In flowering plants, mature pollen grains are produced within the developing anthers
of the flowers in two successive phases, microsporogeneseis and microgametogenesesis.
Anther and pollen development involves the coordinated growth and differentiation
of tissues and cell types required for the formation of viable male gametes, but also,
to permit pollen release and ensure successful fertilization. The identification of
the genetic networks and regulatory molecules involved in the formation of the anther
remains a highly unexplored topic in many crops. Evolutionary and functional studies
in crops are helping to unravel new functions for genes originally identified in model
plants (Rojas-Gracia et al., 2017) and the changes in transcriptional profiles associated
with plant domestication (Xiang et al., 2019). Under the global changes in environmental
conditions, pollen development is probably one of the most vulnerable and challenged
stages of plant reproduction (Chaturvedi et al., 2021). In this context, increasing
basic research will provide valuable information to assist the design of biotechnological
tools to mitigate this effect especially in crop systems in the future.
New advances in the functional characterization of genes involved in different aspects
of pollen development and function are described in the following articles of this
Research Topic. In the original research article by Hamza et al., the authors functionally
characterized Pisum sativum ENDOTHECIUM 1 (PsEND1), a pea anther-specific gene that
encodes a protein containing four hemopexin domains. Gain and loss-of-function experiments
showed that PsEND1 is a central player in the maintenance of balanced redox levels
during pollen and anther development. Zhou L. et al. demonstrated pollen-specific
regulatory function of ZmLarp6c1, maize ortholog of Arabidopsis AtLARP6C, particularly
during the progamic phase. The respective Ds-GFP transposable element insertion line
showed reduced transmission through the male wich was associated with altered germination
dynamics and slower growth of mutant pollen tubes. Considering that LARP6C also orchestrates
posttranscriptional reprogramming of gene expression during hydration in Arabidopsis
(Billey et al., 2021), these findings highlight the general regulatory role of LARP6C
in angiosperms. The original research article by Mazuecos-Aguilera et al. describes
the functional study of the INAPERTURATE POLLEN1 (INP1) ortholog from the basal eudicot
Eschscholzia californica (California poppy), a gene involved in pollen aperture formation.
This study substantiates the importance of INP1 homologs for aperture formation across
angiosperms and opens up new avenues for functional studies of other aperture candidate
genes. The role of the Arabidopsis Ankyrin-repeat protein (AT5G66055, AKRP), during
male and female gametogenesis is analyzed by Kulichová et al. using the new mutant
allele akrp-3. AKRP is a plastid-localized protein with a putative function in plastid
differentiation and morphogenesis. The findings provide insight into the role of this
protein in both the differentiation of gametophytes and the coupling of embryo development
with chlorophyll synthesis. In the research article by Dong et al., the authors reported
a new major gene Cla006625 controlling “Genic Male Sterility” (GMS) in watermelon.
Molecular and genomics studies revealed that this gene renamed as ClaPEX1 encodes
a leucine-rich repeat protein, and the recessive mutant of this locus causes pollen
abortion conferring GMS. The targeted RNAi based evidence further confirmed the functionality
for this gene in GMS, and the authors propose potential applications in hybrid seed
production technology to capture heterosis in watermelon. Kakui et al. studied pollen
number variation in Arabidopsis and discovered the first gene responsible for pollen
number, REDUCED POLLEN NUMBER1 (RDP1), encoding the large ribosomal subunit assembly
factor. CRISPR/Cas9-generated rdp1-3 mutants revealed the pleiotropic effect of RDP1
in flowering and pollen development. Subsequent transcriptome analysis supported the
hypothesis that ribosome biogenesis, critical for pollen development, is disturbed
in the rdp1-3 mutant pollen and highlighted three key bHLH transcription factors (ABORTED
MICROSPORES, bHLH010, and bHLH089).
The identification and characterization of regulatory molecules during anther or pollen
development is the subject of the remaining articles in this Research Topic. The role
of hormone dynamics during microgametogenesis has been explored by Záveská Drábková
et al. in several Nicotiana species. The article describes the dynamic changes in
endogenous phytohormones during pollen ontogeny, highlighting that unequal levels
of endogenous hormones and the presence of specific derivatives which may be characteristic
for pollen development in different phylogenetic plant groups. In the original research
article by Zhou D. et al., the author used male-sterile systems of Brassica campestris
(Chinese cabbage) to study anther and pollen development in this species. Differentially
expressed lncRNAs (DELs), miRNAs (DEMs), and genes (DEGs) were identified providing
new insights into molecular regulation especially the ncRNA interaction during pollen
development in Brassica crops.
