PpASCL, the Physcomitrella patens Anther-Specific Chalcone Synthase-Like Enzyme Implicated in Sporopollenin Biosynthesis, Is Needed for Integrity of the Moss Spore Wall and Spore Viability

Proteome plasticity during Physcomitrium patens spore germination - from the desiccated phase to heterotrophic growth and reconstitution of photoautotrophy

The establishment of moss spores is considered a milestone in plant evolution. They harbor protein networks underpinning desiccation tolerance and accumulation of storage compounds that can be found already in algae and that are also utilized in seeds and pollen. Furthermore, germinating spores must produce proteins that drive the transition through heterotrophic growth to the autotrophic plant. To get insight into the plasticity of this proteome, we investigated it at five timepoints of moss (Physcomitrium patens) spore germination and in protonemata and gametophores. The comparison to previously published Arabidopsis proteome data of seedling establishment showed that not only the proteomes of spores and seeds are functionally related, but also the proteomes of germinating spores and young seedlings. We observed similarities with regard to desiccation tolerance, lipid droplet proteome composition, control of dormancy, and β-oxidation and the glyoxylate cycle. However, there were also striking differences. For example, spores lacked any obvious storage proteins. Furthermore, we did not detect homologs to the main triacylglycerol lipase in Arabidopsis seeds, SUGAR DEPENDENT1. Instead, we discovered a triacylglycerol lipase of the oil body lipase family and a lipoxygenase as being the overall most abundant proteins in spores. This finding indicates an alternative pathway for triacylglycerol degradation via oxylipin intermediates in the moss. The comparison of spores to Nicotiana tabacum pollen indicated similarities for example in regards to resistance to desiccation and hypoxia, but the overall developmental pattern did not align as in the case of seedling establishment and spore germination.

CRISPR/Cas9-based genome editing of 14 lipid metabolic genes reveals a sporopollenin metabolon ZmPKSB-ZmTKPR1-1/-2 required for pollen exine formation in maize

Lipid biosynthesis and transport are essential for plant male reproduction. Compared with Arabidopsis and rice, relatively fewer maize lipid metabolic genic male-sterility (GMS) genes have been identified, and the sporopollenin metabolon in maize anther remains unknown. Here, we identified two maize GMS genes, ZmTKPR1-1 and ZmTKPR1-2, by CRISPR/Cas9 mutagenesis of 14 lipid metabolic genes with anther stage-specific expression patterns. Among them, tkpr1-1/-2 double mutants displayed complete male sterility with delayed tapetum degradation and abortive pollen. ZmTKPR1-1 and ZmTKPR1-2 encode tetraketide α-pyrone reductases and have catalytic activities in reducing tetraketide α-pyrone produced by ZmPKSB (polyketide synthase B). Several conserved catalytic sites (S128/130, Y164/166 and K168/170 in ZmTKPR1-1/-2) are essential for their enzymatic activities. Both ZmTKPR1-1 and ZmTKPR1-2 are directly activated by ZmMYB84, and their encoded proteins are localized in both the endoplasmic reticulum and nuclei. Based on protein structure prediction, molecular docking, site-directed mutagenesis and biochemical assays, the sporopollenin biosynthetic metabolon ZmPKSB-ZmTKPR1-1/-2 was identified to control pollen exine formation in maize anther. Although ZmTKPR1-1/-2 and ZmPKSB formed a protein complex, their mutants showed different, even opposite, defective phenotypes of anther cuticle and pollen exine. Our findings discover new maize GMS genes that can contribute to male-sterility line-assisted maize breeding and also provide new insights into the metabolon-regulated sporopollenin biosynthesis in maize anther.

