Figure 3. a. Chemical structures of cycad derived specialized metabolites found in leaves and coralloid roots: (1) BMAA, (2) Macrozamin and (3) cycacin. (4) hormogonium-inducing factor (HIF) 1-palmitoyl-2-linoleoyl-sn-glycerol produced in precoralloid roots. b. Chemical structures of microbiota derived metabolites found in coralloid roots: (5) Caulobacter- produced indigoidine-like metabolite, (6) cyanobiont produced desmamide A, (7) nostocyclopeptide A1/A3. Note that BMAA (1) is also produced by cyanobacteria in the coralloid roots. c. Host genome gene count differences between the ​​coralloid roots (with Cyanobacteria) vs precoralloid roots (without Cyanobacteria) ofCycas panzhihuaensis. KEGG mapping of enriched genes in coralloid roots (>10000 counts) correspond to endopeptidase inhibitor activity (CYCAS_000803, CYCAS_008980, CYCAS_000805), manganese ion binding and nutrient reservoir activity (CYCAS_020515), response to stress, stimulus or wounding (CYCAS_000805), and Serine-type endopeptidase inhibitor activity (CYCAS_000805). Enriched genes in precoralloid roots (>10000 counts) correspond to endopeptidases inhibitor activity (CYCAS_018813, CYCAS_001064, CYCAS_008980). Data from Liu et al . (2022).
Regarding the community ecology of the coralloid, recent studies have confirmed the existence of multiple microorganisms including fungi and viruses inside the coralloid root and other nitrogen-fixers such as Hyphomicrobiales (Rhizobiales) (Bustos-Diaz et al ., in review and references therein;). While the cyanobiont appears to be the main nitrogen fixer, the role of the microbial community is not well characterized, although efforts are underway (Liu et al ., 2023; Ndlovu et al ., 2023). Along these lines, specialized metabolites produced under nitrogen starvation by associated bacteria in interaction with the cyanobiont in Dioon edule have been reported (Gutiérrez-García et al ., 2019). It has also been suggested that the community itself might be recruited from the soil as a consortia because coralloid root microbial composition differs greatly from other tissues and the surrounding rhizosphere (Suárez-Moo et al ., 2019; Zheng & Gong, 2019). Investigations into the recruitment of the community, the possibility of some taxa inherited via the seed, and the dynamics within coralloid roots are still in their infancy and will likely produce many important advances in symbiosis ecology and specialized metabolism in the coming decade.
The long standing hypothesis that coralloid roots are an ancestral trait, based on the near ubiquity of coralloid roots in all living cycad species, was recently challenged. In the absence of coralloid root fossils, a proxy method used to deduce the existence of symbiotic nitrogen fixation using nitrogen isotopic ratios of fossil cycad leaves found independent origins of the symbiosis in living Zamiaceae and Cycadaceae (Kipp et al ., 2024). The authors concluded that a 35 million year old fossil Zamia entered in symbiosis with nitrogen-fixing bacteria, while inconclusive results were obtained fromBowenia leaf fossils from 50 million years ago. Interestingly, older fossils from extinct cycad genera showed no signs of being capable of symbiotic nitrogen fixation. The authors hypothesize that morphological similarities between coralloid roots from cycads in both families result from convergent evolution which, if true, would make cycads a rich system for investigations into plant morphology and evo-devo. Combined with the ongoing and recent advances in the chemical ecology and genetics of symbiosis described above, these data provide a full picture of the ecology, evolution, physiology, genetics, and development of root symbiosis.
Table 1. Twenty-four conserved cyanobacterial symbiotic associated genes shared among C. panzhihuaensis , the ferns Azolla filiculoides and Azolla cf. caroliniana , the liverwortBlasia pusilla and the hornwort Anthoceros punctatus . Gene sequences can be downloaded from (https://db.cngb.org/codeplot/datasets/PwRftGHfPs5qG3gE), data from Liu et al ., (2022).