Tag Archives: self-incompatibility

Live! It’s The Pollination Show!

Live time-lapse imaging of pollination in Brassica rapa

Live time-lapse imaging of pollination in Brassica rapa

When pollen grains are released from anthers and then captured on the surface of the stigma, they obtain water and other resources from the stigma for germination and pollen tube elongation. Once the pollen tube penetrates the outer layer of the stigmatic cell wall, it grows in the apoplastic space down to the ovary for fertilization. In the ovary, two sperm cells are released from the tip of the pollen tube; one of these fertilizes the egg cell and the other the central cell, termed double fertilization, resulting in seed development. Because pollination is mediated by wind, insects and birds, pollen of other species, pathogens and dust, as well as pollen of the same species, may arrive at the stigma surface. Therefore stigmas require the ability to select suitable pollen to bring about successful fertilization.

Although pollination has been studied for many years, the molecular mechanisms involved are still largely unclear. An accurate knowledge of morphological aspects of pollination is also still far from complete. A new paper in Annals of Botany focuses on pollen behaviour during pollination. For the morphological characterization of pollination, time-lapse image analysis was used to record detailed pollen behaviour during self- and cross-pollinations in Brassica rapa. This approach demonstrated that pollen exhibits various behaviours on an individual stigma, in both self- and cross-pollinations, and the ratios of the different types of pollen behaviour are critical for successful pollination.

From these observations of pollen behaviour it is clear that supply of the correct amount of water to pollen is one of the key stages in successful pollination, and this process consists of multiple components, of hydration, rehydration and dehydration systems, and involves transport of water to and from pollen grains. The precise pollination and self-incompatibility system response in Brassica can be accomplished only when the appropriate balance and co-ordination of these processes are achieved.

 

Time-lapse imaging of self- and cross-pollinations in Brassica rapa. Annals of Botany (2013) 112 (1): 115-122. doi: 10.1093/aob/mct102
Pollination is an important process in the life cycle of plants and is the first step in bringing together the male and female gametophytes for plant reproduction. While pollination has been studied for many years, accurate knowledge of the morphological aspects of this process is still far from complete. This study therefore focuses on a morphological characterization of pollination, using time-series image analysis of self- and cross-pollinations in Brassica rapa. Time-lapse imaging of pollen behaviour during self- and cross-pollinations was recorded for 90 min, at 1 min intervals, using a stereoscopic microscope. Using time-series digital images of pollination, characteristic features of pollen behaviours during self- and cross-pollinations were studied. Pollen exhibited various behaviours in both self- and cross-pollinations, and these were classified into six representative patterns: germination, expansion, contraction, sudden contraction, pulsation and no change. It is noteworthy that in ‘contraction’ pollen grains shrunk within a short period of 30–50 min, and in ‘pulsation’ repeated expansion and contraction occurred with an interval of 10 min, suggesting that a dehydration system is operating in pollination. All of the six patterns were observed on an individual stigma with both self- and cross-pollinations, and the difference between self- and cross-pollinations was in the ratios of the different behaviours. With regard to water transport to and from pollen grains, this occurred in multiple steps, before, during and after hydration. Thus, pollination is regulated by a combination of multiple components of hydration, rehydration and dehydration systems. Regulated hydration of pollen is a key process for both pollination and self-incompatibility, and this is achieved by a balanced complex of hydration, dehydration and nutrient supply to pollen grains from stigmatic papilla cells.

 

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Cultivar identification and maintenance in self-sterile Rubus

Cultivar identification and maintenance in self-sterile Rubus

Cultivar identification and maintenance in self-sterile Rubus

Preservation of cultivar purity is a particular challenge for plants that are self-incompatible, and have easily germinating seeds and vigorously spreading rhizomes. Kostamo et al. develop and test molecular and morphological identification methods for arctic bramble, Rubus arcticus. They find that the best morphological parameters are the length-to-width ratio of the middle leaflet and leaf margin serration. A particular characteristic, fingertip touch, is shown by electron microscopy to be related to the density and quality of the leaf hairs. Red raspberry (R. idaeus) SSR marker no. 126 is useful for differentiation of the eight arctic bramble cultivars tested. These identification methods are critical to secure the maintenance and management of R. arcticus, and the approaches adopted are equally applicable to other species with similar biology.

