Sequential Deposition and Remodeling of Cell Wall Polymers During Tomato Pollen Development

Abstract

The cell wall of a mature pollen grain is a highly specialized, multilayered structure. The outer, sporopollenin-based exine provides protection and support to the pollen grain, while the inner intine, composed primarily of cellulose, is important for pollen germination. The formation of the mature pollen grain wall takes place within the anther with contributions of cell wall material from both the developing pollen grain as well as the surrounding cells of the tapetum. The process of wall development is complex; multiple cell wall polymers are deposited, some transiently, in a controlled sequence of events. Tomato (Solanum lycopersicum) is an important agricultural crop, which requires successful fertilization for fruit production as do many other members of the Solanaceae family. Despite the importance of pollen development for tomato, little is known about the detailed pollen gain wall developmental process. Here, we describe the structure of the tomato pollen wall and establish a developmental timeline of its formation. Mature tomato pollen is released from the anther in a dehydrated state and is tricolpate, with three long apertures without overlaying exine from which the pollen tube may emerge. Using histology and immunostaining, we determined the order in which key cell wall polymers were deposited with respect to overall pollen and anther development. Pollen development began in young flower buds when the premeiotic microspore mother cells (MMCs) began losing their cellulose primary cell wall. Following meiosis, the still conjoined microspores progressed to the tetrad stage characterized by a temporary, thick callose wall. Breakdown of the callose wall released the individual early microspores. Exine deposition began with the secretion of the sporopollenin foot layer. At the late microspore stage, exine deposition was completed and the tapetum degenerated. The pollen underwent mitosis to produce bicellular pollen; at which point, intine formation began, continuing through to pollen maturation. The entire cell wall development process was also punctuated by dynamic changes in pectin composition, particularly changes in methyl-esterified and de-methyl-esterified homogalacturonan.

The tapetal tissue is essential for the maintenance of redox homeostasis during microgametogenesis in tomato

“…Callose deposition occurs around the sporogenous cells prior to meiosis initiation and later, between meiotic products (Jaffri & MacAlister, 2021 ). After the completion of meiosis, callose is quickly degraded after the release of callases (β‐1,3‐glucanases) by the tapetum.…”

The Hydroxyproline O-arabinosyltransferase FIN4 is required for tomato pollen intine development

“…To determine the developmental timing of FIN4 expression during floral development, we carried out semi-quantitative RT-PCR using floral tissue of multiple developmental stages. We have previously established a correlation between bud length and pollen developmental stage for the MicroTom tomato variety (Jaffri and MacAlister, 2021). We found FIN4 was not expressed in floral buds preceding the specification of the microspore mother cells (1 mm buds), but was expressed in 2 mm buds containing microspore mother cells.…”

“…We have previously established that in 6 mm MicroTom flower buds, pollen is at the bicellular stage of development and intine deposition has begun, particularly at the aperture sites and in 8 mm buds the pollen is nearly mature and the intine is well established throughout the pollen (Jaffri and MacAlister, 2021). To understand the nature of the fin4 intine defect, we used calcofluor white, a fluorescent dye with a high affinity for cellulose (β-1,4-glucan), often used to visualize the pollen intine layer to label sections from 6 and 8 mm flower buds (Fang et al, 2008; Herburger & Holzinger, 2016; Jaffri & Macalister, 2021; W. L. Li et al, 2017; Renzaglia et al, 2020; Takebe et al, 2020). As we had previously found, WT pollen in the 6 mm buds had begun intine deposition, visible as calcofluor white fluorescence, particularly accumulating at the future aperture sites (Fig.…”

Genome Wide Identification and Characterization of Wheat GH9 Genes Reveals Their Roles in Pollen Development and Anther Dehiscence

“…In our previous study, following the formation of the cell plate, pectin and xyloglucan gradually combined, excess membrane began to disappear, cellulose content increased and eventually cell walls formed [ 54 , 55 , 56 ]. During meiotic cytokinesis, cellulose accumulates at both the division place and external wall of the cell [ 57 ]. At the prophase I stage, callose deposition happened in the pollen mother cell (PMC), forming a thick callose cell wall until the end of meiosis cell cycle [ 58 ].…”