Auxin production in diploid microsporocytes is necessary and sufficient for early stages of pollen development

<div><p>Gametophytic development in Arabidopsis depends on nutrients and cell wall materials from sporophytic cells. However, it is not clear whether hormones and signaling molecules from sporophytic tissues are also required for gametophytic development. Herein, we show that auxin produced by the flavin monooxygenases YUC2 and YUC6 in the sporophytic microsporocytes is essential for early stages of pollen development. The first asymmetric mitotic division (PMI) of haploid microspores is the earliest event in male gametophyte development. Microspore development in <i>yuc2yuc6</i> double mutants arrests before PMI and consequently <i>yuc2yuc6</i> fail to produce viable pollens. Our genetic analyses reveal that <i>YUC2</i> and <i>YUC6</i> act as sporophytic genes for pollen formation. We further show that ectopic production of auxin in tapetum, which provides nutrients for pollen development, fails to rescue the sterile phenotypes of <i>yuc2yuc6</i>. In contrast, production of auxin in either microsporocytes or microspores rescued the defects of pollen development in <i>yuc2yuc6</i> double mutants. Our results demonstrate that local auxin biosynthesis in sporophytic microsporocytic cells and microspore controls male gametophyte development during the generation transition from sporophyte to male gametophyte.</p>