Studies of moss reproductive development indicate that auxin biosynthesis in apical stem cells may constitute an ancestral function for focal growth control
New Phytol. 2020 Sep 8, Online ahead of print
Landberg K, Šimura J, Ljung K, Sundberg E, Thelander M

Abstract
The plant hormone auxin is a key factor for regulation of plant development, and this function was most likely reinforced during early land plant evolution. We have extended the available tool box to allow detailed studies of how auxin biosynthesis and responses are regulated in moss reproductive organs, their stem cells and gametes to better elucidate the function of auxin in early land plants morphogenesis. We measured auxin metabolites and identified IPyA as the main biosynthesis pathway in Physcomitrium (Physcomitrella) patens and established knock-out, overexpressor and reporter lines for biosynthesis genes which were analyzed alongside previously reported auxin sensing and transport reporters. Vegetative and reproductive apical stem cells synthesize auxin. Sustained stem cell activity depend on an inability to sense the auxin produced while progeny of the stem cells responds to the auxin, aiding in the control of cell division, expansion and differentiation. Gamete precursors are dependent on a certain degree of auxin sensing, while the final differentiation is a low auxin sensing process. Presented data indicate that low auxin activity may represent a conserved hallmark of land plant gametes, and that local auxin biosynthesis in apical stem cells may be part of an ancestral mechanism to control focal growth.

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