Cytoplasmic calcium in silk trichomes after pollen grain reception and post-pollination changes of the electric potential in pistil tissues of maize

Abstract

Pollination stimulates egg cell maturation in the maize ear [1]. Thus, we investigated early post-pollination events in Zea mays to indicate putative elements of the signalling pathway. Pollen grain hydration, germination and pollen tube growth proceeded on the silk fragments in a humid chamber similarly as in planta. Cytoplasmic Ca²⁺ level was monitored by laser scanning microscopy in the stigma trichomes after application of pollen grains or mechanical stimuli. [Ca²⁺]_c increased at pollination site in 80% of the trichomes when pollen hydration and germination occurred. This increase usually appeared 40–140 s after pollination (AP). If pollen hydration failed, [Ca²⁺]_c was stable in 90% of the trichomes. The same was found after application of quartz sand to the trichomes or gently touch with a needle. Changes of [Ca²⁺]_c in the pollinated silk trichomes seem to be related to pollen hydration. As other ion fluxes during pollen reception on the stigma could evoke electrophysiological events, the electric potential was measured in pistil tissues. Within 20 min after application of maize pollen or quartz sand, the electric potential of the silk showed three series of spikes. In the nucellus surrounding the embryo sac with the egg cell, similar electric changes were delayed by 4 min and followed the initial hyperpolarisation by 10–20 mV. The electric signal propagated through the silk towards the ovule at the rate of 3–5 cm min⁻¹. Our results indicate that cytophysiological changes in pistil tissues detected as early variations of the electric potential are involved in stigma-to-ovule signalling in maize after pollination.

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.