Touching ground

Like most moss species Physcomitrella patens does not look particularly impressive: growing on mud and soil near pools of water, the plants rarely grow larger than 5 mm. But when it comes to understanding the origins of plants conquering terrestrial environment the moss serves as a model organism for studies on plant evolution, development and physiology.

In a paper to be published in the December 1 edition of Current Biology, IST Austria Professor Jiři Friml together with colleagues unveils the mechanisms of directional auxin transport in early diverging land plants as exemplified by the moss Physcomitrella patens.

The emergence and radiation of multicellular land plants was driven by crucial innovations in their body plans. The directional transport of the phytohormone auxin represents a key, plant-specific mechanism for polarization and patterning in complex seed plants. The findings of Friml and his colleagues show that already in the early diverging land plant lineage auxin transport by PIN transporters is operational and diversified into ER-localized and plasma membrane-localized PIN proteins, ER being an abbreviation of endoplasmic reticulum, an organelle in which proteins and membrane lipids are produced and exported. Gain-of-function and loss-of-function analyses reveal that PIN-dependent intercellular auxin transport in Physcomitrella patens mediates crucial developmental transitions in tip-growing filaments and waves of polarization and differentiation in leaf-like structures. The results trace the origins of polarization and auxin-mediated patterning mechanisms and highlight the crucial role of polarized auxin transport during the evolution of multicellular land plants.

The evolutionary comparison of auxin transport mechanisms provides insights into the origin of plant-specific mechanisms that underlie cell polarity and tissue polarization. Concluding, PIN-mediated auxin transport was a key part of the molecular toolkit that allowed land plants to evolve the structural and developmental complexity crucial to their adaptation to the terrestrial environment 450 million years ago, when mosses diverged from green algae in the intertidal zone of the early oceans; the PIN-mediated auxin transport therefore helped to give rise to the enormous variety of land plants that populate Earth today.