Abstract

In flowering plants, male gametes are immotile and carried by dry pollen grains to the female organ. Dehydrated pollen is thought to withstand abiotic stress when grains are dispersed from the anther to the pistil, after which sperm cells are delivered via pollen tube growth for fertilization and seed set. Yet, the underlying molecular changes accompanying dehydration and the impact on pollen development are poorly understood. To gain a systems perspective, we analyzed published transcriptomes and proteomes of developing Arabidopsis thaliana pollen. Waves of transcripts are evident as microspores develop to bicellular, tricellular, and mature pollen. Between the “early”- and “late”-pollen-expressed genes, an unrecognized cluster of transcripts accumulated, including those encoding late-embryogenesis abundant (LEA), desiccation-related protein, transporters, lipid-droplet associated proteins, pectin modifiers, cysteine-rich proteins, and mRNA-binding proteins. Results suggest dehydration onset initiates after bicellular pollen is formed. Proteins accumulating in mature pollen like ribosomal proteins, initiation factors, and chaperones are likely components of mRNA-protein condensates resembling “stress” granules. Our analysis has revealed many new transcripts and proteins that accompany dehydration in developing pollen. Together with published functional studies, our results point to multiple processes, including (1) protect developing pollen from hyperosmotic stress, (2) remodel the endomembrane system and walls, (3) maintain energy metabolism, (4) stabilize presynthesized mRNA and proteins in condensates of dry pollen, and (5) equip pollen for compatibility determination at the stigma and for recovery at rehydration. These findings offer novel models and molecular candidates to further determine the mechanistic basis of dehydration and desiccation tolerance in plants.

Advances

1. The normal development of the male gametophyte in many flowering plants undergoes stress-related changes. Pollen dehydrates before grain dispersal, then rehydrates to enable pollen function. The bases of dehydration and its impact on pollen development are not understood.
2. Accumulation of late-embryogenesis-abundant mRNAs in bicellular pollen (BCP) suggests dehydration onset after pollen mitosis I. Transcriptome and proteome datasets revealed a new wave of transcripts that peaks in late BCP.
3. Transcripts coexpressed in late BCP show dehydration is accompanied by changes to transporters, pectin modifiers, membrane trafficking, and cysteine-rich proteins.
4. Proteins required for respiration and energy production are abundant in developing and mature pollen.
5. Some pollen-expressed RNA-binding proteins bind mRNA and form mRNA-protein granules.
6. The protein translation machinery accumulating in dry pollen is likely stored in mRNA-protein stress granules until rehydration.
7. Uncovering transcripts and proteins that accompany pollen dehydration is revealing new working models and genes to study dehydration and desiccation tolerance in plant parts.