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

Abstract

A shoot is a reiterated structure consisting of stems and leaves and is the prevailing body plan in most land plant lineages. Vascular plants form shoots in the diploid generation, whereas mosses do so in the haploid generation.¹ However, whether these plants use similar molecular mechanisms in shoot development and how the genetic networks for shoot development evolved is not clear. In our recent paper,² we examined polar auxin transport in several mosses, which is essential for shoot development in angiosperms. Surprisingly, we did not detect polar auxin transport in the gametophytic shoots of mosses, but did detect it in the sporophytes, which have no shoot structure, indicating that shoots in vascular plants and mosses are most likely regulated differently. Here we discuss the convergent evolution of shoots and diverged auxin regulation in land plants.

Shoot Evolved Several Times in Land Plants and Regulated Differently

Shoots, defined as structures composed of an axis, such as a stem, subtended by laminar organs, such as leaves, have evolved several times in parallel during the evolution of land plants.^3,4 The fossil record indicates that shoots of lycophytes and euphyllophytes, including angiosperms, evolved independently.³ The putative regulatory networks of transcription factors during shoot development in lycophytes, which have been inferred from their expression patterns, are partly conserved⁵ but have substantially diverged from those of angiosperms.⁶ On the other hand, the regulatory mechanism of auxin is likely conserved among vascular plants.⁷

Mosses, a monophyletic bryophyte lineage, form shoot-like structures called gametophores in the haploid generation. This evolved independently from lycophyte and euphyllophyte shoots, both of which are formed in the diploid generation. Auxin responses, such as axis elongation, apical dominance and gravitropic responses, have been similarly observed in the moss gametophyte and angiosperm sporophyte,^8–10 and auxin polar transport has been detected in moss rhizoids,¹¹ as well as sporophytes (Fig. 1).^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. An inhibitor of polar auxin transport did not affect gametophore development, but did induce alterations in the axis development of the sporophytes. The expression pattern of a GUS reporter gene driven by the GH3 auxin responsive promoter of soybean¹⁴ was consistent with the lack of polar auxin transport. These results suggest that polar auxin transport is not essential in gametophore development but is a common feature of sporophyte development in land plants. This also indicates that the development of a gametophore is, at least partly, regulated differently from that of an angiosperm shoot.

Figure 1.

Auxin polar transport was suggested in (A) moss protonemata,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.

The difference in transcription factor networks between vascular plant shoots and gametophores was discovered concurrent with our study. Class 1 KNOX genes are homeodomain-coding transcription factors, which are essential in regulating the initiation and maintenance of shoot apical meristems in angiosperms by regulating phytohormones.^15–19 Class 1 KNOX genes presumably play a similar function in ferns.²⁰ Analyses of deletion mutants of class 1 KNOX genes in P. patens indicate that these genes are not involved in gametophore development but function only in sporophyte development, although their downstream target genes are different from those of angiosperms.²¹ Furthermore, ASYMMETRIC LEAVES1, ROUGH SHEATH2, PHANTASTICA (ARP) and YABBY genes, which interact with class 1 KNOX genes for leaf development in angiosperms, are not found in the P. patens genome.²² Together these results indicate that significant parts of the shoot development regulatory system have not been conserved between angiosperms and mosses.

Difference of Auxin Responses between Flowering Plants and Moss

Although no basipetal auxin transport was detected in the gametophores, the auxin responsive promoters GH3 and DR5, which have been identified in angiosperms, are responsive to exogenous auxin in the P. patens gametophore.^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.²⁴ When we transiently expressed a plasmid carrying a DR5 promoter-uidA reporter (GUS) in P. patens protoplasts,² the induction rate difference between 50 µM of NAA and 0 µM of NAA was only 122%, indicative of a weak response of the DR5 promoter to the increasing level of auxin in the moss. DR5 is a synthetic auxin responsive promoter, which consists of tandem direct repeats of the six-base pair smallest auxin response element (AuxRE), TGTCTC, and showed more than 100-fold induction upon auxin treatment in Arabidopsis thaliana.²⁵ Sequences similar to AuxRE were found in the promoter region of P. patens AUX/IAA genes. Further study will be necessary to understand whether angiosperms and mosses use a similar element for auxin response, as divergence of the cis-regulatory element should be one of the factors accounting for the different auxin response of DR5 and, indeed, for the evolution of auxin responses among land plants.

Addendum to: Fujita T, Sakaguchi H, Hiwatashi Y, Wagstaff SJ, Ito M, Deguchi H, Sato T, Hasebe M. Convergent evolution of shoots in land plants: Lack of auxin polar transport in moss shoots. Evol Dev. 2008;10:176–186. doi: 10.1111/j.1525-142X.2008.00225.x.