M. Herrero

Glycoprotein composition along the pistil of Malus x domestica and the modulation of pollen tube growth

BackgroundThe characteristics of pollen tube growth are not constant, but display distinct patterns of growth within the different tissues of the pistil. In the stigma, the growth rate is slow and autotrophic, whereas in the style, it is rapid and heterotrophic. Very little is known about the interactions between these distinct maternal tissues and the traversing pollen tube and the role of this interaction on the observed metabolism. In this work we characterise pollen tube growth in the apple flower and look for differences in glycoprotein epitope localization between two different maternal tissues, the stigma and the style.ResultsWhile immunocytochemically-detected arabinogalactan proteins were present at high levels in the stigma, they were not detected in the transmitting tissue of the style, where extensins were abundant. Whereas extensins remained at high levels in unpollinated pistils, they were no longer present in the style following pollen tube passage. Similarily, while abundant in unpollinated styles, insoluble polysaccharides such as β-glucans, were depleted in pollinated pistils.ConclusionsThe switch from autotropic to heterotrophic pollen tube growth correlates spatially with a change of glycoprotein epitopes between the stigma and the style. The depletion of extensins and polysaccharides following pollen tube passage in the style suggest a possible contribution to the acceleration of heterotrophic pollen tube growth, which would imply an active contribution of female tissues on prezygotic male–female crosstalk.This work was supported by Ministerio de Ciencia e Innovacion (MICINN)-FEDER [AGL2006-13529-C02-01, AGL 12621-C02-01, AGL 2012–40239], and Gobierno de Aragon [group A43].

Global warming and sexual plant reproduction

The sexual reproductive phase in plants might be particularly vulnerable to the effects of global warming. The direct effect of temperature changes on the reproductive process has been documented previously, and recent data from other physiological processes that are affected by rising temperatures seem to reinforce the susceptibility of the reproductive process to a changing climate. But the reproductive phase also provides the plant with an opportunity to adapt to environmental changes. Understanding phenotypic plasticity and gametophyte selection for prevailing temperatures, along with possible epigenetic changes during this process, could provide new insights into plant evolution under a global-warming scenario.

Pistil strategies controlling pollen tube growth

The progamic phase appears especially well suited for pollen-pistil interaction. During this phase the pistil supports pollen germination and tube growth, and provides an adequate environment, nutrition and directional cues. However, this support does not occur indiscriminantly and some mechanisms operating in the pistil constrain pollen tube growth. An active, regulated constraint is the self-incompatibility reaction, but moderate restrictions of pollen tube growth also occur in compatible matings. These moderate restrictions involve reduced support by the pistil and they operate through two main strategies; one is by decreasing the amount of support and the other is by varying the time at which this support is provided. In this minireview, we examine the evidence that is accumulating for both support and constraint of pollen tube growth by the pistil and discuss the benefits of this dual system.

Dynamics of pollen tube growth under different competition regimes

Pollen tube dynamics following different competition regimes were studied in sweet cherry (Prunus avium L.). In the process from pollination to fertilization, a constant reduction in the number of pollen tubes that travel along the style is observed. There could be two main causes of this reduction. One is a physical or physiological constraint consisting of the progressive decrease in the reserves and space available for pollen tube growth along the transmitting tissue of the style, and the other is genetic interaction both among the male gametophytes and between the male gametophytes and the female tissues of the flower. To evaluate the roles that these two forces play in reducing the number of pollen tubes that travel along the style, pistils were subjected to various pollen competition regimes by applying different mixtures of live and dead pollen onto the stigmata. The results obtained were similar when the experiment was repeated with different genotypes over 2 years, both in the laboratory and in the field. The role of stylar constriction is important, but it is not the only cause of pollen tube attrition because with low pollen loads fewer pollen tubes reach the different parts of the style than could fit therein. The fact that under different pollen competition regimes the number of pollen tubes is reduced by the same proportion in each stylar level indicates that genetic interactions play an important role in the control of pollen tube attrition.

Influence of genotype-temperature interaction on pollen performance

Pollen competition and selection have significant evolutionary consequences, but very little is known about how they can be modulated. We have examined in cherry (Prunus avium L.) how pollen performance is affected by the genotype of the pollen and by the environmental conditions under which it grows, namely the pistilar tissue and temperature. The different pollen donor genotypes tested in this work differed in their behaviour both in vitro and in vivo and this behaviour was modulated depending on the female recipient they grew on. Furthermore, there was a significant temperature‐genotype interaction that affected the pollen tube population census that succeeded in reaching the base of the style. The combination of these three factors, while enabling a capacity of response to variations in environmental pressures, could maintain variability in pollen performance avoiding the fixation of the genes that control pollen tube growth rate.

Influence of intraovular reserves on ovule fate in apricot (Prunus armeniaca L.)

Abstract In many plant species with multiovulate ovaries, a considerable reduction in the number of ovules takes place. However, the underlying physiological causes are not clear. In Prunus spp., although flowers present two ovules, usually only one seed is produced. We have followed the development and degeneration of the two ovules in apricot (Prunus armeniaca L.) and examined the extent to which carbohydrates within the ovule might be involved in determining the fate of the ovule. While the primary ovule grows in the days following anthesis, growth of the secondary ovule is arrested. Starch distribution along the different ovular tissues exhibits several changes that are different in the two ovules. Primary ovule growth is inversely related to starch content and this growth takes place independently of pollination since it occurs in the same way in pollinated and unpollinated flowers. In the secondary ovule, starch disappears simultaneously from all ovular structures and callose is layered at the chalazal end of the nucellus. The size of the secondary ovule does not change significantly from anthesis to degeneration, and callose starts to accumulate 5 days after anthesis. Likewise, this process occurs independently of pollination. These results are discussed in terms of the implications of the starch content of ovules in fertilization success and ovule fate.

