Live-cell imaging of early events following pollen perception in self-incompatible Arabidopsis thaliana

Early events occurring at the surface of the female organ are critical for plant reproduction, especially in species with a dry stigma. After landing on the stigmatic papilla cells, the pollen hydrates and germinates a tube, which penetrates the cell wall and grows towards the ovules to convey the male gametes to the embryo sac. In self-incompatible species within the Brassicaceae, these processes are blocked when the stigma encounters an incompatible pollen. Based on the generation of self-incompatible Arabidopsis lines and by setting up a live imaging system, we showed that control of pollen hydration has a central role in pollen selectivity. The faster the pollen pumps water from the papilla during an initial period of 10 min, the faster it germinates. Furthermore, we found that the self-incompatibility barriers act to block the proper hydration of incompatible pollen and, when hydration is promoted by high humidity, an additional control prevents pollen tube penetration into the stigmatic wall. In papilla cells, actin bundles focalize at the contact site with the compatible pollen but not with the incompatible pollen, raising the possibility that stigmatic cells react to the mechanical pressure applied by the invading growing tube.

Pollination Type Recognition from a Distance by the Ovary Is Revealed Through a Global Transcriptomic Analysis

Sexual reproduction in flowering plants involves intimate contact and continuous interactions between the growing pollen tube and the female reproductive structures. These interactions can trigger responses in distal regions of the flower well ahead of fertilization. While pollination-induced petal senescence has been studied extensively, less is known about how pollination is perceived at a distance in the ovary, and how specific this response is to various pollen genotypes. To address this question, we performed a global transcriptomic analysis in the ovary of a wild potato species, Solanum chacoense, at various time points following compatible, incompatible, and heterospecific pollinations. In all cases, pollen tube penetration in the stigma was initially perceived as a wounding aggression. Then, as the pollen tubes grew in the style, a growing number of genes became specific to each pollen genotype. Functional classification analyses revealed sharp differences in the response to compatible and heterospecific pollinations. For instance, the former induced reactive oxygen species (ROS)-related genes while the latter affected genes associated to ethylene signaling. In contrast, incompatible pollination remained more akin to a wound response. Our analysis reveals that every pollination type produces a specific molecular signature generating diversified and specific responses at a distance in the ovary in preparation for fertilization.

Characterization of the role of several COPI complex isoforms during the early acceptance of compatible pollen grains in Arabidopsis thaliana

COPI is a seven subunit coatomer complex, consisting of α, β, β′, γ, δ, ε, and ξ; in A. thaliana, COPI is necessary for retrograde transport from the Golgi to the Endoplasmic Reticulum, Golgi maintenance, and cell-plate formation in plant cells. Vesicle recruitment to the pollen contact point is required for pollen hydration and pollen tube penetration. To determine what other aspects of trafficking may be involved in the stigmatic papillae acceptance of compatible pollen, knock-out lines of several isoforms of the COPI complex were characterized in their roles during compatible pollination. Isoforms that were studied included α1-COPI, β-COPI, β′-COPI, γ-COPI and ε-COPI. Each mutant line was characterized in regards to pollen grain adherence, pollen tube penetration, and seed set. Of the mutant lines examined, α1-copi had the strongest phenotype with issues with compatible pollen grain adherence, tube germination and reduction in seed set while other lines had milder but visible retardation in compatible pollen acceptance. The data presented here are the first study of the role of the COPI complex in compatible pollinations and that certain subunit isoforms are required for compatible pollen acceptance.

Evaluation of self-(in)compatibility in the almond (Prunus amygdalus batsch) genotype population from the Slankamen hill, Serbia

Due to the importance of obtaining almond cultivars adapted to the agroecological conditions of Serbia, in the period 2005-2006 pollen viability and self-(in)compatibility in 19 almond genotypes selected from the seedling population on Slankamen hill, were studied. All analyzed almond genotypes had good (50-70%) or high (over 70%) pollen germination. The study of self-(in)compatibility was done by monitoring of the fruit set in the field and observing self-pollen growth by fluorescence microscopy. Self-incompatibility was confirmed in all the 19 genotypes by both methods. Pollen tube penetration was stopped mostly at the upper third of the style of all genotypes, with characteristic irregularities.

Seedless Watermelons Produced Via Soft-X-Irradiated Pollen

Watermelon fruit that results from pollination with pollen irradiated with soft-X-ray contains only empty seed, although the fruit develops to a normal size. In this study the processes of fertilization and embryo formation were compared between unirradiated and irradiated pollen in order to reveal the mechanisms by which seedless fruits are generated. The use of soft-X-irradiated pollen resulted in normal pollen tube penetration into the synergid and discharge of sperm cells. Two to three days after pollination, the sperm nucleus was present alongside the egg nucleus before fusion. The polar nuclei divided and the endosperm cell spread in the embryo sac before zygote division. A globular embryo was observed on day 7 to 10 after pollination. Subsequently, the embryo failed to differentiate to organ tissue and degenerated. These results indicate that double fertilization occurred after pollination with the irradiated pollen and that abortion of the embryo results from soft-X-ray induced chromosomal abnormalities in generative nucleus.

Sporophytic self-incompatibility in diploid and tetraploid races of Rutidosis leptorrhynchoides (Asteraceae)

Controlled pollinations were used to examine the mating system of diploid and tetraploid individuals of the endangered grassland herb Rutidosis leptorrhynchoides F.Muell. Crosses among unrelated plants gave 1.5–2 times as many fruit as crosses between plants that were half-sibs, while selfed crosses generally resulted in no fruit. Three classes of compatibility reaction were observed within outcross treatments: (1) reciprocal compatibility, (2) one-way compatibility and (3) reciprocal incompatibility. This is diagnostic of sporophytic control of self-incompatibility, which is characteristic of the Asteraceae. This is supported by the occurrence of a dry stigma and trinucleate pollen. Analysis of the behaviour of self- and outcross pollen on the stigma by using fluorescence microscopy shows that rejection of self-pollen does not all occur at one point but at a number of stages, with cumulative reductions in the adherence of pollen to the stigma, pollen germination, pollen tube penetration of the stigma and fertilisation. On the basis of both fruit set and pollen behaviour data, for any level of relatedness, tetraploid plants are about 20% less likely to be compatible with each other than diploid plants. This presumably reflects the greater likelihood of matching S alleles given the greater potential for polymorphism at the individual level. The occurrence of two plants, one diploid and one tetraploid, that set large amounts of fruit on selfing shows that self-incompatibility can break down.