Characterization and expression analysis of chalcone synthase gene family members suggested their roles in the male sterility of a wheat temperature-sensitive genic male sterile (TGMS) line

Chalcone synthase (CHS), also known as the plants-specific type III polyketide synthases (PKSs), catalyzes the first key step in the biosynthesis of plant flavonoids. Flavonoids are one of the most important secondary metabolites which participate in flower pigmentation and pollen fertility. Recent reports have demonstrated the role of the CHS family in plant pollen exine formation. This study focused on the potential roles of CHS in the pollen exine formation of wheat. In the present study, a genome-wide investigation of the CHS family was carried out, and 87 CHS genes were identified in wheat. TaCHS3, TaCHS10, and TaCHS13 are wheat orthologs of Arabidopsis LESS ADHESIVE POLLEN (LAP5); TaCHS58, TaCHS64, and TaCHS67 are wheat orthologs of AtLAP6. TaCHS3, TaCHS10, and TaCHS67 showed anther-specific patterns. The expression of TaCHS3, TaCHS10, and TaCHS67 was positively co-expressed with sporopollenin biosynthetic genes, including TaCYP703A2, TaCYP704B1, TaDRL1, TaTKPR2, and TaMS2. Coincidently, the expression of TaCHS3, TaCHS10, and TaCHS67, together with those sporopollenin biosynthetic genes, were repressed at the tetrads and uninucleate stages in the temperature-sensitive genic male-sterile (TGMS) line BS366 under sterile conditions. Wheat anther-specific CHS genes might participate in the exine formation of BS366 through co-expressing with sporopollenin biosynthetic genes, which will undoubtedly provide knowledge of the roles of CHS in wheat pollen development.

A Ceratopteris EXCESS MICROSPOROCYTES1 suppresses reproductive transition in the fern vegetative leaves

Land plant sexual reproduction involves the transition of cells from somatic to reproductive identity during post-embryonic development. In Arabidopsis, the leucine-rich repeat receptor-like kinase EXCESS MICROSPOROCYTES1 (EXS/EMS1) restricts the number of sporogenous cells during the transition from diploid tissue to haploid spore production by promoting the formation of the tapetum cell layer within the anther. Although all land plants studied contain EMS1 genes, its function is unknown beyond a few angiosperms. In the model fern Ceratopteris (Ceratopteris richardii), we discovered an EMS1 homolog (CrEMS1) that functions to suppress formation of reproductive structures on vegetative leaves of the fern sporophyte, a role not found in angiosperms. Suppression of CrEMS1 by RNAi did not affect sporogenesis on reproductive leaves but did affect antheridium production of the fern gametophyte. Expression patterns of CrEMS1 across developmental stages suggest threshold levels of CrEMS1 control the specification of reproductive organs during both generations of the fern. Additional EMS1 homologs present in the fern genome suggest a dynamic role of EMS1 receptors in the evolution of reproductive development in vascular plants.

Genome-wide analysis of the GDSL esterase/lipase family genes in Physcomitrium patens and the involvement of GELP31 in spore germination

In plants, the ability to produce hydrophobic substances that would provide protection from dehydration was required for the transition to land. This genome-wide investigation outlines the evolution of GDSL-type esterase/lipase (GELP) proteins in the moss Physcomitrium patens and suggests possible functions of some genes. GELP proteins play roles in the formation of hydrophobic polymers such as cutin and suberin that protect against dehydration and pathogen attack. GELP proteins are also implicated in processes such as pollen development and seed metabolism and germination. The P. patens GELP gene family comprises 48 genes and 14 pseudogenes. Phylogenetic analysis of all P. patens GELP sequences along with vascular plant GELP proteins with reported functions revealed that the P. patens genes clustered within previously identified A, B and C clades. A duplication model predicting the expansion of the GELP gene family within the P. patens lineage was constructed. Expression analysis combined with phylogenetic analysis suggested candidate genes for functions such as defence against pathogens, cutin metabolism, spore development and spore germination. The presence of relatively fewer GELP genes in P. patens may reduce the occurrence of functional redundancy that complicates the characterization of vascular plant GELP genes. Knockout lines of GELP31, which is highly expressed in sporophytes, were constructed. Gelp31 spores contained amorphous oil bodies and germinated late, suggesting (a) role(s) of GELP31 in lipid metabolism in spore development or germination. Future knockout studies of other candidate GELP genes will further elucidate the relationship between expansion of the family and the ability to withstand the harsh land environment.

