Cross Pollen Research Articles

Solanum carolinense L.

<i>Solanum carolinense</i> L.

The influence of self-pollen deposition on female reproductive success in a self-incompatible plant, Akebia quinata.

Geitonogamy is inevitable in hermaphrodite and monecious. Even for self-incompatible species, the negative effects of self-pollen are unavoidable when geitonogamous or self-mating occurs. However, the influence of self-pollen on consecutive development of flowers (e.g., fruiting and seeding) was seldom evaluated. Here, the self-incompatible monecious species, Akebia quinata, was used to estimate the influence of self-pollen deposition. We evaluated the extent of pollen limitation and geitonogamous mating under natural conditions by count of stigmatic pollen load and pollen tracking experiment. Hand pollination with different amount and combinations of self vs. cross pollen grains was applied to detect the response of fruit and seed set. The results showed that geitonogamy and pollen limitation occurred under natural conditions in A. quinata. Carpel numbers, ratio of self- and cross-pollen, and the interactive effect of ratio of self- and cross-pollen and total mixed pollen numbers, and not total pollen grain number, determined the effect of self-pollen on female reproductive success. The effect of self-pollen depended on its intensity. In general, the transfer of self-pollen significantly affected young fruit set. However, a little self-pollen together with cross-pollen did not reduce young fruit production. Although self-incompatible plants have evolved physiological mechanisms that reduce self-fertilization, our results provide new insights into the effects of self-pollen and the adaptive significance of self-incompatible monecious species.

Pollination services in a macadamia cultivar depend on across‐orchard transport of cross pollen

Abstract Pollination services are critical in entomophilous crops. Relationships between pollinator visits and fruit set and mass are complex and optimal strategies to improve pollination are poorly defined. We often do not understand what proportions of the crop result from self‐ or cross‐pollination, how cross‐pollination affects crop quality and how far cross‐pollen is transported. We quantified proportions of self‐ and cross‐paternity in nuts of the partially self‐incompatible macadamia cultivar 816 at increasing distances from cross‐pollen sources. We estimated the distance of effective pollen movement, evaluated how pollination by different parents affected nut size and nutritional quality, and assessed whether distance from a cross‐pollen source determined proportions of cross‐pollinated nuts, nut size and number of nuts harvested. Almost all nuts resulted from cross‐pollination, even at 184 m (23 rows) from a cross‐pollen source. The cross‐pollen cultivars in closest proximity were usually the pollen parents. Cross‐pollination increased nut size and altered kernel nutritional quality. The number of harvested nuts decreased with increasing distance from a cross‐pollen source in one orchard. Nut size was generally not affected by distance from a cross‐pollen source. Synthesis and applications. Macadamia nut production resulted almost entirely from cross‐pollination. Crop production was dependent on across‐orchard transport of cross pollen, sometimes over at least 184 m. This shows the importance of pollen genotype, not just pollen quantity, in ensuring an effective pollination service. The number of harvested nuts decreased at 24 m from a cross‐pollen source, suggesting that different cultivars should be inter‐planted more closely in macadamia orchards. We need to better understand pollinator behaviour, including what genotype of pollen the different flower visitors carry, and how far and in what direction they transport pollen across orchards, to optimise crop yield and quality.

Efficiency of Herkogamy in Narcissus bulbocodium (Amaryllidaceae).

Within the theoretical framework of the correlation pleiades, floral phenotypic integration has been proposed as a consequence of selection mediated by pollinators acting on floral characters. Here, we analyzed that assumption by studying the floral biology and pollination of the late-winter species Narcissus bulbocodium L. We found that the flowers of N. bulbocodium are pollinator-dependent (mainly on Bombus terrestris) in terms of achieving optimal levels of seed production (xenogamy mean seed-to-ovule ratio 64%). Flowers are phenotypically integrated, and only the inclusion of the stigma within the corona seems to have a positive and significant influence on the deposition of the pollen. It has been hypothesized that by including the stigma within the corolla, the flower has some control over the contact between stigma and pollinators that could lead to an “ordered herkogamy” as a way to promote outcross and avoid self-interference. Therefore, herkogamy was also studied, and while most previous studies have assessed the evolutionary significance of herkogamy by considering its relationship with outcrossing rates, we approach this phenomenon from a novel direction assessing the relationship between a proxy for herkogamy and the precision of the pollination process. Our results seem to support the existence of an optimal herkogamy distance that could maintain maximum levels of both pollen export and (cross) pollen capture. On the basis of the broad variability of herkogamy that we have found in N. bulbocodium and other data in the literature, we discuss the universality of the adaptive origin of herkogamy.

