Auxin controls body plan patterning in land plants and it has been proposed to play a similar role in the development of brown algae (Phaeophyta) despite their distant evolutionary relationship with land plants. In flowering plants and many multicellular brown algae, the establishment of the apical-basal pattern is produced through an initial asymmetric cell division pattern. The apical cell grows into the shoot meristem or thallus, while the basal cell develops into the suspensor and root pole or rhizoids in land plants and brown algae, respectively. In land plants, the phytohormone auxin plays a crucial role in the establishment of this apical-basal axis and the subsequent embryo patterning. The mechanism of auxin action in brown algae remains controversial because of contradictory conclusions from pharmacological studies. These disparities may have arisen due to differences in the strains used, the time of their collection, and differences in experimental conditions such as pH and light. Bogaert et al. (10.1104/pp.18.01041) have used Dictyota dichotoma as a model system to show that auxin plays a role during the apical-basal patterning of the embryo of D. dichotima. Indole-3-acetic acid (IAA) was detectable in D. dichotoma germlings and mature tissue. Although two-celled D. dichotoma zygotes normally develop a rhizoid from one pole and a thallus meristem from the other, the addition of exogenous auxins to one-celled embryos affected polarization, and both poles of the spheroidal embryo developed into rhizoids instead. An in silico survey of auxin genes suggested a diverse range of auxin biosynthesis and transport genes in D. dichotoma, including PIN-LIKES (PILS) and ATP-binding cassette subfamily (ABCB) transporters. An inhibitor of ABC-B/multi-drug resistance/P58 glycoprotein (ABCB) subfamily of transporters in land plants, affected rhizoid formation by increasing rhizoid branching and inducing ectopic rhizoids. Together with reports on auxin function in basal lineages of green algae, these results suggest that auxin function predates the divergence between the green and brown lineage and the transition towards land plants.
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