Embedded Files
Flower development is one of the most intensively studied developmental processes in plants. It is a central process in the life cycle of angiosperms, as flowers are the reproductive organs responsible for sexual reproduction and seed formation. The transition from vegetative to reproductive growth and the subsequent formation of floral organs are tightly controlled by genetic regulatory networks that integrate developmental cues, environmental signals, and hormonal pathways.
We are the first group to identify the Nucleostemin-like 1 (NSN1) gene from Arabidopsis and have demonstrated that NSN1 is required for normal apical and floral meristem development. NSN1 is mainly present in the nucleoli and is detectable in the nucleoplasm. The NSN1 gene is expressed in the inflorescence meristem and floral primordia. Mutant plants of nsn1 develop defective flowers on inflorescences that are characteristically terminated with the formation of a terminal carpelloid flowers (TCF). Double mutant plants develop typical flowers of the ag mutant with only sepals and pedals whereas the TCF defect of nsn1 is completely suppressed. Double mutants of nsn1, apetala 2 (ap2) develop defective flowers with enhanced TCF phenotypes. Thus, NSN1 acts to repress AG and plays an additive role with AP2 in floral organ specification. (Wang et al 2012).
During anther development, callose deposition is one of the earliest hallmarks of meiosis in the microsporocytes (pollen mother cells, PMCs). A thick callose layer is deposited between the plasma membrane of PMCs and the surrounding tapetum. This thick callose wall serves as a physical barrier to separate the meiocytes from one another and from somatic tissues. The callose wall also provides a mold for the subsequent primexine and pollen wall formation. After meiosis, four haploid microspores are encased in a callose wall to form a microspore tetrad. Enzymatic degradation of callose by callase (a β-1,3-glucanase secreted from the tapetum) is required for the release of individual microspores. We have demonstrated that mutant plants defective in Callose Synthase 5 (CalS5) could not deposit callose in the PMCs and microspore tetrads and produce deformed and degenerated pollen grains that are mostly male sterile. CalS9, CalS10, CalS11, and CalS12 also contribute to pollen development. Orthologs of CalS5 in other plants (such as rice) also play a key role in pollen development. Our results also show that KOMPEITO (KOM), a Rhomboid-like (Rho) serine protease, is required for CalS protein accumulation and callose deposition in the callose wall during pollen development in Arabidopsis (Kanaoka et al., 2022). Thus, callose is a transient but indispensable component of cell walls during plant gametogenesis. Understanding the molecular control of callose metabolism provides insights into plant fertility and offers potential strategies for crop improvement through the manipulation of reproductive processes.
Fig 1. NSN1 regulates floral meristem development.
(A) GFP-tagged NSN1 is localized to the nucleoli in BY-2 cells. (B) Heterozygous nsn1/+ plants have no growth phenotypes but are defective in reproduction. Homozygous nsn1 plants are tiny and yield only a few seeds per sillique. (C-D) The defects in flowers are more severe in nsn1 ap2-2 double mutant than ap2-2 single mutant. (E-G) Heterozygous nsn1/+ plants produce terminal capelloid flowers (TCF), while homozygous nsn1 flowers are severely defective. (H) NSN1 and AP2 both suppress the AG function to regulate inflorescence meristem (IM) determinacy and floral organ identity.
Fig 2. CalS5 is required for male sterility and pollen development.
(A-B) Vegetative plants and flower size of cals5 mutants are normal, but their length and size of silliques are reduced. (D-E) Carpels of cals5 mutants are normal and fertile, but their anthers are shrunken. (F-G) Anthers of cals5 mutants contain few round-shaped pollen grains and are 95% male sterile.
Fig 3. CalS1 is required for callose deposition during pollen development.
