Unfertilized ovary pushes wheat flower open for cross-pollination

Journal of Experimental Botany, Vol. 69, No. 3 pp. 399–412, 2018 doi:10.1093/jxb/erx410 Advance Access publication 30 November 2017 This paper is available online free of all access charges (see http://jxb.oxfordjournals.org/open_access.html for further details) RESEARCH PAPER Unfertilized ovary pushes wheat flower open for cross-pollination Takashi Okada*, J. E. A. Ridma M. Jayasinghe, Moureen Nansamba, Mathieu Baes, Patricia Warner, Allan Kouidri, David Correia, Vy Nguyen, Ryan Whitford and Ute Baumann School of Agriculture, Food and Wine, University of Adelaide, Plant Genomics Centre, Hartley Grove, Urrbrae, SA 5064, Australia Received 1 August 2017; Editorial decision 25 October 2017; Accepted 27 October 2017 Editor: Zoe Wilson, University of Nottingham, UK Abstract Bread wheat is strongly autogamous; however, an opportunity for outcrossing occurs when self-pollination fails and florets open. The first phase of floret opening at anthesis is short and induced by lodicule turgidity. Some wheat florets re-open post-anthesis for several days, known as the ‘second opening’, for which the underlying mechanisms are largely unknown. We performed detailed physiological, anatomical, and histological investigations to understand the biological basis of the flower opening process. Wheat florets were observed open when the ovary was unfertilized. Unfertilized ovaries significantly increased in radial size post-anthesis, pushing the lemma and palea apart to open the florets. The absence of fertile pollen was not directly linked to this, but anther filament elongation coincided with initiation of ovary swelling. The pericarp of unfertilized ovaries did not undergo degeneration as normally seen in developing grains, instead pericarp cells remained intact and enlarged, leading to increased ovary radial size. This is a novel role for the ovary pericarp in wheat flower opening, and the knowledge is useful for facilitating cross-pollination in hybrid breeding. Ovary swelling may represent a survival mechanism in autogamous cereals such as wheat and barley, ensuring seed set in the absence of self-fertilization and increasing genetic diversity through cross-pollination. Keywords: Fertilization, flower, ovary, pericarp, pollination, wheat. Introduction Bread wheat (Triticum aestivum L.) is an important staple crop for human nutrition and it is the third highest crop after maize and rice in world production (FAO, 2015). Wheat is a strong self-pollinating (autogamous) plant, and outcrossing rates in wheat are typically low (<1%), with most seed produced by selfing (Griffin, 1987; Martin, 1990; Hucl, 1996). Autogamy is one reproductive strategy of plants to ensure seed production, which can be advantageous when pollinators and potential mates are scarce (Jain, 1976; Herlihy and Eckert, 2002). Cleistogamy, closed pollination associated with floral morphology and/or pollination behaviour, also helps self-pollination (Frankel and Galun, 1977). These traits are beneficial for agricultural cereal crops to ensure stable yield by minimizing environmental effects on pollination. Outcrossing cereal crops, such as corn and rye, are wind pollinated, and success by wind pollination is highly dependent on prevailing weather conditions. In contrast, only a small fraction of wheat seed is set by wind pollination. This relates to accumulation of cleistogamic floral morphologies and pollination behaviours in wheat, leading to self-pollination. Some of these traits are a likely consequence of wheat’s breeding and selection history. For example, introduction of the ‘green revolution’ semi-dwarf genes Rht-B1/Rht-D1 into modern wheat cultivars not only prevented lodging and improved © The Author(s) 2017. Published by Oxford University Press on behalf of the Society for Experimental Biology. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited. *Correspondence: 400 | Okada et al. rate exists, with high outcrossing varieties tending to have a greater degree of spikelet opening (Hucl, 1996). In barley and wheat, there are two stages of flower opening. The ‘first opening’ is at anthesis and associated with lodicule swelling as described above, and this is well known for many cereal crops (de Vries, 1971; Frankel and Galun, 1977; Virmani, 1994; Heslop-Harrison, 1996). This ‘first opening’ is rather short temporally, lasting only 7–36 min for barley (Kirby et al., 1983) and 8–30 min for wheat (reviewed in de Vries, 1971; Pickett, 1993), and the duration of the ‘first opening’ is largely dependent on cultivars and weather conditions. After this time, florets close again as the lodicule collapses. Within a few days post-anthesis, some barley and wheat florets will open again, and this is known as the ‘second opening’ (Hoshikawa, 1960; Kirby et al., 1983; Pickett, 1993). This time, the ovary not the lodicule is involved in the opening process. Enlargement of the ovary is reported to be the cause of the ‘second opening’, and in most cases the ovaries were unpollinated. However, there is limited knowledge about this process from early studies, and physiological and biological mechanisms associated with the ‘second opening’ are largely unknown. Furthermore, there is a lack of understanding of the flowering mechanisms for autogamous cereal wheat that lead to cross-pollination. In this study, we aimed to investigate the anatomical and physiological mechanisms of the ‘second opening’ in wheat flowering and the biological link with other flowering processes. Observations in the flowering process were undertaken by measuring flower openness and ovary size in the male-sterile lines as well as emasculated male-fertile lines. Unfertilized ovaries significantly increased in radial size but showed only a slight increase in the vertical direction. This size increase exceeds the breadth of the lemma and forces the palea and lemma apart, resulting in an open floret. Suppression of pericarp degradation within unfertilized ovaries leads to the pericarp cells remaining intact and enlarging. We also demonstrated that barley ovaries swell when emasculated and remain unpollinated, but in a slightly different manner from those of wheat. Finally, we discuss the importance of cross-pollination in autogamous wheat. Materials and methods Plant materials The bread wheat (Triticum aestivum) spring type variety Excalibur and male-sterile mutant lines ms1 and ms5 in the cultivar Chris background (Driscoll, 1975; Sasakuma et al., 1978) were used. Plants were grown in growth room facilities (Narrabri Refrigeration & Air Conditioning, Narrabri, Australia) at the University of Adelaide, with temperature and daylight setting at 22 °C/15 °C (12 h day/12 h night: light on from 08.00 h to 20.00 h) for ovary size measurements (...truncated)