The impact of heat stress in plant reproduction

The increment in global temperature reduces crop productivity, which in turn threatens food security. Currently, most of our food supply is produced by plants and the human population is estimated to reach 9 billion by 2050. Gaining insights into how plants navigate heat stress in their reproductive phase is essential for effectively overseeing the future of agricultural productivity. The reproductive success of numerous plant species can be jeopardized by just one exceptionally hot day. While the effects of heat stress on seedlings germination and root development have been extensively investigated, studies on reproduction are limited. The intricate processes of gamete development and fertilization unfold within a brief timeframe, largely concealed within the flower. Nonetheless, heat stress is known to have important effects on reproduction. Considering that heat stress typically affects both male and female reproductive structures concurrently, it remains crucial to identify cultivars with thermotolerance. In such cultivars, ovules and pollen can successfully undergo development despite the challenges posed by heat stress, enabling the completion of the fertilization process and resulting in a robust seed yield. Hereby, we review the current understanding of the molecular mechanisms underlying plant resistance to abiotic heat stress, focusing on the reproductive process in the model systems of Arabidopsis and Oryza sativa.

SEEDSTICK Controls Arabidopsis Fruit Size by Regulating Cytokinin Levels and FRUITFULL

Upon fertilization, the ovary increases in size and undergoes a complex developmental process to become a fruit. We show that cytokinins (CKs), which are required to determine ovary size before fertilization, have to be degraded to facilitate fruit growth. The expression of CKX7, which encodes a cytosolic CK-degrading enzyme, is directly positively regulated post-fertilization by the MADS-box transcription factor STK. Similar to stk, two ckx7 mutants possess shorter fruits than wild type. Quantification of CKs reveals that stk and ckx7 mutants have high CK levels, which negatively control cell expansion during fruit development, compromising fruit growth. Overexpression of CKX7 partially complements the stk fruit phenotype, confirming a role for CK degradation in fruit development. Finally, we show that STK is required for the expression of FUL, which is essential for valve elongation. Overall, we provide insights into the link between CKs and molecular pathways that control fruit growth.

The regulatory framework of developmentally programmed cell death in floral organs: A review

Developmentally programmed cell death (dPCD) is a tightly controlled biological process. In recent years, vital roles of dPCD on regulating floral organ growth and development have been reported. It is well known that flower is an essential organ for reproduction and a turning point of plants' life cycle. Hence, uncovering the complex molecular networks which regulates dPCD processes in floral organs is utmost important. So far, our understanding of dPCD on floral organ growth and development is just starting. Herein, we summarize the important factors that involved in the tapetal degeneration, pollen tube rupture, receptive synergid cell death, nucellar degradation, and antipodal cell degradation. Meanwhile, the known factors that involved in transmitting tract formation and self-incompatibility-induced PCD were also introduced. Furthermore, the genes that associated with anther dehiscence and petal senescence and abscission were reviewed as well. The functions of various types of factors involved in floral dPCD processes are highlighted principally. The regulatory panorama described here can provide us some insights about flower-specific dPCD process.

The RNA-dependent DNA methylation pathway is required to restrict SPOROCYTELESS/NOZZLE expression to specify a single female germ cell precursor in Arabidopsis

In higher plants, the female germline is formed from the megaspore mother cell (MMC), a single cell in the premeiotic ovule. Previously, it was reported that mutants in the RNA-dependent DNA methylation (RdDM) pathway might be involved in restricting the female germline to a single nucellus cell. We show that the DRM methyltransferase double mutant drm1drm2 also presents ectopic enlarged cells, consistent with supernumerary MMC-like cells. In wild-type ovules, MMC differentiation requires SPOROCYTELESS/NOZZLE (SPL/NZZ), as demonstrated by the spl/nzz mutant failing to develop an MMC. We address the poorly understood upstream regulation of SPL/NZZ in ovules, showing that the RdDM pathway is important to restrict SPL/NZZ expression. In ago9, rdr6 and drm1drm2 mutants, SPL/NZZ is expressed ectopically, suggesting that the multiple MMC-like cells observed might be attributable to the ectopic expression of SPL/NZZ. We show that the ovule identity gene, SEEDSTICK, directly regulates AGO9 and RDR6 expression in the ovule and therefore indirectly regulates SPL/NZZ expression. A model is presented describing the network required to restrict SPL/NZZ expression to specify a single MMC.