Frequency of Polyploids of Solanum tuberosum Dihaploids in 2X × 2X Crosses

When breeding diploid potatoes, tetraploid progeny can result from the union of 2n eggs and 2n pollen in 2x-2x crosses. Thirty-three crosses were made to examine tetraploid progeny frequency in 2x-2x crosses. All crosses were between S. tuberosum dihaploids and diploid self-compatible donors, M6 and DRH S6-10-4P17. Using chloroplast counting for ploidy determination, the frequency of tetraploid progeny was as high as 45% in one of the 33 crosses. Based upon single nucleotide polymorphism (SNP) genotyping, the tetraploid progeny were attributed to bilateral sexual polyploidization (BSP), which is caused by the union of 2n egg and 2n pollen. Dihaploids were identified that produce lower frequencies of 2n eggs. The results of this study suggest that S. tuberosum dihaploids with a high frequency of 2n eggs should be avoided in 2x - 2x crosses for diploid breeding programs.

The functional roles of 3D heterostyly and floral visitors in the reproductive biology of Turnera subulata (Turneroideae: Passifloraceae)

Heterostyly is a floral polymorphism that reduces conflicts between sexual functions (sexual interference), such as self-pollination in self-incompatible flowers and loss of pollen to incompatible stigmas. In many distylous pollination systems, there is a remarkable structural fit between the stigmas of each morph and the part of the pollinator body where compatible (disassortative) pollen is deposited. Nevertheless, inter-morph pollen transfer is often asymmetrical, with short-styled flowers (S-morph) receiving less compatible pollen than long-styled flowers (L-morph). One way to reduce the problem of sexual interference in short-styled flowers is to place stigmas outside of the flower centre, as seen in three-dimensionally (3D) heterostylous flowers. Heterostyly in Turnera has been extensively studied; however, 3D heterostyly has not been previously reported in this genus. The aims of this study were: 1) to elucidate the pollination system of T. subulata; and 2) to assess whether 3D heterostyly promotes disassortative pollination and reduces self-pollination in short-styled flowers in this taxon. The study population of Turnera subulata exhibited an isoplethic (1:1) ratio, with two well-defined morphs, and a high floral accuracy index for both morphs. Results from an experiment in which we manipulated the 3D orientation of the style showed that compatible crosses were favoured by the 3D flower morphology, consistent with a high floral accuracy. Our results also demonstrate the functional importance of 3D heterostyly in reducing intramorph pollination, as it increases the amount of cross pollen reaching the S-morph flowers. Our results provide the first observation of the presence of 3D heterostyly in this well-studied species and demonstrate the importance of this kind of morphological specialization in ensuring efficiency in an ecologically generalized pollination system.

Breeding systems in Cyrtanthus (Amaryllidaceae): variation in self-sterility and potential for ovule discounting.

Breeding systems of plants determine their reliance on pollinators and ability to produce seeds following self-pollination. Self-sterility, where ovules that are penetrated by self-pollen tubes that do not develop into seeds, is usually considered to represent either a system of late-acting self-incompatibility or strong early inbreeding depression. Importantly, it can lead to impaired female function through ovule or seed discounting when stigmas receive mixtures of self and cross pollen, unless cross pollen is able to reach the ovary ahead of self pollen ('prepotency'). Self-sterility associated with ovule penetration by self-pollen tubes appears to be widespread among the Amaryllidaceae. We tested for self-sterility in three Cyrtanthus species - C.contractus, C.ventricosus and C.mackenii - by means of controlled hand-pollination experiments. To determine the growth rates and frequency of ovule penetration by self- versus cross-pollen tubes, we used fluorescence microscopy to examine flowers of C.contractus harvested 24, 48 and 72h after pollination, in conjunction with a novel method of processing these images digitally. To test the potential for ovule discounting (loss of cross-fertilisation opportunities when ovules are disabled by self-pollination), we pollinated flowers of C.contractus and C.mackenii with mixtures of self- and cross pollen. We recorded full self-sterility for C.contractus and C.ventricosus, and partial self-sterility for C.mackenii. In C.contractus, we found no differences in the growth rates of self- and cross-pollen tubes, nor in the proportions of ovules penetrated by self- and cross-pollen tubes. In this species, seed set was depressed (relative to cross-pollinated controls) when flowers received a mixture of self and cross pollen, but this was not the case for C.mackenii. These results reveal variation in breeding systems among Cyrtanthus species and highlight the potential for gender conflict in self-sterile species in which ovules are penetrated and disabled by pollen tubes from self pollen.