Sporogenesis in Physcomitrium patens: Intergenerational collaboration and the development of the spore wall and aperture

Although the evolution of spores was critical to the diversification of plants on land, sporogenesis is incompletely characterized for model plants such as Physcomitrium patens. In this study, the complete process of P. patens sporogenesis is detailed from capsule expansion to mature spore formation, with emphasis on the construction of the complex spore wall and proximal aperture. Both diploid (sporophytic) and haploid (spores) cells contribute to the development and maturation of spores. During capsule expansion, the diploid cells of the capsule, including spore mother cells (SMCs), inner capsule wall layer (spore sac), and columella, contribute a locular fibrillar matrix that contains the machinery and nutrients for spore ontogeny. Nascent spores are enclosed in a second matrix that is surrounded by a thin SMC wall and suspended in the locular material. As they expand and separate, a band of exine is produced external to a thin foundation layer of tripartite lamellae. Dense globules assemble evenly throughout the locule, and these are incorporated progressively onto the spore surface to form the perine external to the exine. On the distal spore surface, the intine forms internally, while the spiny perine ornamentation is assembled. The exine is at least partially extrasporal in origin, while the perine is derived exclusively from outside the spore. Across the proximal surface of the polar spores, an aperture begins formation at the onset of spore development and consists of an expanded intine, an annulus, and a central pad with radiating fibers. This complex aperture is elastic and enables the proximal spore surface to cycle between being compressed (concave) and expanded (rounded). In addition to providing a site for water intake and germination, the elastic aperture is likely involved in desiccation tolerance. Based on the current phylogenies, the ancestral plant spore contained an aperture, exine, intine, and perine. The reductive evolution of liverwort and hornwort spores entailed the loss of perine in both groups and the aperture in liverworts. This research serves as the foundation for comparisons with other plant groups and for future studies of the developmental genetics and evolution of spores across plants.

MS1/MMD1 homologues in the moss Physcomitrium patens are required for male and female gametogenesis

The Arabidopsis Plant HomeoDomain (PHD) proteins AtMS1 and AtMMD1 provide chromatin-mediated transcriptional regulation essential for tapetum-dependent pollen formation. This pollen-based male gametogenesis is a derived trait of seed plants. Male gametogenesis in the common ancestors of land plants is instead likely to have been reminiscent of that in extant bryophytes where flagellated sperms are produced by an elaborate gametophyte generation. Still, also bryophytes possess MS1/MMD1-related PHD proteins. We addressed the function of two MS1/MMD1-homologues in the bryophyte model moss Physcomitrium patens by the generation and analysis of reporter and loss-of-function lines. The two genes are together essential for both male and female fertility by providing functions in the gamete-producing inner cells of antheridia and archegonia. They are furthermore expressed in the diploid sporophyte generation suggesting a function during sporogenesis, a process proposed related by descent to pollen formation in angiosperms. We propose that the moss MS1/MMD1-related regulatory network required for completion of male and female gametogenesis, and possibly for sporogenesis, represent a heritage from ancestral land plants.

Dependence on clade II bHLH transcription factors for nursing of haploid products by tapetal-like cells is conserved between moss sporangia and angiosperm anthers

Clade II basic helix-loop-helix transcription factors (bHLH TFs) are essential for pollen production and tapetal nursing functions in angiosperm anthers. As pollen has been suggested to be related to bryophyte spores by descent, we characterized two Physcomitrium (Physcomitrella) patens clade II bHLH TFs (PpbHLH092 and PpbHLH098), to test if regulation of sporogenous cells and the nursing cells surrounding them is conserved between angiosperm anthers and bryophyte sporangia. We made CRISPR-Cas9 reporter and loss-of-function lines to address the function of PpbHLH092/098. We sectioned and analyzed WT and mutant sporophytes for a comprehensive stage-by-stage comparison of sporangium development. Spore precursors in the P. patens sporangium are surrounded by nursing cells showing striking similarities to tapetal cells in angiosperms. Moss clade II bHLH TFs are essential for the differentiation of these tapetal-like cells and for the production of functional spores. Clade II bHLH TFs provide a conserved role in controlling the sporophytic somatic cells surrounding and nursing the sporogenous cells in both moss sporangia and angiosperm anthers. This supports the hypothesis that such nursing functions in mosses and angiosperms, lineages separated by c. 450 million years, are related by descent.

