arbuscular mycorrhizal fungi (AMF), offers an alternative or complementary approach to conventional plant breeding for improving tomato plant drought tolerance. Studies were therefore carried out to investigate the effect of two species of AMF, Glomus intraradices and Glomus mossea, in single and combined application, on growth, physiological, biochemical and molecular aspects of tomato plants grown under drought stress. In these studies, the establishment of AMF within the tomato plants was determined by assessing the root colonization in plants grown in cocopeat with or without phosphorus application. The impact on growth, water status, pigment content, proline concentration, oxidative stress markers, catalase activity, abscisic acid (ABA) related genes and aquaporin genes were also assessed under drought stress. Omission of phosphate in the growth media significantly (P ≤ 0.001) enhanced root colonization in all AMF treatments and the highest root colonization (76.67%) was observed in mixed inoculation without phosphate addition. Plant inoculated with either AMF, but without phosphate addition produced significantly higher dry shoot weight (DSW) compared to non-inoculated plants with phosphate addition. DSW was significantly lowered in all drought stressed plants, but plants inoculated with AMF had higher DSW under watered and drought stress conditions (P ≤ 0.001). Plants inoculated with AMF showed less pigment damage, maintained higher leaf relative water content and accumulated significantly (P ≤ 0.028) xviii higher free proline in their tissues under drought stress. The levels of hydrogen peroxide and malondialdehyde (MDA) were significantly lower in tissues of plants inoculated with G. intraradices (P ≤ 0.033) while catalase activity was significantly (P ≤ 0.001) higher in plants inoculated with AMF under drought stress. The expression of abscisic acid (ABA) related genes, LeNCED1 (3-fold) and Le4 (13-fold) was upregulated under drought stress in non-inoculated plants, but unaffected in plants inoculated with G. intraradices and downregulated in plants inoculated with either G. mossea or mixed AMF (P ≤ 0.002). Under watered condition, expression of tomato aquaporin genes was generally increased in plants inoculated with AMF. Under drought stress however, the expression of aquaporin genes was reduced or unaffected in plants inoculated with AMF, but enhanced in non-inoculated plants. The results of this study indicate the potential of AMF in improving the growth of tomato plants under normal conditions (of watering and phosphate) as well as under stress conditions (drought and phosphate deficiency). The AMF induced drought stress tolerance is associated with enhanced accumulation of free proline, increased antioxidant enzyme activities and differential regulation of ABA biosynthetic gene and aquaporin genes.