The critical role of honeyeaters in the pollination of the catspaw Anigozanthos humilis (Haemodoraceae)

The pollination biology of Anigozanthos humilis (Haemodoraceae) was studied within a Banksia woodland reserve using a combination of field techniques and genetic analysis. Motion-triggered cameras were deployed on 25 flowering plants to identify visitors to flowers, quantify visitation rates and assess visitor behaviour. Entire A. humilis plants were caged to exclude potential floral visitors with six treatments: (i) bird and honey possum exclusion, allowing access by insects; (ii) bird and insect exclusion, allowing access by honey possums; (iii) total animal exclusion; (iv) open, allowing access by birds, insects and honey possums; (v) hand-pollination with cross-pollen; and (vi) hand-pollination with self-pollen. Open pollinated seed were genotyped to assess mating system parameters including outcrossing rate. From 23424 h of surveillance with camera traps, 109 visits were recorded, 106 (97%) of which were western spinebills (Acanthorhynchus superciliosus), with three visits by brown honeyeaters (Lichmera indistincta). We recorded an average of 8 (±4) visits per plant, equivalent to one visit every eleven days over the 2016 flowering season (mean = 63 days). Mean seed set per fruit was negligible (0.2–0.3 seed per fruit) for all pollination treatments except open- and hand-pollination with cross pollen. Mean seed set per fruit for hand cross-pollinated flowers (37.1 seed per fruit) was 37 times that of open pollinated flowers (1.0 seed per fruit). Outcrossing rate estimates were not significantly different to one. Our results indicate that at least for the study population and season, A. humilis was largely dependent on western spinebills for delivery of outcross pollen and resulting seed set, but visitation rates were low and pollen limitation was severe. Given the known sensitivity of western spinebills to habitat fragmentation, our results suggest flow on effects that may negatively impact on reproduction, and means that A. humilis may be particularly vulnerable to environmental changes that impact on bird pollinators.

Variation in floral organs in wild populations of Brassica rapa L.

In the hermaphroditic plants, floral morphological traits can significantly affect reproductive system by favouring the deposit of self or cross pollen. In the Brassica species, the self-incompatibility mechanism is undoubtedly the main cause of their allogamy; however, floral organs variation can modulate or reinforce the cross pollination level of the plants. In this study, we investigated floral variation among wild populations of Brassica rapa L. from Algeria. We estimated 20 floral traits, including sepals, petals, pistil, stamens and nectaries as well as ovules production. Floral parameters were measured using stereomicroscope. Each flower's ovule number and stigma-anther separation were also quantified under dissecting microscope. We found that all the flower parameters vary among populations or among individuals, especially stigma surface, nectary surface and stigma-anther separation. These three parameters have a coefficient of variation greater than 18,9%. Moreover, the individual variation is different according to the populations. These results suggest that floral morphology may affect pollination efficiency and as a consequence seed production in B. rapa wild populations but with different contributions according to the floral traits.

‘Arbequina’ olive is self-incompatible

Abstract ‘Arbequina’ is the olive cultivar preferred by growers for the design of the trendy high-density orchards due to its low vigor, high yield, and extraordinary oil quality. Although, ‘Arbequina’ is considered by many authors as self-compatible, increasing evidence questions its full self-compatibility. Our results clearly illustrate that, contrary to earlier assumptions, ‘Arbequina’ olive behaves as a self-incompatible variety. Self-pollen rejection in the style, low levels of self-fertilization and decreased initial and final fruit set after self-pollination in comparison to cross-pollination treatments clearly indicate that ’Arbequina’ is self-incompatible. Furthermore, even seed and fruit size were depressed in response to self-pollination, despite the lighter crop produced in self-pollinated shoots. This was observed in both ‘on’ and ‘off’ ‘Arbequina’ trees. Both pollinizers tested, ‘Manzanillo’ and ‘Picual’, matched results obtained under free open-pollination and significantly increased the yield obtained under self-pollination. Our results suggest the convenience of establishing pollinizers for large, high-density ’Arbequina’ orchards, especially in new areas of olive cultivation, where wind pollination may be not enough to ensure beneficial cross pollen flow.