Several of the articles submitted to this Research Topic investigated global developmental
aspects of pollen development and its impact on plant performance. Xue et al., developed
several live imaging methods for the study of anther development. They created the
marker line ProUBQ10:H2B:VENUS and used it to study the development of the middle
layer in the anther of Arabidopsis thaliana. The results showed that the middle layer
was derived from both inner and outer secondary parietal cells, indicating that the
cell fate determination of the middle layer was non-cell-autonomous in Arabidopsis.
In the new research report by Xiao et al., the authors investigated the adaptive and
evolutionary features of “Delayed Autonomous Selfing” (DAS) in Salvia umbractica.
The observations and findings from the field and controlled experiments showed outcrossing
using insect pollinators first, wich failed to fertilize, lead to execution of DAS
to ensure fertilization for successful fruit and seed production. The authors presented
strong supporting evidence by detailed documentation of changes in the reproductive
organs' specific behaviors linked with morphological and developmental activities
of these two built-in alternative pollination options in Salvia species. In the research
article by Calić et al., the authors investigated the influence of long-term storage
temperature on pollen viability of four Serbian autochthon apple cultivars. Interestingly,
the pollen could be efficiently maintained at −20°C and later used for breeding purposes.
The results will surely contribute to the preservation of these old autochthon cultivars
as unique genetic resources with important ecological and economic value. Jaffri and
MacAlister utilized histology and immunostaining to show the structure of the tomato
pollen wall, characterized dynamic changes in pectin composition, and established
a developmental timeline of its formation. Following meiosis, the microspores losing
their cellulose primary wall remain connected by a temporary callose wall in tetrads.
Release of early microspores initiates sporopollenin secretion to form exine, wich
is completed in late microspores. The tomato pollen wall formation is finished by
the formation of intine from pollen mitosis I to pollen maturation. Grienenberger
and Quilichini highlighted significant progress in the field of sporopollenin research;
they examined the cross-disciplinary efforts to solve the sporopollenin composition
puzzle and presented a working model of sporopollenin's molecular structure and biosynthesis.
They further discussed the controversies and remaining knowledge gaps, including the
degree of aromaticity, cross-linkage profiles, and extent of chemical conservation
of sporopollenin among land plants. Finally, the authors highlighted opportunities
for practical utilization of this extraordinary biomaterial.
Pollen biotechnology offers a wide range of possibilities for plant breeding. Doubled-haploid
technology, based on the reprogramming of immature pollen grains or microspores toward
embryogenesis, promises to accelerate crop breeding programs and shorten the time
to obtain new varieties. Investigations in recent years have shown the complex regulatory
mechanisms underlying microspore embryogenesis (Testillano, 2019), opening promising
avenues for improving its efficiency in crop species of economic and environmental
interest. In addition, genetically engineered male-sterile plants offer a valuable
trait for plant breeding programs for many crops. Recently, CRISPR/Cas9 editing technology
has become an efficient and versatile option to obtain new plant varieties and accelerate
breeding practices. In the review by et al., the authors recapitulated past and present
research on obtaining male-derived haploid progeny by microspore embryogenesis. The
authors evaluated basic breeding applications of this process, explored the utility
of genomics and gene editing technologies for protocol development, and provided considerations
to overcome genotype specificity and morphogenic recalcitrance in non-model plant
systems. In the article by Pandey et al., the authors reviewed the molecular mechanisms
controlling the alternation of generations between the sporophytic and gametophytic
stages from an evolutionary perspective. The article compares the genetic factors
and mechanisms regulating the separation of the two developmental programs and discusses
its biotechnological applications for accelerating the breeding of crop plants.
The goal of this Research Topic was to highlight the latest advances in pollen research
and the potential of pollen in the development of biotechnological applications for
plant breeding. Fifteen articles have been published on this Research Topic including
two reviews, one mini-review, and 12 original research articles covering different
aspects of pollen biology, biotechnology, and breeding applications. Remarkably, many
of the research articles were carried out on important crops including studies in
apple, pea, maize, watermelon, or tomato. Globally this Research Topic of articles
successfully represents some major advances in pollen research across different plant
species contributing to increasing knowledge in the field and to the generation of
new opportunities to implement crop improvement programs in the coming years.
Author Contributions
All authors listed have made a substantial, direct, and intellectual contribution
to the work and approved it for publication.
Conflict of Interest
The authors declare that the research was conducted in the absence of any commercial
or financial relationships that could be construed as a potential conflict of interest.
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