Apical and basal auxin sources pattern shoot branching in a moss

Shoot branching mechanisms where branches arise in association with leaves – referred to as lateral or axillary branching – evolved by convergence in the sporophyte of vascular plants and the gametophyte of bryophytes, and accompanied independent events of plant architectural diversification. Previously, we showed that three hormonal cues, including auxin, have been recruited independently to co-ordinate branch patterning in flowering plant leafy shoots and moss gametophores (Coudert, Palubicki et al., 2015). Moreover, auxin-mediated apical dominance, which relies on local auxin production, has been proposed as a unifying molecular regulatory mechanism of branch development across land plants. Whilst our previous work in the moss Physcomitrium patens has gathered indirect evidence supporting the notion that auxin synthesized in gametophore apices regulates branch formation at a distance, direct genetic evidence for a role of auxin biosynthesis in gametophore branching control is still lacking. Here, we show that gametophore apex decapitation promotes branch emergence through massive and rapid transcriptional reprogramming of auxin-responsive genes and altering auxin biosynthesis gene activity. Specifically, we identify a subset of P. patens TRYPTOPHAN AMINO-TRANSFERASE (TAR) and YUCCA FLAVIN MONOOXYGENASE-LIKE (YUC) auxin biosynthesis genes expressed in apical and basal regions of the gametophore, and show that they are essential for branch initiation and outgrowth control. Our results demonstrate that local auxin biosynthesis coordinates branch patterning in moss and thus constitutes a shared and ancient feature of shoot architecture control in land plants.

Involvement of Auxin Biosynthesis and Transport in the Antheridium and Prothalli Formation in Lygodium japonicum

The spores of Lygodium japonicum, cultured in the dark, form a filamentous structure called protonema. Earlier studies have shown that gibberellin (GA) induces protonema elongation, along with antheridium formation, on the protonema. In this study, we have performed detailed morphological analyses to investigate the roles of multiple phytohormones in antheridium formation, protonema elongation, and prothallus formation in L. japonicum. GA4 methyl ester is a potent GA that stimulates both protonema elongation and antheridium formation. We found that these effects were inhibited by simultaneous application of abscisic acid (ABA). On the other hand, IAA (indole-3-acetic acid) promoted protonema elongation but reduced antheridium formation, while these effects were partially recovered by transferring to an IAA-free medium. An auxin biosynthesis inhibitor, PPBo (4-phenoxyphenylboronic acid), and a transport inhibitor, TIBA (2,3,5-triiodobenzoic acid), both inhibited protonema elongation and antheridium formation. L. japonicum prothalli are induced from germinating spores under continuous white light. Such development was negatively affected by PPBo, which induced smaller-sized prothalli, and TIBA, which induced aberrantly shaped prothalli. The evidence suggests that the crosstalk between these plant hormones might regulate protonema elongation and antheridium formation in L. japonicum. Furthermore, the possible involvement of auxin in the prothalli development of L. japonicum is suggested.

Local auxin biosynthesis acts downstream of brassinosteroids to trigger root foraging for nitrogen

Lateral roots (LRs) dominate the overall root surface of adult plants and are crucial for soil exploration and nutrient acquisition. When grown under mild nitrogen (N) deficiency, flowering plants develop longer LRs to enhance nutrient acquisition. This response is partly mediated by brassinosteroids (BR) and yet unknown mechanisms. Here, we show that local auxin biosynthesis modulates LR elongation while allelic coding variants of YUCCA8 determine the extent of elongation under N deficiency. By up-regulating the expression of YUCCA8/3/5/7 and of Tryptophan Aminotransferase of Arabidopsis 1 (TAA1) under mild N deficiency auxin accumulation increases in LR tips. We further demonstrate that N-dependent auxin biosynthesis in LRs acts epistatic to and downstream of a canonical BR signaling cascade. The uncovered BR-auxin hormonal module and its allelic variants emphasize the importance of fine-tuning hormonal crosstalk to boost adaptive root responses to N availability and offer a path to improve soil exploration by expanded root systems in plants.

Endogenous auxin directs development of embryonic stem cells into somatic proembryos in Arabidopsis

Somatic embryogenesis (SE) is the process by which embryos develop from in vitro cultured vegetative tissue explants. The synthetic auxin 2,4-dichlorophenoxyacetic acid (2,4-D) is widely used for SE induction, but SE can also be induced by overexpression of specific transcription factors, such as AT-HOOK MOTIF NUCLEAR LOCALIZED 15 (AHL15). 2,4-D and AHL15 both trigger the biosynthesis of the natural auxin indole-3-acetic acid (IAA). However, the role of this endogenously produced auxin in SE is yet not well understood. In this study we show that the induction of embryonic stem cells from explants does not require IAA biosynthesis, whereas an increase in IAA levels is essential to maintain embryo identity and for embryo formation from these stem cells. Further analysis showed that YUCCA (YUC) genes involved in the IPyA auxin biosynthesis pathway are up-regulated in embryo-forming tissues. Chemical inhibition of the IPyA pathway significantly reduced or completely inhibited the formation of somatic embryos in both 2,4-D-and AHL15-dependent systems. In the latter system, SE could be restored by exogenous IAA application, confirming that the biosynthesis-mediated increase in IAA levels is important. Our analyses also show that PIN1 and AUX1 are the major auxin carriers that determine respectively auxin efflux and influx during SE. This auxin transport machinery is required for the proper transition of embryonic cells to proembryos and, later, for correct cell fate specification and differentiation. Taken together, our results indicate that auxin biosynthesis in conjunction with its polar transport are required during SE for multicellular somatic proembryo development and differentiation.