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Production of maternal haploids using a conspecific haploid inducer is routine and highly efficient in maize. However, the underlying mechanism of haploid induction (HI) is unclear. We develop a method to isolate three nuclei from a pollen grain and four microspores from a tetrad for whole-genome sequencing. A high rate of aneuploidy is observed at the three-nucleus stage (6/22 pollens) rather than at the tetrad stage (1/72 microspores) in one HI line CAU5. Frequent aneuploidy is also observed in another two inducer lines, but not in two regular lines, which implies that HI may be associated with pollen aneuploidy. We further sequence the individual embryos and endosperms of 88 maize kernels crossing between regular and inducer lines. Genome-wide elimination of the CAU5-derived chromosome is identified in eight of 81 embryos. Together, these results suggest that continuous chromosome fragmentation occurring post meiosis in the gametophyte may cause haploidy of the embryo.

In the present study, 40 mg L−1 N (NH treatment) and 20 mg L−1 6‐benzylaminopurine (6‐BA; BA treatment) were used to stimulate tiller bud growth in rice (Oryza sativa L.). The hormone changes in the tiller nodes and tiller buds and the N levels in the leaf blades, leaf sheaths, and tiller nodes were measured. The results indicate that N and 6‐BA promoted tiller bud germination and increased the indole‐3‐acetic acid (IAA) and zeatin (Z) plus zeatin riboside (ZR) levels in tiller buds and tiller nodes but decreased the abscisic acid (ABA) levels in tiller buds. Before tiller bud germination, the IAA and Z plus ZR levels increased but the ABA did not change, suggesting that IAA and Z plus ZR but not ABA may be the key regulators of tiller bud growth. From 4 d posttreatment, the tiller bud growth of the BA‐treated plants markedly slowed but the buds of the NH‐treated plants continued to grow quickly. We also found that the N levels in the leaf blades, leaf sheaths, and tiller nodes of the NH‐treated plants increased significantly as compared to those of the control plants (the plants treated with 10 mg L−1 N). However, no significant differences in N levels were observed between the plants of BA and control treatments. This study suggests that N may regulate tiller bud growth in two ways: by regulating N metabolism and regulating endogenous hormones. External 6‐BA promoted tiller bud germination by regulating endogenous hormones, but tiller bud growth was not maintained because it could not increase the N levels in the plants.

Gene imprinting is a widely observed epigenetic phenomenon in maize endosperm; however, whether it also occurs in the maize embryo remains controversial. Here, we used high-throughput RNA sequencing on laser capture microdissected and manually dissected maize embryos from reciprocal crosses between inbred lines B73 and Mo17 at six time points (3-13 days after pollination, DAP) to analyze allelic gene expression patterns. Co-expression analysis revealed sequential gene activation during maize embryo development. Gene imprinting was observed in maize embryos, and a greater number of imprinted genes were identified at early embryo stages. Sixty-four strongly imprinted genes were identified (at the threshold of 9:1) on manually dissected embryos 5-13 DAP (more imprinted genes at 5 DAP). Forty-one strongly imprinted genes were identified from laser capture microdissected embryos at 3 and 5 DAP (more imprinted genes at 3 DAP). Furthermore, of the 56 genes that were completely imprinted (at the threshold of 99:1), 36 were not previously identified as imprinted genes in endosperm or embryos. In situ hybridization demonstrated that most of the imprinted genes were expressed abundantly in maize embryonic tissue. Our results shed lights on early maize embryo development and provide evidence to support that gene imprinting occurs in maize embryos.

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