**1. Introduction**

The reproductive cycle in plants alternates between a haploid gametophytic and a diploid sporophytic generation. During the evolution from green algae (Charophytes) to land plants, the dominance of the gametophytic generation has gradually decreased in favour of the

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sporophytic generation. Originally, the gametophyte and sporophyte were separate inde‐ pendent organisms with very different appearances. In the first non-vascular land plants (liverworts, mosses) the gametophyte was still dominant, but in the first vascular land plants (ferns) the sporophyte prevailed. In ferns, the gametophyte is still an independent organism but with vastly reduced size. In seed-producing higher plants (Spermatophyta: angiosperms, gymnosperms), the gametophyte reduction became extreme (only a few cells) and both male and female gametophytes became physically part of the sporophyte [1, 2]. The emergence of the sporophyte as the dominant phase of the life cycle in seed plants has been attributed to genetic complementation and the capacity of the diploid stage to mask deleterious DNA mutations, an idea that was supported by the fact that land plants had to adapt to a more hostile environment. This argument has been disputed and the exact reason why the diploid sporo‐ phyte stage became dominant in land plants is still being debated [1, 3–5].

The ecological pressure to adapt to a dry environment with exposure to many new environ‐ mental stresses (water stress, UV light and heat) required a lot of morphological and devel‐ opmental changes during the evolution from mosses and ferns (Archegoniatae) to Spermatophyta [6]. The generation of the vascular system, roots, stomata and the hormonal system that regulates these developmental features in Spermatophytes evolved along with adaptation to new environmental challenges [7–9]. The next step in the evolution of land plants was the establishment of sexual reproduction in a land environment and the development of gametophytes with different sizes and sexes (heterospory). Sexual reproduction offers an opportunity to recombine combinations of genetic traits and spread genetic variability between populations. This new-found capacity played a major role during evolution in the adaptation of plants to the terrestrial environment [10–12]. Sexual reproduction became therefore the prevalent reproduction system in both plants and animals [13]. The immobility of the sporophyte in land plants makes pollen and seeds the only vector systems to exchange genetic information between plant populations. Pollen production and pollination are critical in the breeding system of land plants, and the large biodiversity that evolved in plant polli‐ nation mechanisms illustrates the tight linkage with environmental adaptation [11, 14].

The origin of pollen can be traced back to heterosporous Pteridophyta (vascular plants) [15– 17], which have microspores with features that are reminiscent of pollen: similar cell wall (intine and exine), storage reserves for the first stages of growth, reduction or absence of watery vacuoles at maturity [18]. In seed plants, the female gametophyte is immobile and develops totally inside the ovule of the ovary [19]. This makes pollen grains a crucial mobile vector for exchanging genetic information between different plant populations. The male gametophytes form inside the pollen sac in gymnosperms and in the anthers of angiosperm flowers (Figure 1) [2, 20, 21]. Pollen grains need to be dispersed from the anther and travel to the stigma to fertilise the immobile egg cell inside the ovule(s) of the ovary. This ovary can be located in the same flower, another flower of the same plant, a neighbouring plant or a more remote plant. The tough multi-layered pollen wall is an adaptation to protect the male gametes against environmental stresses during presentation and dispersal, while it is at the same time adapted for different pollen dispersal methods [22, 23]. The pollen dispersal methods and breeding systems in plants are amazingly diverse [14, 24]. Following domestication, many crop species are grown in environments that are vastly different from their original growth habitat. The breeding system of many crop species may therefore not be optimal for their current growth habitat, let alone whether it will be adapted to a future world with a different climate. In many staple crops (e.g. rice and wheat) male reproductive development is considered the 'Achilles tendon' of reproductive development, with massive yield losses under unexpected adverse weather conditions (heat, drought, cold) becoming increasingly common occurrences [25–28]. Although the generation of haploid male gametes in angiosperms occurs via a conserved pathway, there are many variations present in different plant species in the way this process proceeds. In many cases, this biodiversity can be associated with adaptations to particular environmental restraints. This paper will explore the complexity in angiosperm pollen development and investigate how it can contribute to a better understanding of abiotic stress tolerance of male reproductive development. The focus of this review paper will be on the interaction between environment and pollen developmental processes and not on the diversity that exists in pollen–stigma compatibility and plant breeding systems. A supplementary glossary of commonly used terms and definitions related to male gametophyte development is supplied for those readers who are less familiar with this subject (See Appendix).