REPRODUCTIVE BIOLOGY OF MANGO (MANGIFERA INDICA) IN A MEDITERRANEAN CLIMATE

The importance of mango growing contrasts with the paucity of information on its reproductive biology. In this work, the reproductive biology of mango was studied in the subtropical conditions of the Southern Spanish Mediterranean coast, using the cultivars ‘Osteen’, ‘Kensington’, ‘Kent’ and ‘Keitt’. In order to establish the appropriate developmental coordinates during the reproductive phase, phenology was studied from the start of the blooming season to fruit harvest. Also to detect the reasons behind the low fruit set in mango, pollen tube pathway and fertilization were analysed, using microscopy techniques. A good proportion of the flowers were male and dropped soon after anthesis. Pollen grains readily germinated, pollen tubes reached the base of the style within one day, penetrated the ovule through the nucellus and reached the embryo sac three days after pollination. Endosperm nuclei soon developed, but the embryo remained in a quiescent stage. A good proportion of the ovules degenerated at different stages of development. Temperature had an effect on pollen performance, and low temperatures hampered pollen germination. Finally, a compatibility assay was performed, showing no differences between self and cross pollen tube growth in the style. However a paternity analysis, with microsatellite markers, showed significant differences in outcrossing rate depending on the availability of pollinizer trees.

Pollen limitation and the contribution of autonomous selfing to fruit and seed set in a rewarding orchid

• Although rewarding orchids are believed to have a high pollination efficiency, pollination success is often low, suggesting that rewarding orchids may be prone to pollen limitation. Assuming that selfing contributes significantly to fruit and seed set (i.e., reproductive assurance) and that the quality of selfed seeds is high (i.e., low inbreeding depression), it can be hypothesized that under these circumstances populations evolve to facultative or even complete self-pollination.• In this study, we performed emasculation and pollen supplementation experiments in the field to assess the extent of outcross pollen limitation and the contribution of autonomous self-pollination to fruit and seed set in the rewarding orchid Epipactis palustris. Hand-pollinations using cross and self-pollen were performed in the greenhouse to investigate the impact of selfing on seed set and seed quality and to assess the magnitude of inbreeding depression.• Fruit set under natural conditions was high: ca. 70% of all flowers set fruit. Percentage fruit set of emasculated plants was 56%, implying that fruit set in about 14% of all flowers was the result of autonomous self-pollination. Pollen supplementation significantly increased fruit and seed set, indicating strong outcross pollen limitation. Hand-pollination with cross pollen resulted in significantly higher seed set and seed quality compared with manual self-pollination, indicating high levels of early inbreeding depression (δ = 0.46 ± 0.08).• Overall, these results indicate that, despite the high pollination efficiency, populations of this rewarding orchid species were strongly pollen limited. Costs of early inbreeding depression, on the other hand, were high, which may prevent rapid evolution of complete selfing.

Promiscuous breeding behaviour in relation to reproductive success in Grewia asiatica L. (Malvaceae)

Grewia asiatica L., an Asian shrub, is indigenous to India. Despite its immense importance in the Ayurvedic and traditional systems of medicine scientific and commercial exploration has been neglected. In this direction, we undertook systematic studies to understand the sexual system, breeding behaviour and reproductive phenology deployed by the species. Our results suggest that the species practices mixed mating as a consequence of temporal dichogamy of protandrous type and herkogamy with stigma slightly above stamens. Sequential floral evocation in acropetal order and profuse visitation by pollinators predispose G. asiatica to receive cross pollen. In a situation of non-receipt of pollen through insect pollinators, self-fertilization is prevalent by recurvation of receptive stigma towards dehiscent anthers. Seed set efficiency and fruit maturation are independent of pollen genotype (self/cross). Pre-emergent reproductive success (PERS), a measure of number of viable seeds that enter the ambient environment, was around 21%, which is indicative of moderate reproductive success. A self-compatibility index (SCI) and self-fertility index (SFI) showed values of 0.66 and 0.55, respectively, indicating the self-fertile and self-compatible nature of G. asiatica. The present study provides a detailed exposition of various reproductive features and breeding system prevalent in G. asiatica. These aspects are fundamental to understand the efficiency or failure of a species for genetic variation, crop improvement and evolutionary success.