The Toughest Material in the Plant Kingdom: An Update on Sporopollenin

The extreme chemical and physical recalcitrance of sporopollenin deems this biopolymer among the most resilient organic materials on Earth. As the primary material fortifying spore and pollen cell walls, sporopollenin is touted as a critical innovation in the progression of plant life to a terrestrial setting. Although crucial for its protective role in plant reproduction, the inert nature of sporopollenin has challenged efforts to determine its composition for decades. Revised structural, chemical, and genetic experimentation efforts have produced dramatic advances in elucidating the molecular structure of this biopolymer and the mechanisms of its synthesis. Bypassing many of the challenges with material fragmentation and solubilization, insights from functional characterizations of sporopollenin biogenesis in planta, and in vitro, through a gene-targeted approach suggest a backbone of polyhydroxylated polyketide-based subunits and remarkable conservation of biochemical pathways for sporopollenin biosynthesis across the plant kingdom. Recent optimization of solid-state NMR and targeted degradation methods for sporopollenin analysis confirms polyhydroxylated α-pyrone subunits, as well as hydroxylated aliphatic units, and unique cross-linkage heterogeneity. We examine the cross-disciplinary efforts to solve the sporopollenin composition puzzle and illustrate a working model of sporopollenin's molecular structure and biosynthesis. Emerging controversies and remaining knowledge gaps are discussed, including the degree of aromaticity, cross-linkage profiles, and extent of chemical conservation of sporopollenin among land plants. The recent developments in sporopollenin research present diverse opportunities for harnessing the extraordinary properties of this abundant and stable biomaterial for sustainable microcapsule applications and synthetic material designs.

The land plant-specific MIXTA-MYB lineage is implicated in the early evolution of the plant cuticle and the colonization of land

The evolution of a lipid-based cuticle on aerial plant surfaces that protects against dehydration is considered a fundamental innovation in the colonization of the land by the green plants. However, key evolutionary steps in the early regulation of cuticle synthesis are still poorly understood, owing to limited studies in early-diverging land plant lineages. Here, we characterize a land plant specific subgroup 9 R2R3 MYB transcription factor MpSBG9, in the early-diverging land plant model Marchantia polymorpha, that is homologous to MIXTA proteins in vascular plants. The MpSBG9 functions as a key regulator of cuticle biosynthesis by preferentially regulating expression of orthologous genes for cutin formation, but not wax biosynthesis genes. The MpSBG9 also promotes the formation of papillate cells on the adaxial surface of M. polymorpha, which is consisitent with its canonical role in vascular plants. Our observations imply conserved MYB transcriptional regulation in the control of the cutin biosynthesis pathway as a core genetic network in the common ancestor of all land plants, implicating the land plant-specific MIXTA MYB lineage in the early origin and evolution of the cuticle.

Reactive oxygen species are required for spore wall formation in Physcomitrella patens

A robust spore wall was a key requirement of terrestrialization by early plants. Sporopollenin in spore and pollen grain walls is thought to be polymerized and cross-linked to other macromolecular components partly through oxidative processes involving H2O2. Therefore, we investigated effects of scavengers of reactive oxygen species (ROS) on formation of spore walls in the moss, Physcomitrella patens. Exposure of sporophytes, containing spores in the process of forming walls, to ascorbate, dimethylthiourea or 4-hydroxy-TEMPO prevented normal wall development in a dose, chemical and stage-dependent manner. Mature spores, exposed while developing to a ROS scavenger, burst when mounted in water on a flat slide under a coverslip (a phenomenon we named "augmented osmolysis" since they did not burst in phosphate-buffered saline or in water on a depression slide). Additionally, walls of exposed spores were more susceptible to alkaline hydrolysis than those of control spores and some were characterized by discontinuities in the exine, anomalies in perine spine structure, abnormal intine and aperture and occasionally wall shedding. Our data support involvement of oxidative cross-linking in spore wall development, including sporopollenin polymerization or deposition, as well as a role for ROS in intine/aperture development.

