Gimme some sugar

Pollinated plant tissues provide sugar for pollen’s journey to the ovary

Written by: Katherine Kelly

Edited by: Sienna Schaeffer, Katherine Hill

Stamen covered in male pollen grains (left, orange) transfers pollen to female stigma (middle, purple). Image credit: Paul Morris.

Springtime is upon us in the northern hemisphere. Soon enough, flowers will bloom and pollen will fill the air with the promise of fruit to come later in the year. These billions of ‘plant sperm’ have a long journey ahead before making any seeds. Landing on the right part of the right flower is only the beginning of the long and exhausting process to get the pollen to the egg cell of the plant so that fertilization can occur. Although plant reproduction shares some similarities with mammalian reproduction, we still have much to learn about the sex lives of plants. A new study published in Plant Physiology has brought us a little closer to understanding this essential springtime phenomenon.

Pollination requires careful coordination between the female and male tissues of the plant to help steer the pollen to the ovary. To begin the journey, a pollen grain has to land on the stigma, the entry point to the female organ of a plant (Fig. 1). Once pollen grains land on the tip of the stigma, the female tissues of the plant sense their presence and hydrate the pollen. This hydration causes the pollen to germinate, sprouting a tiny tube appropriately named a “pollen tube.”

Figure 1. Reproduction in a flowering plant (left) compared to a human (right). The anatomical equivalents in the human reproductive system compared to plant reproduction are: pollen grain/sperm cell, egg cell/egg cell, stigma/cervix, style/uterus or fallopian tubes. In mammals, the sperm travels through the uterus to the fallopian tubes to reach the egg cell. In plants, the pollen tunnels through the stigma, grows down the walls of style, and fertilizes an egg cell at the base of the style. (http://www.biologydiscussion.com, ilovebio.wikispaces.com)

The pollen tube then begins to tunnel into the stigma. The ovary containing the egg cells of the plant is situated below the stigma at the base of a long tube called the style. Unlike mammalian sperm, the pollen of flowering plants do not have the whip-like tail that propels the sperm forward. The movement of the pollen tube is instead produced by a stream of materials within the cell constantly being forced to the front of the tube. The pressure created by this stream pushes the front of the pollen tube forward through the central channel of the style, gradually edging toward the ovary. In an impressive feat of nature, pollen tubes can grow at speeds of one centimeter per hour — the fastest growing plant cells ever observed. To achieve this rapid growth rate, sugars are constantly broken down to fuel the synthesis of new tissue for the pollen tube.

Figure 2. Cross-section of tobacco flowers stained to visualize pollen tubes (Goetz et al. 2016.). (A) Surface of stigma two hours after pollination; (B) four hours after pollination; (C, D) 24 hours after pollination. Individual pollen tubes most evident in C, with germinated pollen grains near the top of the style on the stigma and pollen tubes descending into the style toward the ovary.

However, pollen grains are unable to sustain this rapid growth unless they use fuel other than the small supply of sugars they arrive with. A recent research article published in Plant Physiology by Goetz et al. illustrated this need for extra fuel by growing pollen from a tobacco plant in a petri dish with or without sucrose (a type of sugar). If the pollen grains were not supplemented with sucrose, researchers observed a substantial decrease in the number of germinated pollen grains. In addition, they observed that pollen grown without sucrose had stunted pollen tubes, growing to only one-fifth the length of the pollen tubes grown with sucrose. These results show that successful pollen tube development largely depends on sucrose supplied by the environment outside of the pollen grain.

So, how do pollen tubes get enough sugar to grow to full size in nature? Past studies tracing the flow of sugars between pollen and female tissues suggest the female tissues of the plant provide the essential nutrients for pollen tube growth. One earlier study by a different research group showed that a class of enzymes called invertases, which are responsible for helping absorb sugars into the pollen cell, were highly active during pollen tube growth. Invertases break down sucrose into two smaller component sugars. These sugars are then taken up by the plant cell and can be used for energy.

Based on these earlier findings, Goetz et al hypothesized that invertases found embedded in the outmost layer of the pollen tube are an essential tool for sustaining pollen tube growth. The scientists focused on one specific invertase called Nin88, which had been formerly associated with pollen germination and tube growth. Goetz et al. looked at different lines of tobacco plants with mutations that rendered Nin88 non-functional. They found that the pollen of mutant plants germinated less often than non-mutant plants. These results lent further support to the hypothesis that invertases are essential during pollination.

Goetz et al. then decided to dig even deeper into the process by taking a look inside tobacco plants to see if they could detect invertases throughout pollination. Researchers used a probe to reveal where the invertase was located in the tissue of pollinated and unpollinated plants (Fig. 3).

Prior to pollination, no invertase was detected in the style. Two hours after pollination, a weak invertase signal was detected at the top of the style where the pollen tube had just emerged and was beginning to burrow into the female tissues. After four hours, a strong invertase signal developed near the top of the style where the pollen tubes were actively growing. After 24 hours, the signal had disappeared from the top of the style and was only detectable near the bottom of the style where the pollen tubes were now actively growing.

Figure 3. Cross sections of pollinated, non-pollinated tobacco with Nin88 stained via RNA-probe (Goetz et al. 2016.). (J,K) Plant tissue section from pollinated tobacco plant showing invertase transcripts stained (purple). (K) Detail of boxed area in J showing stained pollen tubes. (L) Non-pollinated plant showing no invertase stained by the RNA-probe, indicating Nin88 was not expressed in the female tissues.

Looking closely at the tissues, the researchers saw that only pollen tubes expressed the transcript for Nin88; invertase was not detected in the female tissues of the plants. This suggested that Nin88 was specific for actively growing cells such as pollen tubes and not the surrounding female tissues. In additional experiments, researchers identified two other proteins that showed similar expression patterns. These proteins allow pollen tubes to absorb sugars from outside the pollen tube. These results provide strong evidence for the pollen tube cleaving and taking up sugar from outside of the pollen tube.

These results led the researchers to propose a mechanism for the development and growth of pollen tube within the female tissue. The researchers concluded that after pollen germinates on the stigma, the female tissues of the plant provide sugar to sustain pollen tube growth to the ovary. Sucrose serves as the major fuel source for the pollen tube through its long journey to the egg cell and the invertase Nin88 appears essential in tobacco plants for use of the sucrose. This differs drastically from the process of mammalian reproduction, where sperm is provided with nutrients within the male ejaculate, not by the vaginal tissues.

As we look closer at the reproductive processes of plants, we are beginning to better understand our distant cousins and the variety of ways organisms reproduce. Ultimately, this research brings us one step closer to understanding the secret sex lives of plants. This spring, as I walk outside, I will make sure take a moment to pause near a flowering plant and ponder the intricate workings happening within.

Check out the full paper here for more information:

Goetz, M. et al. Metabolic Control of Tobacco Pollination by Sugars and Invertases. Plant Physiology 173, 984–997 (2016).

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Interested in the wonderful world of plants? Check out Botany Thoughts by Isabella Armour for some insightful reads.