The Thelander lab: Evolution of developmental control in land plants

Novel tools to study auxin function in moss

The plant hormone auxin is essential for developmental control in both flowering plants and mosses. Comparisons between these two lineages have the potential to reveal details about the origin and evolution of auxin functions, and also about the developmental processes that they control. We develop versatile transgenic tools to facilitate detailed studies of auxin function in moss. We have for example developed a moss-specific version of the R2D2 ratiometric reporter showing where and when during development auxin is sensed. We have also produced a battery of transgenic moss lines making it possible to map where and when auxin is synthesized and to study the consequences of manipulated auxin levels. Using these tools, we have shown that while proliferation of moss apical stem cells require low auxin sensing they still synthesize auxin to control differentiation of their progeny, possibly reflecting an ancestral mechanism to control focal growth in land plants.

Fig 2. Bud from the P. patens R2D2 auxin response reporter line. High nuclear green:magenta signal ratios indicate auxin sensing. For details, see Thelander et al., 2019.

Auxin control of reproductive development, embryos and intergenerational communication in moss

Auxin controls reproductive organ formation and embryo development in flowering plants, but it has been an open question to what extent this applies also to haploid-dominant plants like mosses, and whether the mechanisms involved may have a common origin. We are studying the importance of auxin for male and female reproductive organ development, gamete formation, fertilization, and embryo development in P. patens. We are also exploring the possibility that auxin may be involved in communication and coordination between the haploid and the diploid generation in this moss. This may reveal details about the origin and evolution of auxin control of reproductive development and embryo growth, and may give clues as to how typical diploid-dominant life cycles of extant flowering plants may have evolved from ancestral haploid-dominant life cycles.

Fig 3. The auxin biosynthesis gene PpTARA is expressed during female reproductive organ development. Green is GFP signals indicating PpTARA expression, magenta is chloroplast autofluorescence. For details, see Landberg et al., 2020.

Control of moss sporangia development and the origin of tapetum-assisted pollen formation control

The diploid sporophyte of mosses develops an apical capsule (sporangium) in which cells undergo meiosis to form haploid spores. Flowering plant pollen are believed to have evolved from spores reminiscent of those in extant mosses. Male gamete production in pollen of flowering plants is strictly dependent on diploid tapetum cells surrounding and nursing the developing pollen. We are exploring to what extent tapetum-dependent formation of flowering plant pollen is dependent on conserved mechanisms for regulation and intergenerational communication identifiable during spore formation in moss.

Fig 4. Class II bHLH proteins, essential for tapetum-dependent pollen formation in flowering plants, are needed also for tapetum-dependent spore formation in P. patens sporangia. Sporangia from the Ppbhlh092Ppbhlh098 double mutant fail to develop functional spores. For details, see Landberg et al., 2021.