The thought of pollen dispersed throughout the air might trigger horrific memories of allergies, but the drifting dander is absolutely essential to all life.
Science has long linked this element of reproduction with environmental conditions, but the reasons why and how pollen functions were less understood. Now lingering questions about the nuanced control of plants are being answered.
“Pollen is a very important part of the reproductive process and if we understand how pollen develops and how environmental stresses impinge on this process, we might be able to prevent crop loss due to high temperature or drought stress etc.,” said Shuqun Zhang, a Bond Life Sciences Center investigator.
Zhang has developed a new line of seeds that helped him and his lab identify an influential signaling pathway that triggers a chain reaction associated with normal pollen formation and function.
This research could lead to improvement to a plant’s response to disastrous environmental variables like drought to optimize pollen production and increase the production of food crops.
Left: Pollen grains genotypes MAPK3 and MAPK6 are illuminated by red and yellow dye using a fluorescent microscope. RIGHT: Normally developed pollen grains shown by an electronic microscope scan. | Credit: Shuqun Zhang
Seeds of success
Mutant seeds are the key to this work.
Instead of glowing green in the soil like you might see in a science fiction movie, they are providing important insight on plant reproduction and stress tolerance.
Zhang developed these plants from a mutant strain of Arabidopsis, a model plant used in scientific research. Certain genes were “switched off”to pinpoint where important pollen functions were signaled.
Using this mutant plant and seed system, Zhang found that WRKY34and WRKY2, two proteins that turn on/off genes, are regulated by MPK3and MPK6 “signaling” enzymes. These enzymes basically transform proteins from a non-functional state to a functional state, turning on specific duties or functions. Zhang, a professor of biochemistry at MU, began tinkering with the MPK3 and MKP6 pathways more than twenty years ago during his post-doc at Rutgers University.
Zhang’s research shows the newly identified MPK3/MPK6-WRKY34/WRKY2 pathway is a key switch in the hierarchy of the signaling system in pollen formation.
The research showed that the plant’s defense/stress response and reproductive process are linked, and the influential proteins MPK3 and MPK6 were part of the bigger WRKY34/WRKY2control pathway, which is activated in early pollen production.
The system is so useful that researchers across the country won’t stop asking for the seeds, Zhang said.
“We have a lot of requests for seeds,” Zhang said. “This is a very nice system to study pollen formation and function.”
The cascade of control
The functions of MPK3/MPK6 in plants can be compared to a “mother board” switch. The pathway — MPK3 and MPK6 —are part of a hierarchy of response, turning functions on or off. In other words, it’s a switch that controls a lot of different things. Controlling WRKY34/WRKY2 is one of the many roles played by MPK3 and MPK6.
Shuqun Zhang, University of Missouri Bond Life Sciences investigator.
“Whatever is plugged into it is what comes on,” Zhang said. “We are actually very, very interested in the evolutionarily context, how this came to be.”
This signaling process is just one of many in plants. MPK3 and MPK6 are two out the 20 MPKs, or MAPKs (abbreviated from Mitogen-Activated Protein Kinases) in Arabidopsis. They control plant defense, stress tolerance, growth, and development including pollen formation and functions.
“We determined that this MAPK-WRKY signaling module functions at the early stage of pollen development,” Zhang said.
The “loss of function of this pathway reduces pollen viability, and the surviving pollen has poor germination and reduced pollen tube growth, all of which reduce the transmission rate of the mutant pollen,” according to the research.
Zhang and his lab worked with the MU Division of Biochemistry and Interdisciplinary Plant Group on the research, which published in PLoS Genetics in June of this year.
A world without pollen production and defense
Without pollen, plants would not reproduce — there aren’t any Single Bars in the plant world (that we know of) — and if plant generations don’t propagate, there would be no air or food for human life to sustain.
“The factors such as heat and drought stresses cause problems to the plant’s normal developmental process and that’s how pollen fails to develop,” Zhang said. “If we understand the process, and know how environmental factors impact negatively the process, we can then make plants that can handle environmental stress better.”
Zhang and his lab continue to research the complexities of these pathways. Next on the quest is to answer how MPK3/MPK6 are involved in pollen functions such as guiding the pollen tube growth towards ovule to complete the sexual reproduction process in plants.
“It is possible that MPK3 and MPK6 are activated quickly in response to the guidance signals,” he said. “There’s still a long way to go because very few players in this process have been identified, we try to understand the biological process how they work together.” This research is in collaboration with Dr. Bruce McClure, also professor of Division of Biochemistry.
1. PLoS Genetics (May 2014): Phosphorylation of a WRKY Transcription Factor by MAPKs is Required for Pollen Development and Function in Arabidopsis — Funded by a Hughes Research Fellowship and grants from the National Science Foundation.
2. Plant Physiology (June 2014): Two Mitogen-Activated Protein Kinases, MPK3 and MPK6, are required for Funicular Guidance of Pollen Tubes in Arabidopsis — Funded by a National Science Foundation grant and a NSF Young Investigator Award.