SignificanceFlowering time is a critical determinant of crop adaptation to local environments. As a result of natural and artificial selection, maize has evolved a reduced photoperiod sensitivity to adapt to regions over 90° of latitude in the Americas. Here we show that a distant Harbinger-like transposon acts as a cis-regulatory element to repress ZmCCT9 expression to promote flowering under the long days of higher latitudes. The transposon at ZmCCT9 and another functional transposon at a second flowering-time gene, ZmCCT10, arose sequentially following domestication and were targeted by selection as maize spread from the tropics to higher latitudes. Our results demonstrate that new functional variation created by transposon insertions helped maize to spread over a broad range of latitudes rapidly.

ORCID IDs: 0000-0002-4656-3189 (H.S.); 0000-0003-0518-5924 (C.Z.); 0000-0001-9320-9628 (W.J.).Anther cuticle and pollen exine are protective barriers for pollen development and fertilization. Despite that several regulators have been identified for anther cuticle and pollen exine development in rice (Oryza sativa) and Arabidopsis (Arabidopsis thaliana), few genes have been characterized in maize (Zea mays) and the underlying regulatory mechanism remains elusive. Here, we report a novel male-sterile mutant in maize, irregular pollen exine1 (ipe1), which exhibited a glossy outer anther surface, abnormal Ubisch bodies, and defective pollen exine. Using map-based cloning, the IPE1 gene was isolated as a putative glucose-methanolcholine oxidoreductase targeted to the endoplasmic reticulum. Transcripts of IPE1 were preferentially accumulated in the tapetum during the tetrad and early uninucleate microspore stage. A biochemical assay indicated that ipe1 anthers had altered constituents of wax and a significant reduction of cutin monomers and fatty acids. RNA sequencing data revealed that genes implicated in wax and flavonoid metabolism, fatty acid synthesis, and elongation were differentially expressed in ipe1 mutant anthers. In addition, the analysis of transfer DNA insertional lines of the orthologous gene in Arabidopsis suggested that IPE1 and their orthologs have a partially conserved function in male organ development. Our results showed that IPE1 participates in the putative oxidative pathway of C16/C18 v-hydroxy fatty acids and controls anther cuticle and pollen exine development together with MALE STERILITY26 and MALE STERILITY45 in maize.Male sterility is a common biological phenomenon in plants and widely used in the production of hybrid seeds, which can reduce costs and enhance seed purity (Tester and Langridge, 2010). According to inheritance or origin, male sterility includes three types: genic male sterility, cytoplasmic male sterility, and cytoplasmicgenic male sterility (Rhee et al., 2015). The generation of mature pollen grains relies on anther development. The start of anther formation occurs in differentiated flower tissues (floral meristem), which consist of three histogenic layers: L1, L2, and L3. After continuous cell division and differentiation, L1 forms the epidermis and the L3 layer develops into the stomium and vascular bundles. L2 is the most important layer; it undergoes a series of periclinal and anticlinal divisions and eventually grows into the endothecium, the middle layer, the tapetum, and the pollen mother cells. When anther morphogenesis is completed, the anther has centrally localized pollen mother cells enclosed by four somatic layers, which are, from the surface to the interior, the epidermis, endothecium, middle layer, and tapetum. Then, the pollen mother cells undergo meiosis and mitosis, resulting in trinucleate pollen grains, and the endothecium, middle layer, and tapetum are gradually degraded (Goldberg et al., 1993(Goldberg et al., , 1995Ma, 2005).The anther cuticle and poll...

The floral transition of the maize (Zea mays ssp. mays) shoot apical meristem determines leaf number and flowering time, which are key traits influencing local adaptation and yield potential. dlf1 (delayed flowering1) encodes a basic leucine zipper protein that interacts with the florigen ZCN8 to mediate floral induction in the shoot apex. However, the mechanism of how dlf1 promotes floral transition remains largely unknown. We demonstrate that dlf1 underlies qLB7-1, a quantitative trait locus controlling leaf number and flowering time that was identified in a BC 2 S 3 population derived from a cross between maize and its wild ancestor, teosinte (Zea mays ssp. parviglumis). Transcriptome sequencing and chromatin immunoprecipitation sequencing demonstrated that DLF1 binds the core promoter of two AP1/FUL subfamily MADS-box genes, ZmMADS4 and ZmMADS67, to activate their expression. Knocking out ZmMADS4 and ZmMADS67 both increased leaf number and delayed flowering, indicating that they promote the floral transition. Nucleotide diversity analysis revealed that dlf1 and ZmMADS67 were targeted by selection, suggesting that they may have played important roles in maize flowering time adaptation. We show that dlf1 promotes maize floral transition by directly activating ZmMADS4 and ZmMADS67 in the shoot apex, providing novel insights into the mechanism of maize floral transition.

High temperatures interfere with meiotic recombination and the subsequent progression of meiosis in plants, but few genes involved in meiotic thermotolerance have been characterized. Here we characterize a maize (Zea mays) classic dominant male sterile mutant Ms42, which has defects in pairing and synapsis of homologous chromosomes and DNA double-strand break repair. Ms42 encodes a member of the heat shock protein family, HSP101, which accumulates in pollen mother cells. Analysis of the dominant Ms42 mutant and hsp101 null mutants reveals that HSP101 functions in RAD51 loading, double-strand break repair, and subsequent meiosis. Consistent with these functions, overexpression of Hsp101 in anthers results in robust microspores with enhanced heat tolerance. These results demonstrate that HSP101 mediates thermotolerance during microsporogenesis, shedding light on the genetic basis underlying the adaptation of male meiocytes to high temperatures.

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