Cytoplasmic calcium in silk trichomes after pollen grain reception and post-pollination changes of the electric potential in pistil tissues of maize
Introduction
In many angiosperms, pollination regulates developmental events that make the flower no longer attractive to pollinators but ensure fertilisation and successful seed development (for reviews see [2], [3], [4]). It is assumed that this is mediated by phytohormones. Besides the most visible perianth colour change and wilting or ovary growth, other structural and biochemical responses to pollination have been detected before the actual fertilisation. Gene expression, enzyme activity and protein synthesis increased in various parts of pollinated pistils [5], [6], [7], [8], [9]. Pollination-triggered synergid degeneration has been described in several species (for review see [10]), and pollination stimulated maturation of the ovules and embryo sacs in orchids [11]. Recently, a post-pollination calcium rise and redistribution of calmodulin-like protein were found in the transmitting tract of petunia [12], [13].
Our previous study [1] showed that in maize ears the egg cell maturation is accelerated by pollination. This process was triggered early after the deposition of pollen, and a mechanical stimulus seemed to be of importance as “pollination” with sand had a very similar effect on the egg cells. Presumably, ion fluxes are the first events at the pollination site. Calcium ion is involved in many signalling pathways [14], [15], and it seems also to interact with displacement of other ions which are connected to electrical events [16]. This is true for growing pollen tubes [17], [18] but extensive work is needed on the female tissues participating in pollen reception and pollen tube growth. As found in several species [19], [20], [21], [22], [23], one of the earliest physiological responses to pollination are changes of the electric potential in the stigma and the style. Electrical signalling plays an important role in interorgan communication in wounded or water-stressed plants [24], [25]. Thus, the idea proposed by Lysikov and Dukhovnyi in 1966 [26], i.e. that ovaries receive information as electric signals evoked by pollination, requires a strong confirmation by new data.
This paper deals with very early events which occur after pollination (AP) in Zea mays. We investigated changes of cytoplasmic Ca2+ in the silk trichomes receiving single pollen grains, the changes of electric potential in pistil tissues after pollination, and the propagation of the electric signal from the pollination site to the ovule. Our results indicate that changes in [Ca2+]c at the pollination site are related to pollen hydration and suggest the importance of the electric signalling in the pollinated maize pistil.
Section snippets
Plant material and microscopic observations
Plants of Z. mays L. (line A 188, seeds from the INRA Plant Breeding Station, Domaine de Crouelle, Clermont-Ferrand, France) were grown in a climate controlled chamber with 16 h light per 8 h dark photoperiod (photon flux density of 560 μmol m−2 s−1), at 24/20 °C and 70/80% relative humidity. The first ears appearing on the plants were covered with paper bags before silk emergence and used for experiments at optimum silk receptivity, i.e. at the external silk length of 11–13 cm. Fluorochromatic
Early pollination events on stigma trichome
We compared pollen grain hydration and germination after pollination in planta, after pollination of ears attached to stem fragments, and after single pollen grain application to a trichome in our semi vivo system under a microscope. In all experiments 3–5 min after pollination, meniscus appeared at the contact site of the pollen grain with trichome cells (Fig. 1A). Then the pollen tube tip was formed at about 10 min AP (Fig. 1B). After another 2–5 min, short pollen tubes were observed on pollen
Discussion
In maize, egg cells differentiate within the embryo sacs during the female inflorescence development and during the receptivity period. This process is accelerated by pollination [1]. Looking for putative elements of the pollination-induced signalling between the stigma and ovule, we investigated cytoplasmic Ca2+ at the pollination site and changes of the electric potential in the maize silk and nucellus. Silk fragments were pollinated in a humid chamber in our experiments on calcium, and
Acknowledgements
This work was supported by the French Ministry of Foreign Affairs and the KBN Polish State Committee for Scientific Research (projects 76327, 98084, 01847, KBN grant 6P04C 09212). We thank Drs. A.-F. Antoine and J.-E. Faure for initiation into confocal microscopy, and Dr. T. Wyka for his comments on the manuscript and for correction of the English.
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