The Diversity of the Pollen Tube Pathway in Plants: Toward an Increasing Control by the Sporophyte.

Jorge Lora,María Herrero,José I Hormaza

doi:10.3389/fpls.2016.00107

Abstract

Plants, unlike animals, alternate multicellular diploid, and haploid generations in their life cycle. While this is widespread all along the plant kingdom, the size and autonomy of the diploid sporophyte and the haploid gametophyte generations vary along evolution. Vascular plants show an evolutionary trend toward a reduction of the gametophyte, reflected both in size and lifespan, together with an increasing dependence from the sporophyte. This has resulted in an overlooking of the importance of the gametophytic phase in the evolution of higher plants. This reliance on the sporophyte is most notorious along the pollen tube journey, where the male gametophytes have to travel a long way inside the sporophyte to reach the female gametophyte. Along evolution, there is a change in the scenery of the pollen tube pathway that favors pollen competition and selection. This trend, toward apparently making complicated what could be simple, appears to be related to an increasing control of the sporophyte over the gametophyte with implications for understanding plant evolution.

Highlights

Alternation of generations between gamete-producing multicellular gametophytes and sporeproducing sporophytes is present in all land plants (Hofmeister, 1851)
This situation is reversed in seed plants, in which the sporophyte is the prominent phase and the gametophytic generation is the one that develops and spends most of its life enclosed within the tissues of the sporophyte
It appears clear that the construction of a pollen tube provides a prevalent useful way for the male gametes to travel toward the female gametophyte

Summary

INTRODUCTION

Alternation of generations between gamete-producing multicellular gametophytes and sporeproducing sporophytes is present in all land plants (Hofmeister, 1851). Clear differences in the subsequent contribution of the sporophytic tissues that envelop the female gametophyte and interact with the male gametophyte are observed along evolution This is shown in the male–female meeting place where pollen germinates, and along the pollen tube pathway toward the female gametophyte. Variations in carpel structure are apparent along the evolution We examine these differences, paying especial attention to the interaction between the male gametophyte and the female sporophyte during the different steps of the pollen tube pathway. The implications of these variations during plant evolution are considered to get an integrated view on the evolutionary trends in the control of pollen tube performance in seed plants and the role of the sporophyte in this process

THE CONSERVATION OF A POLLEN TUBE

CHANGING THE MEETING PLACE

Findings

TOWARD A MAJOR CONTROL OF THE SPOROPHYTE