RESUMEN
Abortion of fertilized ovaries at the tip of the ear can generate significant yield losses in maize crops. To investigate the mechanisms involved in this process, 2 maize hybrids were grown in field crops at 2 sowing densities and under 3 irrigation regimes (well-watered control, drought before pollination, and drought during pollination), in all possible combinations. Samples of ear tips were taken 2-6 days after synchronous hand pollination and used for the analysis of gene expression and sugars. Glucose and fructose levels increased in kernels with high abortion risk. Several FASCICLIN-LIKE ARABINOGALACTAN PROTEIN (FLA) genes showed negative correlation with abortion. The expression of ZmFLA7 responded to drought only at the tip of the ear. The abundance of arabinogalactan protein (AGP) glycan epitopes decreased with drought and pharmacological treatments that reduce AGP activity enhanced the abortion of fertilized ovaries. Drought also reduced the expression of AthFLA9 in the siliques of Arabidopsis thaliana. Gain- and loss-of-function mutants of Arabidopsis showed a negative correlation between AthFLA9 and seed abortion. On the basis of gene expression patterns, pharmacological, and genetic evidence, we propose that stress-induced reductions in the expression of selected FLA genes enhance abortion of fertilized ovaries in maize and Arabidopsis.
Asunto(s)
Proteínas de Arabidopsis/genética , Arabidopsis/genética , Regulación de la Expresión Génica de las Plantas , Mucoproteínas/genética , Proteínas de Plantas/genética , Semillas/fisiología , Zea mays/genética , Arabidopsis/fisiología , Proteínas de Arabidopsis/metabolismo , Quimera , Sequías , Glucósidos/farmacología , Mucoproteínas/metabolismo , Óvulo Vegetal/genética , Óvulo Vegetal/fisiología , Floroglucinol/análogos & derivados , Floroglucinol/farmacología , Proteínas de Plantas/metabolismo , Plantas Modificadas Genéticamente , Polinización , Semillas/genética , Zea mays/efectos de los fármacos , Zea mays/fisiologíaRESUMEN
Female gametogenesis in most flowering plants depends on the predetermined selection of a single meiotically derived cell, as the three other megaspores die without further division or differentiation. Although in Arabidopsis thaliana the formation of the functional megaspore (FM) is crucial for the establishment of the gametophytic generation, the mechanisms that determine the specification and fate of haploid cells remain unknown. Here, we show that the classical arabinogalactan protein 18 (AGP18) exerts an active regulation over the selection and survival of megaspores in Arabidopsis. During meiosis, AGP18 is expressed in integumentary cells located in the abaxial region of the ovule. Overexpression of AGP18 results in the abnormal maintenance of surviving megaspores that can acquire a FM identity but is not sufficient to induce FM differentiation before meiosis, indicating that AGP18 positively promotes the selection of viable megaspores. We also show that all four meiotically derived cells in the ovule of Arabidopsis are competent to differentiate into a gametic precursor and that the function of AGP18 is important for their selection and viability. Our results suggest an evolutionary role for arabinogalactan proteins in the acquisition of monospory and the developmental plasticity that is intrinsic to sexual reproduction in flowering plants.