The Moss Physcomitrium (Physcomitrella) patens: A Model Organism for Non-Seed Plants

Since the discovery two decades ago that transgenes are efficiently integrated into the genome of Physcomitrella patens by homologous recombination, this moss has been a premier model system to study evolutionary developmental biology questions, stem cell reprogramming, and the biology of nonvascular plants. P patens was the first non-seed plant to have its genome sequenced. With this level of genomic information, together with increasing molecular genetic tools, a large number of reverse genetic studies have propelled the use of this model system. A number of technological advances have recently opened the door to forward genetics as well as extremely efficient and precise genome editing in P patens Additionally, careful phylogenetic studies with increased resolution have suggested that P patens emerged from within Physcomitrium Thus, rather than Physcomitrella patens, the species should be named Physcomitrium patens Here we review these advances and describe the areas where P patens has had the most impact on plant biology.

Acyl-CoA synthetases from Physcomitrella, rice and Arabidopsis: different substrate preferences but common regulation by MS188 in sporopollenin synthesis

ACOS5, OsACOS12 and PpACOS6 are all capable of fatty acyl-CoA synthetase activity but exhibit different substrate preferences. The transcriptional regulation of ACOS for sporopollenin synthesis appears to have been conserved in Physcomitrella, rice and Arabidopsis during evolution. Sporopollenin is the major constituent of spore and pollen exines. In Arabidopsis, acyl-CoA synthetase 5 (ACOS5) is an essential enzyme for sporopollenin synthesis, and its orthologues are PpACOS6 from the moss Physcomitrella and OsACOS12 from monocot rice. However, knowledge regarding the evolutionary conservation and divergence of the ACOS gene in sporopollenin synthesis remains limited. In this study, we analysed the function and regulation of PpACOS6 and OsACOS12. A complementation test showed that OsACOS12 driven by the ACOS5 promoter could partially restore the male fertility of the acos5 mutant in Arabidopsis, while PpACOS6 did not rescue the acos5 phenotype. ACOS5, PpACOS6 and OsACOS12 all complemented the acyl-CoA synthetase-deficient yeast strain (YB525) phenotype, although they exhibited different substrate preferences. To understand the conservation of sporopollenin synthesis regulation, we constructed two constructs with ACOS5 driven by the OsACOS12 or PpACOS6 promoter. Both constructs could restore the fertility of acos5 plants. The MYB transcription factor MS188 from Arabidopsis directly regulates ACOS5. We found that MS188 could also bind the promoters of OsACOS12 and PpACOS6 and activate the genes driven by the promoters, suggesting that the transcriptional regulation of these genes was similar to that of ACOS5. These results show that the ACOS gene promoter region from Physcomitrella, rice and Arabidopsis has been functionally conserved during evolution, while the chain lengths of fatty acid-derived monomers of sporopollenin vary in different plant species.

Studies of a rice sterile mutant sstl from the TRIM collection

BACKGROUND

Rice (Oryza sativa) is one of the main crops in the world, and more than 3.9 billion people will consume rice by 2025. Sterility significantly affects rice production and leads to yield defects. The undeveloped anthers or abnormal pollen represent serious defects in rice male sterility. Therefore, understanding the mechanism of male sterility is an important task. Here, we investigated a rice sterile mutant according to its developmental morphology and transcriptional profiles.

