Cycad phyllosphere microbiota and interaction with the plant metabolome
Above-ground plant tissues (i.e., the phyllosphere) microbiota may also play a role in plant ecology and fitness. Phyllosphere bacteria of cycads has only been published in Cycas panzhihuaensis(Zheng & Gong, 2019), but did not include leaves, the most prominent cycad organ. The authors found that reproductive structures (ovules, unfertilized and viable seeds) contain less bacterial diversity in comparison with roots (which are enriched in cyanobacteria). Bacterial communities among reproductive organs were highly similar and dominated by the family Enterobacteriaceae. Cycad pollen, however, was significantly enriched with Comamonadaceae, when compared to the other plant organs (Zheng & Gong, 2019). Here we present the first cycad leaf bacteriome and mycobiome (Fig. 4a, b). We find that Enterobacteriaceae are rarely represented in the phyllosphere community of six Zamiaspecies, while Nostocaceae, Beijerinckiaceae and Acetobacteraceae are abundant. The rather surprising abundance of the nitrogen fixing bacteria Desmonostoc suggests functional redundancy with coralloid roots, possibly as a failsafe to ensure the plant acquires sufficient fixed nitrogen (Moyes et al ., 2016) and represents the first evidence of potential additional symbionts in these long-lived plants. Research on leaf fungal endophytes is limited to a few studies, i.e., Encephalartos (Nesamari et al ., 2017) and twoZamia species (Sierra et al ., in review). The endophytic leaf fungi form a polyphyletic assemblage of mostly Ascomycota (Fig. 5b). These initial diversity studies of leaf endo- and epiphytic microbes provide a veritable mine of data for further investigations into the ecological and evolutionary significance of the phyllosphere.
Cycads are known for harboring a number of anti-herbivore and toxic compounds (see Sect. V), whose origin and impact across trophic levels has been a source of great debate (Marler, Snyder & Shaw, 2010 and reference therein). Sierra et al . (in review) recently addressed the association of plant-microbiota (bacteria and fungi) on the intraspecific leaf metabolome composition of two species (Zamia nana and Z. pseudoparasitica ). Among the 49 abundant metabolites, five compounds (Benzodioxoles, Biflavonoids and polyflavonoids, 1,2-diacylglycerols, N-acyl-alpha amino acids and glutamic acid and derivatives), were more expressed in Z. pseudoparasitica compared to Z. nana (Sierra et al . , in review). The metabolite variation was correlated to bacterial and fungal leaf endophyte community composition (Fig. 4c), and significantly associated with the abundance of one bacterial order (Frankiales) and three fungal orders (Hypocreales, Sordariales and Chaetothyriales). We are just beginning to unravel the association of cycad metabolites with phyllosphere microbiota and the ecological consequences for associated insects. Defensive secondary metabolites in cycads seem to be induced by complex plant-microbe reciprocal interactions, which calls for experimental research to uncover the genomic and biochemical underpinnings derived by the host and endophytic microbes, or both.