(A-D) Microspore tetrads of WT (A-B) and cals5 mutant (C-D). Tetrads of WT are encased in callose walls and are separated by clear boundaries. In cals5 mutants, the four microspores form a clump and the boundaries are barely recognizable. No callose is detected in the cell walls the mutant tetrads (D). (I-J) Longitudinal sections of anthers showing normal presence of viable, purple-stained pollen grains in WT (I) and lack of pollen grain in cals5 (J). A few deformed pollen grains are found in cals5 (J). (K-L) Viable pollen grain of WT and deformed pollen grain of cals5. Most of deformed pollen grains degenerate and disappear, while a few of the survived, deformed pollen grains of cals5 could germinate and are fertile.
Fig 4. Kom protease is required for CalS5 activity during pollen development.
(A-B) The kom mutant produces short silliques as in the cals5 mutant plants. (C-D) Microspore tetrads of WT are encased in callose walls. In kom mutant, the four microspores form a clump and the perpheral callose wall surrounding the tetrad is missing, whereas the interstatial walls between spores are intact (D). (E-F) Mature pollen grains of kom are deformed and shrunken as in cals5. (G-J) KOM is expressed in the pollen mother cells (PMC) undergoing meiosis (H), but not in PMC before meiosis (G) nor in microspore tetrads (I). Sense mRNA of KOM is used as the negative control (J). (K) Kom is an atypical serine protease and does not have protease activity. It may forman enzyme complex with another Rhomboid-like serine protease (RBL) to cleave the N-terminus of CalS5 zymogen. Without Kom, CalS5 is not active and its protein is unstable.
References
Cai Z, Su Y, Kong L, Ren G, Zheng L, Zhu M, Qiu B, Hong Z, Ye Y, Xue Y, Wu W, Duan Y (2025) JAGGED and DROOPING LEAF synergistically establish sexual organs and terminate floral meristem in rice. Plant J 123: e70346 (DOI: 10.1111/tpj.70346)
Dong X, Hong Z, Chatterjee J, Kim S, Verma DPS (2008) Expression of callose synthase genes and its connection with Npr1 signaling pathway during pathogen infection. Planta 229: 87-98
Dong X, Hong Z, Sivaramakrishnan M, Mahfouz M, Verma DPS (2005) Callose synthase (CalS5) is required for exine formation during microgametogenesis and pollen viability in Arabidopsis. Plant J. 42: 315-328
Kanaoka M, Shimizu K, Xie B, Urban S, Freeman M, Hong Z, Okada K (2022) KOMPEITO, an atypical Arabidopsis Rhomboid-related gene, is required for callose accumulation and pollen wall development. Int J Mol Sci 23: 5959 (DOI: 10.3390/ijms23115959)
Mjomba FM, Zheng Y, Liu H, Tang W, Hong Z, Wang F, Wu W (2016) Homeobox is pivotal for OsWUS controlling tiller development and female fertility in rice. Genes, Genomics and Genetics (G3, Bethesda) 6: 2013-2021.
Wang X, Gingrich DK, Deng Y, Hong Z (2012a) NSN1, a nucleostemin-like GTPase required for apical and floral meristem development in Arabidopsis. Mol Bio Cell 23: 1446-1456. PMID: 22357616
Wang X, Xie B, Hong Z (2012b) Nucleostemin-like 1 is required for embryogenesis and leaf development in Arabidopsis. Plant Mol Biol 78: 31-44. PMID: 22058024
Wang Z, Wang X, Xie B, Hong Z and Yang Q (2018) Arabidopsis NUCLEOSTEMIN-LIKE 1 (NSN1) regulates cell cycling potentially by cooperating with nucleosome assembly protein AtNAP1;1. BMC Plant Biol 18:99
Xie B, Deng Y, Kanaoka M, Okada K, Hong Z (2011) Expression of Arabidopsis callose synthase 5 (CalS5) in tobacco BY-2 cells: subcellular localization and effect on cell wall permeability. Plant Sci 183: 1-8. PMID: 22195570
Xie B, Wang X, Hong Z (2010) Precocious pollen germination in Arabidopsis lines with altered callose deposition during microsporogenesis. Planta 231: 809-823
Page updated
Report abuse