RESULTS

An untagged T-DNA insertional mutant showed defective pollen and abnormal anthers as compared with its semi-sterile mutant (sstl) progeny segregates. Transcriptomic analysis of sterile sstl-s revealed several biosynthesis pathways, such as downregulated cell wall, lipids, secondary metabolism, and starch synthesis. This downregulation is consistent with the morphological characterization of sstl-s anthers with irregular exine, absence of intine, no starch accumulation in pollen grains and no accumulated flavonoids in anthers. Moreover, defective microsporangia development led to abnormal anther locule and aborted microspores. The downregulated lipids, starch, and cell wall synthesis-related genes resulted in loss of fertility.

CONCLUSIONS

We illustrate the importance of microsporangia in the development of anthers and functional microspores. Abnormal development of pollen grains, pollen wall, anther locule, etc. result in severe yield reduction.

OsPKS1 is required for sexine layer formation, which shows functional conservation between rice and Arabidopsis

The sporopollenin precursors, as a general constituent of sexine, are synthesized in the tapetum and deposited on the pollen surface after transportation and processing. The polyketide synthase condenses the acyl-CoA into a hydroxyalkyl α-pyrone, which is predicted to be a component of the sporopollenin precursors. In this study, we found that the rice POLYKETIDE SYNTHASE 1 (OsPKS1) was the orthologue of Arabidopsis POLYKETIDE SYNTHASE A/LESS ADHESIVE POLLEN 6 (PKSA/LAP6) through sequence alignment. The OsPKS1 knockout mutants obtained by Crispr-Cas9-mediated editing exhibited a complete male sterile phenotype. Cytological observations revealed that abnormal bacula deposition and ubisch body structures for sexine formation led to pollen rupture in ospks1. The expression analysis showed that the OsPKS1 was highly expressed in tapetal cells and anther locules from stage 9 to stage 11 during anther development in rice. Subcellular localization demonstrated that the OsPKS1 protein was preferentially localized to the ER. The genomic sequence of OsPKS1 driven by the PKSA/LAP6 promoter restored the sexine pattern of Arabidopsis pksa/lap6. These results indicated that OsPKS1 is required for sexine layer formation in rice and functionally conserved in the sporopollenin synthesis pathway.

The Physcomitrella patens gene atlas project: large-scale RNA-seq based expression data

High-throughput RNA sequencing (RNA-seq) has recently become the method of choice to define and analyze transcriptomes. For the model moss Physcomitrella patens, although this method has been used to help analyze specific perturbations, no overall reference dataset has yet been established. In the framework of the Gene Atlas project, the Joint Genome Institute selected P. patens as a flagship genome, opening the way to generate the first comprehensive transcriptome dataset for this moss. The first round of sequencing described here is composed of 99 independent libraries spanning 34 different developmental stages and conditions. Upon dataset quality control and processing through read mapping, 28 509 of the 34 361 v3.3 gene models (83%) were detected to be expressed across the samples. Differentially expressed genes (DEGs) were calculated across the dataset to permit perturbation comparisons between conditions. The analysis of the three most distinct and abundant P. patens growth stages - protonema, gametophore and sporophyte - allowed us to define both general transcriptional patterns and stage-specific transcripts. As an example of variation of physico-chemical growth conditions, we detail here the impact of ammonium supplementation under standard growth conditions on the protonemal transcriptome. Finally, the cooperative nature of this project allowed us to analyze inter-laboratory variation, as 13 different laboratories around the world provided samples. We compare differences in the replication of experiments in a single laboratory and between different laboratories.

OsPKS2 is required for rice male fertility by participating in pollen wall formation