Effect of pollen load, source and mixture on reproduction success of four cultivars of Citrullus lanatus (Thunb.) Matsumara and Nakai (cucurbitaceae)

Citrullus lanatus is cultivated in Côte d’Ivoire for its edible kernels which are used as a soup thickener. There are four distinct cultivars (one Bebu and three Wlêwlê). A better understanding of the reproduction system for those annual monoecious cultivars may improve its production. We studied the impact of pollen load, pollen source and diversity on the yield of fruits and seeds.Various pollination treatments were applied during four growing seasons to assess pollen load, pollen source and diversity (cross pollination with single source, cross pollination with multiple sources, self pollination and natural pollination) and pollen mixture. Pollination and reproductive success were determined by a fertilization index and by assessing fruits and seeds production. The self-compatibility and inbreeding depression index were also evaluated.Higher or lower pollen quantity decreases fertilization success of fruits and seeds production while medium pollen load leads to optimal yield. All cultivars were self compatible and higher level of inbreeding depression had been observed. When cross pollinated, yield was higher than natural pollination and self pollination yield. With manual pollination, cross pollination with multiple sources produces more fruits and seeds resulting in increased yield. Pollen diversity also improves fruits and seeds production. Pollen mixture will increase yield if the proportion of cross pollen in the mixture is higher than self pollen proportion.In conclusion, fruits and seeds production of those local self compatible cultivars were influenced by the amount of pollen load, pollen source and diversity, and also by proportion of pollen type in the pollen mixture.These results are therefore necessary in order to predict the evolutionary trajectory of mating systems and to make plant breeding program.

Inbreeding Depression in Solanum carolinense (Solanaceae) under Field Conditions and Implications for Mating System Evolution

The clonal weed Solanum carolinense exhibits plasticity in the strength of its self-incompatibility (SI) system and suffers low levels of inbreeding depression (δ) in the greenhouse. We planted one inbred and one outbred plant from each of eight maternal plants in a ring (replicated twice) and monitored clonal growth, herbivory, and reproduction over two years. Per ramet δ was estimated to be 0.63 in year one and 0.79 in year two, and outbred plants produced 2.5 times more ramets than inbred plants in the spring of year two. Inbred plants also suffered more herbivore damage than outbred plants in both fields, suggesting that inbreeding compromises herbivore resistance. Total per genet δ was 0.85 over the two years, indicating that S. carolinense is unlikely to become completely self-compatible, and suggesting that plasticity in the SI system is part of a stable mixed-mating system permitting self-fertilization when cross pollen limits seed production.

Four Marker Genotypes Related to Pollen and Spikelet Fertility in Wide Crosses of Rice (Oryza sativa L.)

Four Marker Genotypes Related to Pollen and Spikelet Fertility in Wide Crosses of Rice (Oryza sativa L.)

Pollinator‐mediated selfing erodes the flexibility of the best‐of‐both‐worlds mating strategy inBulbine vagans

Summary1. The best‐of‐both‐worlds hypothesis proposes that delayed autonomous selfing assures reproduction when pollinators are absent, but allows outcrossing when pollinators are present without incurring seed and pollen discounting. However, because self‐compatibility is necessary for delayed selfing, discounting can be incurred if pollinators mediate selfing concurrently with outcrossing. The extent that pollinator‐mediated selfing erodes the best‐of‐both‐worlds mating strategy is largely unknown.2. In this study, we assess how delayed selfing and pollinator‐mediated selfing contribute to reproductive output under contrasting pollination environments inBulbine vagans(Asphodelaceae).3. We manipulated floral phenotype under contrasting weather conditions, plant densities and floral display. We emasculated flowers at early‐flowering (allowing only outcrossing) or late‐flowering (eliminating delayed selfing) stages and compared reproductive output with intact flowers (outcrossing + pollinator‐mediated selfing + delayed selfing).4. During inclement weather, few or no pollinators visited flowers. Reproductive output was severely limited by cross pollen, and delayed selfing provided reproductive assurance. During fine weather, cross pollen was also limiting, but pollinator‐mediated selfing boosted reproductive output before delayed selfing could occur.5. On isolated plants, flowers received fewer pollinator visits and reproductive output was limited more by cross pollen compared with grouped plants. Despite these differences, reproductive output of both isolated and grouped plants was boosted to similar levels by pollinator‐mediated selfing before delayed selfing could occur.6. One‐ and three‐flowered plants were similarly limited by cross pollen. On three‐flowered plants, facilitated and geitonogamous selfing boosted reproductive output before delayed selfing could occur. By contrast, on one‐flowered plants geitonogamy was eliminated, and pollinator‐mediated selfing was reduced. This provided mating flexibility by retaining opportunities for subsequent outcrossing, although delayed selfing boosted reproductive output.7. Reproductive output was consistently limited by cross‐pollination. Yet, delayed selfing provided reproductive assurance only during inclement weather and on one‐flowered plants. Under other conditions, pollinators mediated selfing that could render ovules unavailable for outcrossing as selfing occurred and for outcrossing that might occur subsequently. We conclude that best‐of‐both‐worlds flexibility will often be eroded by pollinator‐mediated selfing that inevitability occurs with outcrossing.

