Xylem Sap Metabolite Profile Changes During Phytostimulation of Maize by the Plant Growth-Promoting Rhizobacterium, Azospirillum lipoferum CRT1

Abstract

Plant Growth-Promoting Rhizobacteria (PGPR) enhances host plant growth and tolerance to biotic and abiotic stresses. Despite increased knowledge of their functional activities, reports of their impact on host metabolism and signalling networks are rare. In this study, small organic substances were analysed in the ascending xylem sap of maize plantlets that were inoculated with the PGPR Azospirillum lipoferum. In this feasibility study, xylem sap collection using a Scholander chamber was combined with metabolome analysis by Gas Chromatography-Mass Spectrometry (GC-MS) based profiling. Two genotypes of maize were investigated. Cultivar Seiddi displayed A. lipoferum-mediated increases in lateral root growth and enhanced photosynthetic potential unlike non-responsive cultivar FuturiXX. A total of 119 small organic substances were annotated in maize xylem sap. The content of 17 substances, including primary metabolites, such as sucrose, maltose, glucose, TCA cycle intermediates, amino acids, GABA and shikimate pathway metabolites, decreased in both cultivars after A. lipoferum inoculation and may thus reflect general effects of the maize-A. lipoferum interaction. The content of 28 additional substances, namely glucose, lactic acid, acidic intermediates of the pentose phosphate and ascorbate/aldarate pathways and defense-related hydroxycinnamic acids, specifically changed in the xylem sap of the A. lipoferum-phytostimulated cultivar Seiddi, therefore, suggesting that phytostimulation of maize by A. lipoferum may involve xylem-transported metabolic signalling. Glucose or other metabolites that are retrograde transported through the xylem to the shoot by transpirational pull may act as feedback signals of the root status. Such signals may stimulate leaves to enhance photosynthesis-mediated C-assimilation that is needed to sustain A. lipoferum-triggered root growth. The untargeted metabolome analysis of the xylem, i.e., the xylenome, indicates that the differential interactions of the two maize cultivars Seiddi and FuturiXX with Azospirillum lipoferum could represent a feasible system for the study of the role of xylem transported signals in plant/ PGPR interactions.