Os03g0650400

OsAM1, a homolog of Arabidopsis SWI1 and maize AM1, encodes a protein with a coiled-coil domain in its central region.

Annotated Information

Function

• OsAM1 may function at an earlier stage than MEL1 and any other meiotic elements previously reported in rice. OsAM1 plays a fundamental role in building the proper chromosome structure at the beginning of meiosis.

• PMCs in Osam1 are halted at the leptotene-zygotene transition. A similar phenomenon of meiotic arrest in maize involving an am1-praI mutant suggests the presence of a novel checkpoint in maize , and supports the notion that the leptotene-zygotene transition checkpoint may also exist in rice.

• Both OsAM1 and OsREC8 are involved in early meiotic events, but their relationship has not been fully established. OsREC8 is still detected as very faint signals in Osam1 PMCs, suggesting that the initial recruitment of OsREC8 is independent of OsAM1. This hypothesis is consistent with the fact that OsAM1 and OsREC8 do not show obvious co-localization in wild type at the beginning of prophase I. However, the elongation of OsREC8 signals was impaired in Osam1, suggesting that OsAM1 may facilitate the extension of OsREC8 to form chromosome axes.

• OsAM1 has more similarities to its monocotyledonous homolog AM1. Chromosome behavior of the rice Osam1 mutants seems to be identical to that of maize am1-praI, suggesting that OsAM1 may also be involved in a leptotene-zygotene transition checkpoint.

Mutation

• The mutant plant was normal in vegetative development, although no pollen was produced (Figure 1A and 1B). Normal phenotype plants and sterile plants from the progeny of Osam1 +/− give 3:1 segregation, establishing it as a single recessive mutant (χ2 = 0.83; P > 0.05). The researchers named the mutant Osam1-1. When pollinated with normal pollen from wild-type plants, Osam1-1 plants do not set seeds.

Figure 1 Comparison of the wild-type and Osam1-1 mutant phenotype. (A) A wild-type plant (left) and an Osam1-1 mutant plant (right). (B) A wild-type panicle (left) and an Osam1-1 mutant panicle (right). .

• To determine the anther morphological defects in Osam1- 1, transverse sections of both wild-type and mutant anthers at different developmental stages were examined. The development of rice anthers has been described in detail. Before entering meiosis, the four-layered anther wall, including epidermis, endothecium, middle layer and tapetum, is formed and primary sporogenous cells develop into larger PMCs in the anther locules (Figure 2A). The PMCs gradually become spherical and enter meiosis (Figure 2B and 2C). At the end of meiosis, the PMCs develop into tetrads. Concomitantly, the middle layer and tapetum begin to degenerate. After going through the vacuolated stage (Figure 2D) and two rounds of mitosis (Figure 2E), microspores then generate the mature tricellular pollen grains (Figure 2F). There was no detectable difference between Osam1-1 and wild-type anthers before entering meiosis, and the four layers of anther wall were also normal (Figure 2G). The impairment of anther development was observed in Osam1-1 after PMCs enlarged and entered meiosis (Figure 2H). The tapetal cells had significantly swelled and the PMCs did not complete meiosis (Figure 2I). The development of PMCs in Osam1-1 anthers seemed to be interrupted; they were arrested at the PMC stage with aberrant tapetum development (Figure 2J and 2K). Eventually, all the PMCs as well as tapetum had disappeared, leading to the anthers without pollen (Figure 2L).

Figure 2 Transverse sections of anthers in both wild type and the Osam1-1 mutant. As the development of the Osam1-1 PMCs was arrested, the sections of Osam1-1 anthers were selected according to the length of spikelet. The wild-type sections are shown in A-F, and the Osam1-1 sections are shown in G-L. (A, B, G, H) Early meiosis stage. (C, I) Late meiosis stage. (D, J) Vacuolated pollen stage. (E, K) Pollen mitosis stage. (F, L) Mature pollen stage. E, epidermis; En, endothecium; ML, middle layer; T, tapetum; PMCs, pollen mother cells; MP, mature pollen. Scale bars, 5 μm. .

• In wild-type PMCs, early leptotene chromosomes begin to condense and individual chromosomes appear as thin threads (Figure 3A), while nucleoli move to the periphery of the nuclei in late leptotene (Figure 3B). As zygotene progresses, homologous chromosomes undergo pairing and synapsis (Figure 3C). At pachytene, fully synapsed chromosomes are observed (Figure 3D). SCs are then disassembled, and chiasmata, which correspond to crossovers, hold the homologous chromosomes together at diplotene. During diakinesis, the chromosomes are highly condensed and 12 short bivalents are clearly observed (Figure 3E). At metaphase I, the bivalents become aligned in the center of the cell (Figure 3F), and then homologous chromosomes separate and migrate toward opposite poles at anaphase I (Figure 3G), generating dyads at the end of meiosis I. During meiosis II, the two dyads undergo equational division and produce tetrads (Figure 3H and 3I). The Osam1-1 chromosomes behaved normally during meiotic interphase and leptotene (Figure 4A and 4B). However, the later stages of prophase I were never observed; the meiotic process seemed to have been arrested at leptotene, although some chromosomes continue to condense (Figure 4C). The abnormal PMCs remained in leptotene for some time (Figure 4D). The researchers have checked more than 300 PMCs in the Osam1-1 mutant and all of them showed the same characteristics. They also found that Osam1-2 meiosis is identical to Osam1-1.

Figure 3 Meiosis of male meiocytes in wild type. (A) Early leptotene; (B) late leptotene; (C) zygotene; (D) pachytene; (E) diakinesis; (F) metaphase I; (G) anaphase I; (H) dyad; and (I) Tetrad. Scale bars, 5 μm. .

Figure 4 Meiosis of male meiocytes in the Osam1-1 mutant. (A) Early leptotene. (B and C) Late leptotene. (D) The arrested PMCs. Scale bars, 5 μm. .

• In wildtype meiocytes, two unpaired 5S rDNA signals were visible at leptotene. At zygotene, only one bright signal was observed in each cell, showing that the homologous chromosomes paired well at this stage (Figure 5A). However, two separate signals were always visible in Osam1-1 meiocytes (Figure 5B), indicating that the homologous chromosome pairing is perturbed in this mutant.In wild type, almost all telomeres were adjacent to the nuclear envelope and clustered, showing a typical bouquet configuration (Figure 5C). However, in Osam1-1, the telomeres scattered throughout the nucleus randomly; no bouquet was formed (Figure 5D), further showing that the PMC development had halted at leptotene.

Figure 5 Detection of homologous chromosome pairing and telomere bouquet formation. (A, B) Chromosomes probed with 5S rDNA in wild type (A) and the Osam1-1 mutant (B). (C, D) Telomere bouquet formation revealed by FISH using pAtT4 as the probe in wild type (C) and the Osam1-1 mutant (D). Chromosomes are stained with DAPI. Scale bars, 5 μm. .

Expression

• Western blot analysis showed that the OsAM1 protein was present in young panicles, but not in vegetative organs, suggesting that OsAM1 expression is subjected to regulation at the translational level.

Evolution

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Labs working on this gene

• State Key Laboratory of Plant Genomics and Center for Plant Gene Research, Institute of Genetics and Developmental Biology, Chinese Academy of Sciences, Beijing 100101, China
• Key Laboratory of Plant Functional Genomics of Ministry of Education, Yangzhou University, Yangzhou 225009, China