Assessing allergen levels in peach and nectarine cultivars

BACKGROUND The lipid transfer protein Pru p 3 has been identified as a major peach fruit allergen. However, the putative peach member of the Bet v 1 family, Pru p 1, has been neither identified nor characterized. OBJECTIVES To determine the distribution and solubility properties of the main peach allergens and to quantify Pru p 3 and Pru p 1 levels in peach and nectarine cultivars. METHODS Peach peel and pulp were extracted using different buffers, and extracts were analyzed by means of sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunodetection using polyclonal antibodies against lipid transfer proteins, profilins, and Bet v 1 homologues. Pru p 3 was quantified in peach and nectarine cultivars using a sandwich enzyme-linked immunosorbent assay method. A similar method was developed to quantify Pru p 1. RESULTS A differential distribution between peel and pulp and different solubility properties were found for Pru p 3, Pru p 1, and peach profilin. Mean Pru p 3 levels were 132.86, 0.61, and 16.92 microg/g of fresh weight of peels, pulps, and whole fruits, respectively. The corresponding mean Pru p 1 levels were 0.62, 0.26, and 0.09 microg/g of fresh weight. Most US cultivars showed higher levels of both allergens than Spanish cultivars. CONCLUSIONS The different distribution and solubility properties of the main peach allergens can determine the quality of fruit extracts used as diagnostic tools. These differences, together with the natural variation of Pru p 3 and Pru p 1 levels among peach and nectarine cultivars, can be exploited to reduce peach allergenicity by means of industrial processing and plant breeding.

Gametophytic competition and selection.

A common phenomenon in higher plants is the alternation of two phases in their life cycle: a conspicuous diploid sporophyte that develops from the zygote and a reduced haploid gametophyte that develops from spores produced by meiosis. The gametophytic phase has been progressively reduced both in terms of duration and relative biomass along the evolutionary line (Heslop-Harrison 1979). Its reduction is at its greatest in Angiosperms, where typically, the mature male gametophyte (microgametophyte) consists of three haploid cells (the vegetative cell and the two sperm cells), and the female gametophyte (megagametophyte) of six haploid cells within a single binucleate cell. The pollen grain of flowering plants acts as a vector for the delivery of the two sperm cells (male gametes) to the megagametophyte of the ovule, accomplishing a unique feature of Angiosperms: double fertilization. Several reports deal with the physiology of the male gametophyte from its development in the anther up to fertilization (different aspects reviewed in Knox 1984a,b; Shivanna and Johri 1985; Knox et al. 1986; Dickinson 1987; Heslop-Harrison 1987; Mascarenhas 1989, 1990a,b). For many years it had been assumed that the majority of the genome in the male gametophyte was repressed with only the genes required to germinate, produce the pollen tube, and achieve fertilization being active (Brink and MacGillivray 1924; Heslop-Harrison 1979). The male gametophyte, however, now appears to be not just a simple transmission vector for the genome, but also an independent organism expressing its own genetic information (Mascarenhas 1989, 1990a,b). As such, we could expect selection to operate in the gametophytic phase resulting in a change in gene frequencies in the subsequent generation. Jones, as early as 1928, proposed that ‘in the spermatophytes fertilization is dependent upon the ability of pollen tube to grow, and discrimination between the gametes from different individuals and between different gametes from the same individuals is made before the germ cells come in contact’. Genetic differences among microgametophytes, if expressed, would result in gametophytic competition and gametophytic selection leading to nonrandom fertilization (Snow 1986a; Mulcahy and Mulcahy 1987; Schlichting et al. 1990).

Pollen performance as affected by the pistilar genotype in sweet cherry (Prunus avium L.)

SummaryDifferences in pollen performance in higher plants can result in significant selective advantages for some particular genotypes leading to both gametophytic and sexual selection. However, the possibility of selection among male gametophytes has been questioned since natural selection could lead to the fixation of alleles for the best competing male genotypes. These two apparently conflicting hypotheses could be reconciled if pollen performance, rather than operating in absolute terms, could be modulated by the pistilar genotype. Thus, pollen performance in vivo and in vitro has been compared in four sweet cherry (Primus avium L.) cultivars. Differences among the cultivars studied have been recorded in the speed and final pollen germination percentages both in vivo and in vitro. The results obtained show that the female genotype also modulates the final result of pollen performance. These two factors are not merely additive but, on the contrary, the interaction between them affects pollen behavior in vivo. This fact has clear implications for gametophytic and sexual selection since the best male-female combinations can be favored and this could explain the variability observed for pollen performance in nature.

Male gametophytic selection as a plant breeding tool

Abstract New plant breeding programs are constantly being developed in many horticultural species with the objective of responding to the increasing demand for new and better agronomic products. However, in most of those programs, and especially in those involving woody tree species, it would be desirable to use strategies that allow an early selection of genotypes. Gametophytic selection could be one of those strategies. In this review, three approaches that can be taken to use gametophytic selection in plant breeding programs are considered: genotype screening, effect of gametophytic competition on offspring vigor and manipulation of the reproductive phase to transmit selected traits to the next sporophytic generation. The advantages and limitations of the use of gametophytic selection in plant breeding are examined as well as the progress in this field and the future prospects of the method.