GC-MS based metabolomic profiling of sandalwood induced systemic signalling in host plants through a split root system

Abstract

Indian sandalwood (Santalum album L.) is a xylem-tapping hemiparasitic tree that relies on host plants for water and mineral nutrients. Although post-haustorial nutrient transfer has been well documented, the initial signaling associated with host recognition and systemic responses are still not well understood. The current study used the hydroponic split root system to spatially segregate the root systems of three host species (Mulberry, Casuarina, and Alternanthera) and tested whether exposure to sandalwood would induce systemic signalling in physically separated root systems. Under control conditions, where the roots of the hosts were evenly distributed in two compartments without sandalwood, the metabolic profiles demonstrated the stable and homogeneous pattern of exudation. 37 metabolites were found to be common in the exudates of the roots of Mulberry, while 39 metabolites were found to be common in the exudates of Casuarina and Alternanthera. However, when sandalwood was placed in the left compartments, significant reprogramming of the metabolic profile was observed. When the roots of Mulberry were placed in the presence of sandalwood, the number of common metabolites was found to have reduced drastically to only 5, while unique metabolites were found in the non-exposed and exposed roots. Casuarina showed reduced numbers of common metabolites to 9, while 15 unique metabolites were found in the non-exposed roots. Alternanthera showed similar responses. This indicate the systemic signalling from exposed roots to non-exposed roots system and strong metabolic reprogramming. KEGG annotation supported the presence of L-lysine (C00047) in the root exudates of Casuarina, which was involved in degradation (map00310), amino acid biosynthesis (map01230), and secondary metabolisms, including alkaloid biosynthesis (map01064). The present study show that sandalwood induces a rapid, long-distance metabolic shift in the host root systems regardless of physical exposure, suggesting the perception of chemical sandalwood cues and the activation of root to root signalling.