Screening method and metabolic analysis of plant anti-aging microorganisms via ammonia-induced senescence in the duckweed Wolffia microscopica

Ammonium is the preferred N nutrition over nitrate for some plant species, but it is toxic to many other plant species and induces senescence at high concentrations. The duckweed <i>Wolffia microscopica</i> (Griff.) Kurz is the smallest and fast-growing angiosperm. It is highly sensitive to ammonium and has a short lifespan on media containing 0.5 mM or higher ammonia. This feature makes it a potential model plant to screen for anti-aging microorganisms. By co-culturing <i>W. microscopica</i> with endophytic microorgainisms isolated from rubber tree, we screened out an <i>Aspergillus sclerotiorum</i> strain ITBB2-31 that significantly increased the lifespan and the biomass of <i>W. microscopica</i>. Interestingly, both filter-sterilized and autoclaved exudates of ITBB2-31 increased the lifespan of <i>W. microscopica</i> cultures from 1 month to at least 7 months. Meanwhile, the exudates also showed strong anti-aging effects on cassava and the rubber tree leaves and increased chlorophyll contents by 50% - 350%. However, high contents of filter-sterilized exudates inhibited the growth of <i>W. microscopica</i> while extending its lifespan, indicating that there were heat-sensitive growth-inhibiting agents in the exudates as well. Comparative metabolome analysis of the filter-sterilized and autoclaved exudates revealed multiple heat-stable anti-aging and heat-sensitive growth-inhibiting compounds. Our results suggest that <i>W. microscopica</i> can be served as a rapid and efficient model plant to screen for plant anti-aging microorganisms.