Asunto(s)
Proteínas de Arabidopsis/genética , Arabidopsis/genética , Glicoproteínas de Membrana/genética , Óvulo Vegetal/genética , Arabidopsis/crecimiento & desarrollo , Arabidopsis/metabolismo , Proteínas de Arabidopsis/metabolismo , Gametogénesis en la Planta/genética , Regulación del Desarrollo de la Expresión Génica , Regulación de la Expresión Génica de las Plantas , Glucuronidasa/genética , Glucuronidasa/metabolismo , Meiosis/genética , Glicoproteínas de Membrana/metabolismo , Microscopía Fluorescente , Mucoproteínas/genética , Mucoproteínas/metabolismo , Óvulo Vegetal/crecimiento & desarrollo , Óvulo Vegetal/metabolismo , Plantas Modificadas Genéticamente , Reacción en Cadena de la Polimerasa de Transcriptasa InversaRESUMEN
Research on speciation of marine organisms has lagged behind that of terrestrial ones, but the study of the evolution of molecules involved in the adhesion of gametes in free-spawning invertebrates is an exception. Here I review the function, species-specificity, and molecular variation of loci coding for bindin in sea urchins, lysin in abalone and their egg receptors, in an effort to assess the degree to which they contribute to the emergence of reproductive isolation during the speciation process. Bindin is a protein that mediates binding of the sperm to the vitelline envelope (VE) of the egg and the fusion of the gametes' membranes, whereas lysin is a protein involved only in binding to the VE. Both of these molecules are important in species recognition by the gametes, but they rarely constitute absolute blocks to interspecific hybridization. Intraspecific polymorphism is high in bindin, but low in lysin. Polymorphism in bindin is maintained by frequency-dependent selection due to sexual conflict arising from the danger of polyspermy under high densities of sperm. Monomorphism in lysin is the result of purifying selection arising from the need for species recognition. Interspecific divergence in lysin is due to strong positive selection, and the same is true for bindin of four out of seven genera of sea urchins studied to date. The differences between the sea urchin genera in the strength of selection can only partially be explained by the hypothesis of reinforcement. The egg receptor for lysin (VERL) is a glycoprotein with 22 repeats, 20 of which have evolved neutrally and homogenized by concerted evolution, whereas the first two repeats are under positive selection. Selection on lysin has been generated by the need to track changes in VERL, permitted by the redundant structure of this molecule. Both lysin and bindin are important in reproductive isolation, probably had a role in speciation, but it is hard to determine whether they meet the strictest criteria of "speciation loci," defined as genes whose differentiation has caused speciation.
Asunto(s)
Evolución Molecular , Gastrópodos/fisiología , Glicoproteínas/genética , Mucoproteínas/genética , Erizos de Mar/fisiología , Animales , Proteínas del Huevo/genética , Proteínas del Huevo/metabolismo , Femenino , Gastrópodos/genética , Glicoproteínas/metabolismo , Masculino , Mucoproteínas/metabolismo , Mucoproteínas/fisiología , Polimorfismo Genético , Receptores de Superficie Celular/genética , Receptores de Superficie Celular/metabolismo , Aislamiento Reproductivo , Erizos de Mar/genética , Selección Genética , Especificidad de la EspecieRESUMEN
Classical arabinogalactan proteins (AGPs) are an abundant class of cell surface proteoglycans widely distributed in flowering plants. We have used a combination of enhancer detection tagging and RNA interference (RNAi)-induced posttrancriptional silencing to demonstrate that AGP18, a gene encoding a classical arabinogalactan protein, is essential for female gametogenesis in Arabidopsis thaliana. AGP18 is expressed in cells that spatially and temporally define the sporophytic to gametophytic transition and during early stages of seed development. More than 75% of the T1 transformants resulted in T2 lines showing reduced seed set during at least three consecutive generations but no additional developmental defects. AGP18-silenced T2 lines showed reduced AGP18 transcript levels in female reproductive organs, the presence of 21-bp RNA fragments specific to the AGP18 gene, and the absence of in situ AGP18 mRNA localization in developing ovules. Reciprocal crosses to wild-type plants indicate that the defect is female specific. The genetic and molecular analysis of AGP18-silenced plants containing a single T-DNA RNAi insertion suggests that posttranscriptional silencing of AGP18 is acting both at the sporophytic and gametophytic levels. A cytological analysis of all defective AGP18-RNAi lines, combined with the analysis of molecular markers acting at key stages of female gametogenesis, showed that the functional megaspore fails to enlarge and mitotically divide, indicating that AGP18 is essential to initiate female gametogenesis in Arabidopsis. Our results assign a specific function in plant development to a gene encoding a classical AGP.