OsPKS2, the rice orthologous gene of Arabidopsis PKSB/LAP5, encodes a polyketide synthase that is involved in pollen wall formation in rice. In flowering plants, the pollen wall protects male gametes from various environmental stresses and pathogen attacks, as well as promotes pollen germination. The biosynthesis of sporopollenin in tapetal cell is critical for pollen wall formation. Recently, progress has been made in understanding sporopollenin metabolism during pollen wall development in Arabidopsis. However, little is known about the molecular mechanism that underlies the sporopollenin synthesis in pollen wall formation in rice (Oryza sativa). In this study, we identified that a point mutation in OsPKS2, a plant-specific type III polyketide synthase gene, caused male sterility in rice by affecting the normal progress of pollen wall formation. Two other allelic mutants of OsPKS2 were generated using the CRISPR/Cas9 system and are also completely male sterile. This result thus further confirmed that OsPKS2 controls rice male fertility. We also showed that OsPKS2 is an orthologous gene of Arabidopsis PKSB/LAP5 and has a tapetum-specific expression pattern. In addition, its product localizes in the endoplasmic reticulum. Results suggested that OsPKS2 is critical for pollen wall formation, and plays a conserved but differentiated role in sporopollenin biosynthesis from Arabidopsis.

Great moments in evolution: the conquest of land by plants

500Ma ago the terrestrial habitat was a barren, unwelcoming place for species other than, for example, bacteria or fungi. Most probably, filamentous freshwater algae adapted to aerial conditions and eventually conquered land. Adaptation to a severely different habitat apparently included sturdy cell walls enabling an erect body plan as well as protection against abiotic stresses such as ultraviolet radiation, drought and varying temperature. To thrive on land, plants probably required more elaborate signaling pathways to react to diverse environmental conditions, and phytohormones to control developmental programs. Many such plant-typical features have been studied in flowering plants, but their evolutionary origins were long clouded. With the sequencing of a moss genome a decade ago, inference of ancestral land plant states using comparative genomics, phylogenomics and evolutionary developmental approaches began in earnest. In the past few years, the ever increasing availability of genomic and transcriptomic data of organisms representing the earliest common ancestors of the plant tree of life has much informed our understanding of the conquest of land by plants.

PpORS, an ancient type III polyketide synthase, is required for integrity of leaf cuticle and resistance to dehydration in the moss, Physcomitrella patens

PpORS knockout mutants produced abnormal leaves with increased dye permeability and were more susceptible to dehydration, consistent with PpORS products being constituents of a cuticular structure in the moss. Type III polyketide synthases (PKSs) have co-evolved with terrestrial plants such that each taxon can generate a characteristic collection of polyketides, fine-tuned to its needs. 2'-Oxoalkylresorcinol synthase from Physcomitrella patens (PpORS) is basal to all plant type III PKSs in phylogenetic trees and may closely resemble their most recent common ancestor. To gain insight into the roles that ancestral plant type III PKSs might have played during early land plant evolution, we constructed and phenotypically characterized targeted knockouts of PpORS. Ors gametophores, unless submerged in water while they were developing, displayed various leaf malformations that included grossly misshapen leaves, missing or abnormal midribs, multicellular protuberances and localized necrosis. Ors leaves, particularly abnormal ones, showed increased permeability to the hydrophilic dye, toluidine blue. Ors gametophores lost water faster and were more susceptible to dehydration than those of the control strain. Our findings are consistent with ors leaves possessing a partially defective cuticle and implicate PpORS in synthesis of the intact cuticle. PpORS orthologs are present in a few moss species but have not been found in other plants. However, conceivably an ancestral ORS in early land plants may have contributed to their protection from dehydration.

OsLAP6/OsPKS1, an orthologue of Arabidopsis PKSA/LAP6, is critical for proper pollen exine formation

BACKGROUND

Male fertility is crucial for rice yield, and the improvement of rice yield requires hybrid production that depends on male sterile lines. Although recent studies have revealed several important genes in male reproductive development, our understanding of the mechanisms of rice pollen development remains unclear.

RESULTS

We identified a rice mutant oslap6 with complete male sterile phenotype caused by defects in pollen exine formation. By using the MutMap method, we found that a single nucleotide polymorphism (SNP) variation located in the second exon of OsLAP6/OsPKS1 was responsible for the mutant phenotype. OsLAP6/OsPKS1 is an orthologous gene of Arabidopsis PKSA/LAP6, which functions in sporopollenin metabolism. Several other loss-of-function mutants of OsLAP6/OsPKS1 generated by the CRISPR/Cas9 genomic editing tool also exhibited the same phenotype of male sterility. Our cellular analysis suggested that OsLAP6/OsPKS1 might regulate pollen exine formation by affecting bacula elongation. Expression examination indicated that OsLAP6/OsPKS1 is specifically expressed in tapetum, and its product is localized to the endoplasmic reticulum (ER). Protein sequence analysis indicated that OsLAP6/OsPKS1 is conserved in land plants.

