Peptides/receptors signaling during plant fertilization

Abstract

Double fertilization is a unique and particularly complicated process for the generation alternation of angiosperms. Sperm cells of angiosperms lose the motility compared with that of gymnosperms. The sperm cells are passively carried and transported by the pollen tube for a long journey before targeting the ovule. Two sperm cells are released at the cleft between the egg and the central cell and fused with two female gametes to produce a zygote and endosperm, respectively, to accomplish the so-called double fertilization process. In this process, extensive communication and interaction occur between the male (pollen or pollen tube) and the female (ovule). It is suggested that small peptides and receptor kinases play critical roles in orchestrating this cell-cell communication. Here, we illuminate the understanding of phases in the process, such as pollen-stigma recognition, the hydration and germination of pollen grains, the growth, guidance, and rupture of tubes, the release of sperm cells, and the fusion of gametes, by reviewing increasing data recently. The roles of peptides and receptor kinases in signaling mechanisms underlying cell-cell communication were focused on, and directions of future studies were perspected in this review.

Keywords: cell-cell communication; double-fertilization; kinase; peptide; pollen tube guidance; polytubey block.

Figure 1

Schematic diagram of SCR/SP11 and SRK grouped for pollen-stigma interactions of Brassica self-incompatibility. The peptide triggering inhibition of signaling pathways is highlighted in red. The blue represents receptor complexes and the co-receptors are shown in parentheses. The source of peptides and receptors is shown in parentheses. Abbreviations for all small peptides and receptors are elucidated in the text.

Figure 2

Pollen and stigma recognize each other to initiate the hydration and germination of pollen. In Solanum Lycopersicum, LAT52 interacts with LePRK1/2 to inhibit hydration and germination, and LeSTIG1 interacts with LePRK1/2 to facilitate hydration and germination. However, in Arabidopsis, pollen-specific PCP-Bs bind to FER/ANJ-LLG1 to promote hydration and germination, and stigmatic RALF23/33 interacts with FER/ANJ-LLG1 to maintain ROS accumulation and repress hydration of pollen. The peptides involved in the activation and inhibition of signaling pathways are highlighted in green and red, respectively. The receptor complexes are marked in blue and the co-receptors are shown in parentheses. The source of peptides and receptors is shown in parentheses. Abbreviations for all small peptides and receptors are elucidated in the text.

Figure 3

The integrity of pollen tube growth is maintained, and pollen tubes rupture In Arabidopsis thaliana, pollen-specific RALF4/19 orchestrates pollen tube receptor complexes BUPS1/2-ANX1/2-LLG2/3 and LRX8/9/10/11 to maintain pollen tube growth and integrity in the transmitting tract. After the pollen tube enters the ovule, the ovule-specific RALF34 interacts with BUPS1/2-ANX1/2-LLG2/3, resulting in pollen tube rupture. In Zea Mays, ZmES1/2/3/4 secreted from synergid cells causes pollen tube rupture. The peptides involved in the activation and inhibition of signaling pathways are highlighted in green and red, respectively. The blue represents receptor complexes and the co-receptors are shown in parentheses. Question marks indicate unidentified signal components. The source of peptides and receptors is shown in parentheses. Abbreviations for all small peptides and receptors are elucidated in the text.

Figure 4

The micropyle guidance and polytubey block of pollen tubes. In Arabidopsis thaliana, pollen tubes penetrate the septum, and the RALF6/16/17/36/37 secreted by the pollen tube interacts with the septum-specific FER/ANJ/HERK1 to initiate the polytubey block. XIUQIUs and AtLURE1s secreted by synergid cells diffuse along the funiculus. AtLURE1s interact with PRK6 and/or MIK1/2-MDIS of the pollen tube to guide the pollen tube to grow towards the micropyle. The peptides are highlighted in green and red to represent activation or suppression, respectively. The blue represents receptor complexes and the co-receptors are shown in parentheses. Question marks indicate unidentified signal components. The source of peptides and receptors is shown in parentheses. Abbreviations for all small peptides and receptors are elucidated in the text.

Figure 5

Pollen tube is received, polytubey block, and rupture. At the micropyle, FER/ANJ/HERK1-LLG1 derived from synergid cells is involved in the recognition and reception of pollen tube; they also interact with tube-specific RALF6/16/17/36/37 to cause polytubey block. After the pollen tube enters the ovule, the interaction between ovule-specific RALF34 and BUPS1/2-ANX1/2-LLGs of the pollen tube results in the rupture of the tube in Arabidopsis. ZmES1/2/3/4 secreted by synergids is related to the rupture of the pollen tube in Zea Mays. The peptides are highlighted in green and red to represent activation or suppression, respectively. The receptor complexes are marked in blue and the co-receptors are shown in parentheses. Question marks indicate unidentified signal components. The source of peptides and receptors is shown in parentheses. Abbreviations for all small peptides and receptors are elucidated in the text.