Untargeted metabolomics in halophytes: The role of different metabolites in New Zealand mangroves under multi-factorial abiotic stress conditions

Mangroves are halophytes which live in harsh coastal environments, and can serve as excellent model organisms to understand mechanisms of stress tolerance. Metabolomics is a useful approach to investigate the role different metabolites play during physiological responses of plants to abiotic stress factors. Previous studies indicate that levels of non-structural carbohydrates in plants may be associated with stress tolerance. Here, we manipulated levels of non-structural carbohydrates (NSCs) in the New Zealand mangrove, Avicennia marina subsp. australasica, through a light swapping regime. In a subsequent drought x salinity experiment, we then monitored leaf metabolite profiles of two NSC groups (high-NSC and low-NSC). Our results show that fourteen metabolites, belonging to multiple biochemical pathways, were significantly affected by one or more of these factors. The manipulation of non-structural carbohydrates led to increased abundance of amino acids in the low-NSC (L-NSC) plants compared to the high-NSC (H-NSC) plants. Under high drought conditions, the L-NSC plants had higher abundances of leucine and valine in comparison to those with the H-NSC phenotype. The L-NSC plants also had higher abundances of putrescine and aminoadipic acid when exposed to high salinity. Under the combination of drought and salinity, a-ketoglutarate was reduced in plants with the H-NSC phenotype, and soluble sugars accumulated compared to those with the L-NSC phenotype. The increased soluble sugar content in the H-NSC plants can facilitate osmotic adjustment, thereby aiding their survival during low soil water potential conditions. We also detected the presence of stress-protective phenolic compounds (syringic and sinapic acids) not previously reported in mangroves. Relationships between plant growth and the mangrove metabolome were also established, with levels of glucose and phosphoenol pyruvate being primarily responsible for phenotypic associations. Our findings show that the metabolites detected in this study, and their respective metabolic pathways, play a significant role in salinity and drought stress tolerance of A. marina subsp. australasica, providing new information to better understand mangrove mortality.