CNGCs break through—A rice cyclic nucleotide-gated channel paves the way for pollen tube growth

Lower plants generate mobile sperm cells that must reach their female counterparts by swimming. This requirement for water is a disadvantage for these plants as compared with angiosperms, for which the dry pollen attaches to a stigma and becomes hydrated, enabling the emerging pollen tube to grow in the protected environment of the pistil. After adhesion, hydration, and germination of the pollen at the stigmatic papilla cells, the pollen tube enters the stigma and grows in the intercellular space between papilla cells towards the style and transmission tract (TT). The TT contains a nutrient-rich extracellular matrix (ECM) and guides the pollen tube to the ovary. After penetrating the septum, the pollen tube grows through the funiculus and then enters the ovule though the micropyle to deliver the two nonflagellate sperm cells to the two female gametes, leading to double fertilization, a prerequisite to seed formation [1]. The 2017 study by Xu et al. [2] reveals, for the first time, the importance of a Ca signal generated by rice cyclic nucleotide-gated channel 13 (OsCNGC13) in the pistil to induce programmed cell death (PCD), which facilitates proper pollen tube growth. Furthermore, they showed this step significantly affects the yield of rice grains. The events during pollen grain–stigma interaction and pollen tube reception are relatively well studied [1, 3], while much less is known about the growth of the pollen tube through the style and TT tissue—particularly the signaling between the pollen tube and the pistil tissue(s). Intracellular signaling in the pollen tube during pollen tube growth has been studied extensively [4]. The role of Ca2+ is well established: in the pollen tube, a Ca2+ gradient is essential for pollen tube guidance [5]. External Ca2+ from the pistil must be taken up by the pollen tube and is required for its growth. Several potential Ca2+ channels that are expressed in the pollen tube have been identified in Arabidopsis. These include two glutamate receptor-like (GLR) channels (GLR1.2 and GLR3.7) [6] and six cyclic nucleotide-gated channels (CNGCs; CNGC7, 8, 9, 10, 16, and 18), of which CNGC18 has been shown to be a Ca2+-conducting channel that is essential for tip growth in pollen tubes [7] and pollen tube guidance (Fig 1) [5]. Pollen tubes can grow in vitro; however, for the pollen tube to make its way through the style to the ovary, some communication with the sporophyte is necessary. But so far, few signaling components have been identified that drive the interaction between the TT and the pollen tube. On the pollen tube side, two membrane-localized leucine rich repeat (LRR) receptor kinases, LePRK1 and 2, have been identified that may interact with different ligands in different pistil tissues [8]. Other examples include the pollen tube-localized GLRs, GLR1.2 and