sporophytic generation. Originally, the gametophyte and sporophyte were separate inde‐ pendent organisms with very different appearances. In the first non-vascular land plants (liverworts, mosses) the gametophyte was still dominant, but in the first vascular land plants (ferns) the sporophyte prevailed. In ferns, the gametophyte is still an independent organism but with vastly reduced size. In seed-producing higher plants (Spermatophyta: angiosperms, gymnosperms), the gametophyte reduction became extreme (only a few cells) and both male and female gametophytes became physically part of the sporophyte [1, 2]. The emergence of the sporophyte as the dominant phase of the life cycle in seed plants has been attributed to genetic complementation and the capacity of the diploid stage to mask deleterious DNA mutations, an idea that was supported by the fact that land plants had to adapt to a more hostile environment. This argument has been disputed and the exact reason why the diploid sporo‐

The ecological pressure to adapt to a dry environment with exposure to many new environ‐ mental stresses (water stress, UV light and heat) required a lot of morphological and devel‐ opmental changes during the evolution from mosses and ferns (Archegoniatae) to Spermatophyta [6]. The generation of the vascular system, roots, stomata and the hormonal system that regulates these developmental features in Spermatophytes evolved along with adaptation to new environmental challenges [7–9]. The next step in the evolution of land plants was the establishment of sexual reproduction in a land environment and the development of gametophytes with different sizes and sexes (heterospory). Sexual reproduction offers an opportunity to recombine combinations of genetic traits and spread genetic variability between populations. This new-found capacity played a major role during evolution in the adaptation of plants to the terrestrial environment [10–12]. Sexual reproduction became therefore the prevalent reproduction system in both plants and animals [13]. The immobility of the sporophyte in land plants makes pollen and seeds the only vector systems to exchange genetic information between plant populations. Pollen production and pollination are critical in the breeding system of land plants, and the large biodiversity that evolved in plant polli‐ nation mechanisms illustrates the tight linkage with environmental adaptation [11, 14].

The origin of pollen can be traced back to heterosporous Pteridophyta (vascular plants) [15– 17], which have microspores with features that are reminiscent of pollen: similar cell wall (intine and exine), storage reserves for the first stages of growth, reduction or absence of watery vacuoles at maturity [18]. In seed plants, the female gametophyte is immobile and develops totally inside the ovule of the ovary [19]. This makes pollen grains a crucial mobile vector for exchanging genetic information between different plant populations. The male gametophytes form inside the pollen sac in gymnosperms and in the anthers of angiosperm flowers (Figure 1) [2, 20, 21]. Pollen grains need to be dispersed from the anther and travel to the stigma to fertilise the immobile egg cell inside the ovule(s) of the ovary. This ovary can be located in the same flower, another flower of the same plant, a neighbouring plant or a more remote plant. The tough multi-layered pollen wall is an adaptation to protect the male gametes against environmental stresses during presentation and dispersal, while it is at the same time adapted for different pollen dispersal methods [22, 23]. The pollen dispersal methods and breeding systems in plants are amazingly diverse [14, 24]. Following domestication, many crop species

phyte stage became dominant in land plants is still being debated [1, 3–5].

704 Abiotic and Biotic Stress in Plants - Recent Advances and Future Perspectives

**Figure 1.** Schematic drawing of an ideal stamen (A), anther at microspore stage (B) and just after anther opening (C) with their components and functions. Water and some nutrients are transported by the vascular bundle from the mother plant via the filament towards the anther. Nutrients move to the tapetum via the connective tissue and compo‐ nents synthesised by the tapetum are then released into the loculus, where they are absorbed by the developing grains, and they are either utilised immediately or stored temporarily in the locular fluid, vacuole or amyloplasts.
