AtbZIP 34 is required for Arabidopsis pollen wall patterning 3 and the control of several metabolic pathways in developing pollen 4

9 Abstract Sexual plant reproduction depends on the pro10 duction and differentiation of functional gametes by the 11 haploid gametophyte generation. Currently, we have a 12 limited understanding of the regulatory mechanisms that 13 have evolved to specify the gametophytic developmental 14 programs. To unravel such mechanisms, it is necessary to 15 identify transcription factors (TF) that are part of such 16 haploid regulatory networks. Here we focus on bZIP TFs 17 that have critical roles in plants, animals and other king18 doms. We report the functional characterization of Ara19 bidopsis thaliana AtbZIP34 that is expressed in both 20 gametophytic and surrounding sporophytic tissues during 21 flower development. T-DNA insertion mutants in Atb22 ZIP34 show pollen morphological defects that result in 23 reduced pollen germination efficiency and slower pollen 24 tube growth both in vitro and in vivo. Light and fluores25 cence microscopy revealed misshapen and misplaced 26 nuclei with large lipid inclusions in the cytoplasm of atb27 zip34 pollen. Scanning and transmission electron micros28 copy revealed defects in exine shape and micropatterning 29 and a reduced endomembrane system. Several lines of 30 evidence, including the AtbZIP34 expression pattern and 31 the phenotypic defects observed, suggest a complex role in 32 male reproductive development that involves a sporophytic 33 role in exine patterning, and a sporophytic and/or game34 tophytic mode of action of AtbZIP34 in several metabolic 35 pathways, namely regulation of lipid metabolism and/or 36 cellular transport. 37 38