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Evolution and speciation in Sorbus

Evolution and speciation in Sorbus

Evolution and speciation in Sorbus

Interspecific hybridization and polyploidy are key processes in plant evolution and are responsible for ongoing genetic diversification in the genus Sorbus (Rosaceae). Ludwig et al. study mating systems of diploid, triploid and tetraploid taxa in a diversity ‘hotspot’ for Sorbus in south-west England and find mating inter-relationships to be complex, and to provide the driving force for hybridization and ongoing genetic diversification. In particular, the presence of self-incompatibility (SI) in triploid pseudogamous apomicts imposes a requirement for interspecific cross-pollination, thereby facilitating continuing diversification and evolution through rare sexual hybridization events. This is the first report of naturally occurring pseudogamous apomictic SI plant populations, and suggests that interspecific pollination, in combination with a relaxed endosperm balance requirement, is the most likely route to the persistence of these populations.

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Passerine pollination in a winter-flowering tree

Passerine pollination in a winter-flowering tree

Passerine pollination in a winter-flowering tree

Birds can be alternative pollinators for winter-flowering plants outside the tropics where low temperatures limit insect activity. Feng et al. observe pollinator visitation to loquat (Eriobotrya japonica, Rosaceae) and exclude birds and other animals from the flowers. They determine that in late winter two passerine birds (Pycnonotus sinensis and Zosterops japonicus) are effective pollinators, and that the perigynous flowers reward passerines with relatively large volumes of dilute nectar. The results suggest a possible association between perigyny and bird pollination.

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Mechanisms of self-incompatibility in grasses (Review)

Mechanisms of self-incompatibility in grasses (Review)q

Mechanisms of self-incompatibility in grasses (Review)

In grasses, self-incompatibilty (SI) is based on two loci, S and Z, but the mechanisms are much less well understood relative to other gametophytic SI systems. Klass et al. review recent progress towards elucidating grass SI, and by identifying Ca binding and kinase domains from subtracted Lolium stigma cDNA libraries they strengthen the case for involvement of calcium signalling and phosphorylation in the process. This is supported by the inhibition of Lolium SI by Ca2+ channel blockers, and by findings of increased phosphorylation activity during SI responses.

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S-RNase-based self-incompatibility (Review)

S-RNase-based self-incompatibility (Review)

S-RNase-based self-incompatibility (Review)

S-RNase-based self-incompatibility (SI) occurs in the Solanaceae, Rosaceae and Plantaginaceae, with S-RNases determining the specificity of pollen rejection in the pistil and S-locus F-box proteins fulfilling this function in pollen. McClure et al. introduce the genetics of SI and briefly describe the characteristics of S-RNases and pollen F-box proteins. Two alternative mechanisms have been proposed whereby compatibility is explained either as a result of degradation of non-self S-RNase or by its compartmentalization so that it does not have access to the pollen tube cytoplasm. These models are not necessarily mutually exclusive, but each makes different predictions about whether pollen compatibility or incompatibility is the default.

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Pollen–pistil interactions in the Asteraceae (Review)

Pollen–pistil interactions in the Asteraceae (Review)

Pollen–pistil interactions in the Asteraceae (Review)

Pollen–pistil interactions are an essential prelude to fertilization in angiosperms, and self-incompatibility (SI) is the best understood of these at a molecular level. Allen et al. review studies in the Asteraceae, and consider that recent cellular and molecular work in Senecio squalidus (Oxford ragwort) have challenged the belief that sporophytic SI and pollen–pistil interactions in Asteraceae and Brassicaceae are similar. The availability of a pool of pistil-specific genes for S. squalidus offers an opportunity to elucidate the molecular mechanisms of pollen–pistil interactions and SI in the Asteraceae.

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SI-mediated actin alterations in pollen (Review)

SI-mediated actin alterations in pollen (Review)

SI-mediated actin alterations in pollen (Review)

Self-incompatibility (SI) is generally controlled by the S-locus, and comprises allelic pollen and pistil S-determinants. Poulter et al. review and discuss our current understanding of the cytoskeletal alterations induced in incompatible pollen during SI and their relationship with programmed cell death in Papaver rhoeas, focussing on data relating to the formation of F-actin punctate foci. Analysis has identified differences between proteins associated with F-actin from SI-induced pollen samples and those associated with F-actin in untreated pollen. This provides candidate proteins implicated in either the formation or stabilization of the punctate actin structures formed during SI.


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S-RNase-based self-incompatibility in Petunia

S-RNase-based self-incompatibility in Petunia
S-RNase-based self-incompatibility in Petunia

In Solanaceae-type self-incompatibility, the specificity of self/non-self interactions between pollen and pistil is controlled by two polymorphic genes at the S-locus, with S-locus F-box gene (SLF or SFB) controlling pollen specificity and S-RNase gene controlling pistil specificity. Meng et al.  summarize their recent results on the identification and functional studies of S-RNase and SLF in Petunia inflata, and a protein-degradation model is proposed to explain the biochemical mechanism for specific rejection of self-pollen tubes by the pistil.

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