Autogamy in Boronia falcifolia: consequences for inbreeding and persistence in fragmented coastal heaths

AbstractCoastal heath is becoming increasingly fragmented, resulting in reduced pollinator abundance affecting species viability. The reproductive ecology and inbreeding of a common coastal heath species, Boronia falcifolia (Rutaceae), was investigated. Pollination observations and experiments were carried out at one site, and inbreeding was assessed using genetic markers at eight populations. Four pollination treatments were assessed: cross‐pollination, self‐pollination, autogamous pollination and open (insect) pollination. Boronia falcifolia was found to be highly self‐compatible, frequently setting seeds from autogamous pollination. There were no significant differences between the autogamous, self and cross pollen treatments for any measures of reproductive success (pollen tube growth, fruit set, swollen locules and seed production). Insect activity resulted in fewer seeds per fruit than hand cross pollination, although pollinator visits were few. All populations studied were highly inbred. Autogamous self‐pollination is widespread in B. falcifolia and this combined with its ability to vegetatively regenerate enhances its potential for survival when pollinators are few.

'A life or death decision' for pollen tubes in S-RNase-based self-incompatibility

Mate choice is an essential process during sexual plant reproduction, in which self-incompatibility (SI) is widely adopted as an intraspecific reproductive barrier to inhibit self-fertilization by many flowering plants. Genetic studies show that a single polymorphic S-locus, encoding at least two components from both the pollen and pistil sides, controls the discrimination of self and non-self pollen. In the Solanaceae, Plantaginaceae, and Rosaceae, an S-RNase-based SI mechanism is involved in such a discrimination process. Recent studies have provided some important clues to how a decision is made to accept cross pollen or specifically to reject self pollen. In this review, the molecular features of the pistil and pollen S-specificity factors are briefly summarized and then our current knowledge of the molecular control of cross-pollen compatibility (CPC) and self-pollen incompatibility (SPI) responses, respectively, is presented. The possible biochemical mechanisms of the specificity determinant between the pistil and pollen S factors are discussed and a hypothetical S-RNase endosome sorting model is proposed to illustrate the distinct destinies of pollen tubes following compatible and incompatible pollination.

Brassica self-incompatibility

Self-incompatibility (SI) has emerged as an evolutionary strategy to enhance the genetic variability of plant species. In Brassica, it is controlled by a single multiallelic locus, the S-locus, encoding a receptor kinase (SRK) expressed in the stigma papilla cells and its ligand, a small protein (SCR) located in the pollen coat. Pollen rejection is achieved only when the receptor recognizes SCR coming from the same S-allele. If a single papilla cell is simultaneously pollinated by a self- and a cross-pollen grain, it is capable of distinguishing between the two and responding accordingly, rejecting self while accepting cross pollen. This phenomenon reveals that SI response is strictly localized and does not involve the whole papilla cell. It also suggests that the distribution of SRK inside the cell may play an important role in regulating this dual response. We have recently demonstrated that SRK is mostly intracellular, only small amounts being present in distinct domains of the plasma membrane (PM), where interaction with SCR occurs. Following ligand recognition, the receptor-ligand complex is endocytosed and degraded. Based on this, we propose a model of the significance of SRK intracellular trafficking for the functioning and specificity of SI response.