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  1. Summary

    Phased secondary siRNAs (phasiRNAs) are broadly present in the reproductive tissues of flowering plants, with spatial–temporal specificity. However, the ARGONAUTE (AGO) proteins associated with phasiRNAs and their miRNA triggers remain elusive.

    Here, through histological and high‐throughput sequencing analyses, we show that rice AGO1d, which is specifically expressed in anther wall cells before and during meiosis, associates with both miR2118 and miR2275 to mediate phasiRNA biogenesis.

    AGO1d preferentially binds to miR2118‐triggered 21‐nucleotide (nt) phasiRNAs with a 5′‐terminal uridine, suggesting a dual role in phasiRNA biogenesis and function. Depletion of AGO1d causes a reduction of 21‐ and 24‐nt phasiRNAs and temperature‐sensitive male sterility. At lower temperatures, anthers of theago1dmutant predominantly show excessive tapetal cells with little starch accumulation during pollen formation, possibly caused by the dysregulation of cell metabolism.

    These results uncover an essential role of AGO1d in rice anther development at lower temperatures and demonstrate coordinative roles of AGO proteins during reproductive phasiRNA biogenesis and function.

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    The phytohormone cytokinin plays a significant role in nearly all aspects of plant growth and development. Cytokinin signaling has primarily been studied in the dicot model Arabidopsis, with relatively little work done in monocots, which include rice (Oryza sativa) and other cereals of agronomic importance. The cytokinin signaling pathway is a phosphorelay comprised of the histidine kinase receptors, the authentic histidine phosphotransfer proteins (AHPs) and type‐B response regulators (RRs). Two negative regulators of cytokinin signaling have been identified: the type‐A RRs, which are cytokinin primary response genes, and the pseudo histidine phosphotransfer proteins (PHPs), which lack the His residue required for phosphorelay. Here, we describe the role of the ricePHPgenes. Phylogenic analysis indicates that the PHPs are generally first found in the genomes of gymnosperms and that they arose independently in monocots and dicots. Consistent with this, the three ricePHPsfail to complement an Arabidopsisphpmutant (aphp1/ahp6). Disruption of the three ricePHPsresults in a molecular phenotype consistent with these elements acting as negative regulators of cytokinin signaling, including the induction of a number of type‐A RR and cytokinin oxidase genes. The triplephpmutant affects multiple aspects of rice growth and development, including shoot morphology, panicle architecture, and seed fill. In contrast to Arabidopsis, disruption of the ricePHPsdoes not affect root vascular patterning, suggesting that while many aspects of key signaling networks are conserved between monocots and dicots, the roles of at least some cytokinin signaling elements are distinct.

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