CONCLUSIONS

OsLAP6/OsPKS1 is a critical molecular switch for rice male fertility by participating in a conserved sporopollenin precursor biosynthetic pathway in land plants. Manipulation of OsLAP6/OsPKS1 has potential for application in hybrid rice breeding.

Insights into Land Plant Evolution Garnered from the Marchantia polymorpha Genome

The evolution of land flora transformed the terrestrial environment. Land plants evolved from an ancestral charophycean alga from which they inherited developmental, biochemical, and cell biological attributes. Additional biochemical and physiological adaptations to land, and a life cycle with an alternation between multicellular haploid and diploid generations that facilitated efficient dispersal of desiccation tolerant spores, evolved in the ancestral land plant. We analyzed the genome of the liverwort Marchantia polymorpha, a member of a basal land plant lineage. Relative to charophycean algae, land plant genomes are characterized by genes encoding novel biochemical pathways, new phytohormone signaling pathways (notably auxin), expanded repertoires of signaling pathways, and increased diversity in some transcription factor families. Compared with other sequenced land plants, M. polymorpha exhibits low genetic redundancy in most regulatory pathways, with this portion of its genome resembling that predicted for the ancestral land plant. PAPERCLIP.

Sexual reproduction, sporophyte development and molecular variation in the model moss Physcomitrella patens: introducing the ecotype Reute

Rich ecotype collections are used for several plant models to unravel the molecular causes of phenotypic differences, and to investigate the effects of environmental adaption and acclimation. For the model moss Physcomitrella patens collections of accessions are available, and have been used for phylogenetic and taxonomic studies, for example, but few have been investigated further for phenotypic differences. Here, we focus on the Reute accession and provide expression profiling and comparative developmental data for several stages of sporophyte development, as well as information on genetic variation via genomic sequencing. We analysed cross-technology and cross-laboratory data to define a confident set of 15 mature sporophyte-specific genes. We find that the standard laboratory strain Gransden produces fewer sporophytes than Reute or Villersexel, although gametangia develop with the same time course and do not show evident morphological differences. Reute exhibits less genetic variation relative to Gransden than Villersexel, yet we found variation between Gransden and Reute in the expression profiles of several genes, as well as variation hot spots and genes that appear to evolve under positive Darwinian selection. We analyzed expression differences between the ecotypes for selected candidate genes in the GRAS transcription factor family, the chalcone synthase family and in genes involved in cell wall modification that are potentially related to phenotypic differences. We confirm that Reute is a P. patens ecotype, and suggest its use for reverse-genetics studies that involve progression through the life cycle and multiple generations.

Development and genetics in the evolution of land plant body plans

The colonization of land by plants shaped the terrestrial biosphere, the geosphere and global climates. The nature of morphological and molecular innovation driving land plant evolution has been an enigma for over 200 years. Recent phylogenetic and palaeobotanical advances jointly demonstrate that land plants evolved from freshwater algae and pinpoint key morphological innovations in plant evolution. In the haploid gametophyte phase of the plant life cycle, these include the innovation of mulitcellular forms with apical growth and multiple growth axes. In the diploid phase of the life cycle, multicellular axial sporophytes were an early innovation priming subsequent diversification of indeterminate branched forms with leaves and roots. Reverse and forward genetic approaches in newly emerging model systems are starting to identify the genetic basis of such innovations. The data place plant evo-devo research at the cusp of discovering the developmental and genetic changes driving the radiation of land plant body plans.This article is part of the themed issue 'Evo-devo in the genomics era, and the origins of morphological diversity'.