Abstract
During evolution of land plants, the haploid gametophytic stage has been strongly reduced in size and the diploid sporophytic phase has become the dominant growth form. Both male and female gametophytes are parasitic to the sporophyte and reside in separate parts of the flower located either on the same plant or on different plants. For fertilization to occur, bi-cellular or tri-cellular male gametophytes (pollen grains) have to travel to the immobile female gametophyte in the ovary. To survive exposure to a hostile atmosphere, pollen grains are thought to enter a state of complete or partial developmental arrest (DA). DA in pollen is strongly associated with acquisition of desiccation tolerance (DT) to extend pollen viability during air travel, but occurrence of DA in pollen is both species-dependent and at the same time strongly dependent on the reigning environmental conditions at the time of dispersal. Several environmental stresses (heat, drought, cold, humidity) are known to affect pollen production and viability. Climate change is also posing a serious threat to plant reproductive behavior and crop productivity. It is therefore timely to gain a better understanding of how DA and pollen viability are controlled in plants and how pollen viability can be protected to secure crop yields in a changing environment. Here, we provide an overview of how DA and pollen viability are controlled and how the environment affects them. We make emphasis on what is known and areas where a deeper understanding is needed.
Highlights
During evolution, land plants evolved from aquatic green algae (Charophytes) to colonize the Earth’s land mass
Viability depends on the degree of dehydration and the level of desiccation tolerance (DT) and developmental arrest (DA) and this is heavily influenced by environmental conditions such as temperature and humidity
Compatible pollen that are dispersed in a low DA state, i.e., with a water content higher than 30%, can germinate quickly and fertilize the ovule
Summary
Land plants evolved from aquatic green algae (Charophytes) to colonize the Earth’s land mass. The two pollen types differ in the percentage of cellular water, accumulation of biochemical components and morphology of pollen at the time of dispersal (Pacini and Dolferus, 2016) In both pollen types, viability depends on the degree of dehydration and the level of DT and DA and this is heavily influenced by environmental conditions such as temperature and humidity. In higher plants, seed and pollen both develop inside – and are dispersed from – involucre reproductive sporophytic structures: the anther in the case of pollen and the ovary in the case of the ovule (after fertilization, the ovary develops into the fruit while the ovule develops into the seed) At maturity, both pollen and seeds are dispersed in the environment in an arrested state (Franchi et al, 2011). The ABA catabolic enzyme ABA 8 -hydroxylase is expressed in the protective coleorhiza tissue of the embryo root and plays a role in dormancy release
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