Yamagata University clarifies the mechanism that enables pollen to survive for long periods — Activity resumes rapidly upon obtaining water

A research group including Assistant Professor Mio Shibuta of the Faculty of Science at Yamagata University has clarified the mechanism that enables pollen to survive for long periods with its activity suspended, while instantly reactivating when necessary. They discovered that when pollen obtains water, structural changes in the cell nucleus and gene activity resume, allowing cell division to progress in a stepwise manner. This achievement is expected to find applications in the agricultural field, such as improving pollination efficiency and stabilizing crop production. The findings were published in Plant Direct on June 2.

Some angiosperm (flowering plant) pollen matures in a dehydrated state and can survive for a long time by temporarily halting its activity. Its lifespan and desiccation tolerance vary by species, with some losing functionality within a short timeframe.

On the other hand, regardless of the species, pollen extends a pollen tube within a short time after pollination to transport two sperm cells involved in double fertilization. The mechanism that reconciles this "state of suspended activity" with a "state capable of rapidly restarting activity" had not been fully understood.

Sperm cells are generated by the cell division of the generative cell present within the pollen. The nucleus of the generative cell in mature pollen takes on a chromatin structure resembling chromosomes in the mitotic phase, where DNA is condensed into thread-like structures. However, how this structure relates to the actual cell cycle state and reactivation remained unknown.

This time, the research group utilized Lilium longiflorum, which is highly suitable for observing cellular structures. In lily pollen, it is known that the generative cell completes the DNA replication necessary for cell division during the maturation phase.

First, they verified whether the generative cell in the pollen (the cell that divides inside the pollen tube to form two sperm cells) is actually entering the mitotic phase (M phase), as its appearance suggests.

When generative cells were extracted from pollen and observed using a confocal laser microscope, it was demonstrated that the generative cells had not entered the M phase but were instead arrested in the G2 phase, which is the pre-mitotic stage after completing DNA replication.

Furthermore, when analyzing generative cells derived from pollen after water absorption, the team detected molecular markers unique to the start of the M phase, while simultaneously observing the loosening of chromatin. It became clear that the cell cycle progresses alongside chromatin decondensation, ultimately leading to the division of the generative cell.

Analysis using transcription inhibitors further revealed that the initial nuclear changes corresponding to the preparation stages for chromatin decondensation and cell cycle progression do not depend on the resumption of transcription after water absorption.

Under transcription inhibitor treatment, the chromosome structure unique to the M phase was not formed, demonstrating that the resumption of transcription is indispensable for the division of the generative cell.

The study clarified a mechanism where absorbing water loosens the chromatin structure, rapidly restarting gene functions and allowing cell division and pollen tube elongation to progress step-by-step.

It was revealed that the generative cell inside pollen is in a unique state. Namely, while it displays an M-phase chromatin structure, it is actually in the G2 phase. Additionally, it was found that reactivation after water absorption consists of an initial transcription-independent stage and a subsequent transcription-dependent stage.

Shibuta stated: "Because the process of pollen resuming its activity progresses within an extremely short timeframe, capturing that exact moment was a major challenge. This time, by performing repeated microscopic observations using lily pollen, which has a large genome size and allows for detailed observation of nuclear structures, we were able to visualize the intranuclear changes from the dehydrated state to reactivation. We believe this study represents a step forward in understanding the fundamental mechanism of how pollen temporarily suspends its vital activities and subsequently restarts them so rapidly."

Journal Information
Publication: Plant Direct
Title: Generative Cell Division, but Not Early Nuclear Reorganization, Requires Transcriptional Reactivation During Pollen Tube Growth
DOI: 10.1002/pld3.70175