Germline Development and Fertilization Mechanisms in Maize

Abstract

Maize is the most important agricultural crop used for food, feed, and biofuel as well as a raw material for industrial products such as packaging material. To increase yield and to overcome hybridization barriers, studies of maize gamete development, the pollen tube journey, and fertilization mechanisms were initiated more than a century ago. In this review, we summarize and discuss our current understanding of the regulatory components for germline development including sporogenesis and gametogenesis, the progamic phase of pollen germination and pollen tube growth and guidance, as well as fertilization mechanisms consisting of pollen tube arrival and reception, sperm cell release, fusion with the female gametes, and egg cell activation. Mechanisms of asexual seed development are not considered here. While only a few molecular players involved in these processes have been described to date and the underlying mechanisms are far from being understood, maize now represents a spearhead of reproductive research for all grass species. Recent development of essentially improved transformation and gene-editing systems may boost research in this area in the near future.

Maize Genic Male-Sterility Genes and Their Applications in Hybrid Breeding: Progress and Perspectives

“…The information about GMS obtained in Arabidopsis and rice provides opportunities to identify and utilize male sterility in economically important crops such as maize, barley, and wheat where GMS systems are not as well characterized (Ferná ndez Gó mez and Wilson, 2014;Gomez et al, 2015). So far, hundreds of maize GMS mutants have been generated and described (Timofejeva et al, 2013), and there have been many reviews focused on the cytologic morphology of anther/pollen cells and germline development in maize (Walbot and Egger, 2016;Zhou et al, 2017). However, only a small number of maize GMS genes have been identified and functionally characterized.…”

High expression in maize pollen correlates with genetic contributions to pollen fitness as well as with coordinated transcription from neighboring transposable elements

“…In flowering plants, sexual reproduction requires the development of reduced, haploid gametophytes from sporophytic, diploid parents. The mature female gametophyte, the embryo sac, includes the binucleate central cell and the egg cell (reviewed in [1,2]), each of which is fertilized by a sperm cell to generate the triploid endosperm and diploid embryo, respectively. The mature male gametophyte, pollen, consists of a vegetative cell harboring two sperm cells (reviewed in [3,4]).…”

“…In maize, a pollen tube must grow up to 30 cm through the silk to reach the female gametophyte, often competing with multiple pollen tubes to eventually enter the embryo sac and release its sperm cells for fertilization (reviewed in [2,5]). Across the angiosperms, this competitive context for pollen tube development differs, depending on the pollen population as well as sporophytic characters (reviewed in [10]).…”

Differential gene expression indicates involvement of F-box proteins and E3 ligases in sexualversusapomictic germline specification inBoechera

“…Beside its damaging properties, and thus triggering oxidative stress responses, ROS are also known to function as signalling cues between and within cells for example during male germline development (Kelliher and Walbot, 2012;De Storme and Geelen, 2014). Similarly, cell-to-cell signalling mechanisms are also required during female germline specification and differentiation (Grossniklaus and Schneitz, 1998;Koltunow and Grossniklaus, 2003;Schmidt et al, 2015;Zhou et al, 2017). In accordance with that, some DEGs can be attributed to cell signalling mechanisms, such as the homologue of ATHK1, which potentially acts as nonethylene phytohormone receptor (Tran et al, 2007).…”