Convergent evolution of shoots in land plants: lack of auxin polar transport in moss shoots

Abstract

The shoot is a repeated structure made up of stems and leaves and is the basic body plan in land plants. Vascular plants form a shoot in the diploid generation, whereas nonvascular plants such as mosses form a shoot in the haploid generation. It is not clear whether all land plants use similar molecular mechanisms in shoot development or how the genetic networks for shoot development evolved. The control of auxin distribution, especially by polar auxin transport, is essential for shoot development in flowering plants. We did not detect polar auxin transport in the gametophytic shoots of several mosses, but did detect it in the sporophytes of mosses without shoot structure. Treatment with auxin transport inhibitors resulted in abnormal embryo development, as in flowering plants, but did not cause any morphological changes in the haploid shoots. We fused the soybean auxin-inducible promoter GH3 with a GUS reporter gene and used it to indirectly detect auxin distribution in the moss Physcomitrella patens. An auxin transport inhibitor NPA did not cause any changes in the putative distribution of auxin in the haploid shoot. These results indicate that polar auxin transport is not involved in haploid shoot development in mosses and that shoots in vascular plants and mosses are most likely regulated differently during development.

The evolutionary development of plant body plans

“…In older sporophytes, acropetal IAA flux exceeded basipetal flux (and had different IAA inhibitor sensitivities), suggesting that acropetal and basipetal IAA transport involves different cellular pathways, much like in angiosperms (Poli et al 2003). In contrast, the auxin transport inhibitor NPA does not cause changes in the distribution of IAA in Physcomitrella gametophytes (Fujita et al 2008), suggesting that the bryophyte lineages either evolved different IAA transport systems independently, or represent an evolutionary transformation series, with hornworts being the most ancient and the mosses being the most derived (Fig. 4).…”

Homologues of the Arabidopsis thaliana SHI/STY/LRP1 genes control auxin biosynthesis and affect growth and development in the moss Physcomitrella patens

“…Auxin also functions as a hormone in bryophytes, and IAA application triggers physiological responses such as the chloronemato-caulonema transition (Johri and Desai, 1973), stem elongation (Fujita et al, 2008) and rhizoid emergence from the leafy shoot (Ashton et al, 1979). With the completion of the genome sequence of the moss Physcomitrella patens, it became clear that genes involved in auxin transport, perception and signalling in seed plants are present also in mosses (Rensing et al, 2008).…”

Convergences and divergences in polar auxin transport and shoot development in land plant evolution

“…2,12 These observations suggest that auxin actions and polar transport are conserved among land plants. 7 However, in our paper, 2 we provided evidence against the involvement of polar auxin transport in the gametophore of several mosses, including Physcomitrella patens, whose genome sequences are mostly determined. 13 Our direct measurements of auxin transport in the mosses did not detect directional auxin transport in gametophores.…”

“…2,23 This suggests that gametophores have the capacity to respond to auxin level changes, and the difference in auxin levels may be regulated by de novo auxin synthesis, auxin metabolism and auxin diffusion or short-range lateral auxin transport, rather than long-range basipetal transport. In fact, the moss genome includes most auxin-related components, such as auxin biosynthesis, metabolism, transport and signal transduction.…”

“…26 and directly detected in (B) rhizoids (r), and sporophytes (s), but not in gametophores (g). 2 Bars indicate 0.1 and 2 mm, respectively.…”