The Importance of Cytokinins during Reproductive Development in Arabidopsis and Beyond

Fertilization and seed formation are fundamental events in the life cycle of flowering plants. The seed is a functional unit whose main purpose is to propagate the plant. The first step in seed development is the formation of male and female gametophytes and subsequent steps culminate in successful fertilization. The detailed study of this process is highly relevant because it directly impacts human needs, such as protecting biodiversity and ensuring sustainable agriculture to feed the increasing world population. Cytokinins comprise a class of phytohormones that play many important roles during plant growth and development and in recent years, the role of this class of phytohormones during reproduction has become clear. Here, we review the role of cytokinins during ovule, pollen and seed formation at the genetic and molecular levels. The expansion of knowledge concerning the molecular mechanisms that control plant reproduction is extremely important to optimise seed production.

Arabidopsis REM16 acts as a B3 domain transcription factor to promote flowering time via directly binding to the promoters of SOC1 and FT

Actin depolymerizing factor (ADF) is a key modulator for dynamic organization of actin cytoskeleton. Interestingly, it was found that the ADF1 gene silencing delays flowering, but its mechanism remains unclear. In this study, ADF1 was used as a bait to screen its interacting proteins by the yeast two-hybrid (Y2H) system. One of them, the REM16 transcription factor was identified. As one of the AP2/B3-like transcriptional factor family members, the REM16 contains two B3 domains and its transcript levels kept increasing during the floral transition stage. Overexpression of REM16 accelerates flowering while silencing of REM16 delays flowering. Gene expression analysis indicated that the key flowering activation genes such as CONSTANS (CO), FLOWERING LOCUS T (FT), LEAFY (LFY) and SUPPRESSOR OF OVEREXPRESSION OF CONSTANS (SOC1) were upregulated in the REM16 overexpression lines, while the transcription of the flowering suppression gene FLOWERING LOCUS C (FLC) was decreased. In contrast, the REM16 gene silencing lines contained lower transcript levels of the CO, FT, LFY and SOC1 but higher transcript levels of the FLC compared with the wild-type plants. It was proved that REM16 could directly bind to the promoter regions of SOC1 and FT by in vitro and in vivo assays. Genetic analysis supported that REM16 acts upstream of SOC1 and FT in flowering pathways. All these studies provided strong evidence demonstrating that REM16 promotes flowering by directly activating SOC1 and FT.

BPC transcription factors and a Polycomb Group protein confine the expression of the ovule identity gene SEEDSTICK in Arabidopsis

The BASIC PENTACYSTEINE (BPC) GAGA (C-box) binding proteins belong to a small plant transcription factor family. We previously reported that class I BPCs bind directly to C-boxes in the SEEDSTICK (STK) promoter and the mutagenesis of these cis-elements affects STK expression in the flower. The MADS-domain factor SHORT VEGETATIVE PHASE (SVP) is another key regulator of STK. Direct binding of SVP to CArG-boxes in the STK promoter are required to repress its expression during the first stages of flower development. Here we show that class II BPCs directly interact with SVP and that MADS-domain binding sites in the STK promoter region are important for the correct spatial and temporal expression of this homeotic gene. Furthermore, we show that class I and class II BPCs act redundantly to repress STK expression in the flower, most likely by recruiting TERMINAL FLOWER 2/LIKE HETEROCHROMATIN PROTEIN 1 (TFL2/LHP1) and mediating the establishment and the maintenance of H3K27me3 repressive marks on DNA. We investigate the role of LHP1 in the regulation of STK expression. In addition to providing a better understanding of the role of BPC transcription factors in the regulation of STK expression, our results suggest the existence of a more general regulatory complex composed of BPCs, MADS-domain factors and Polycomb Repressive Complexes that co-operate to regulate gene expression in reproductive tissues. We believe that our data along with the molecular model described here could provide significant insights for a more comprehensive understanding of gene regulation in plants.

Reproductive Multitasking: The Female Gametophyte

Fertilization of flowering plants requires the organization of complex tasks, many of which become integrated by the female gametophyte (FG). The FG is a few-celled haploid structure that orchestrates division of labor to coordinate successful interaction with the sperm cells and their transport vehicle, the pollen tube. As reproductive outcome is directly coupled to evolutionary success, the underlying mechanisms are under robust molecular control, including integrity check and repair mechanisms. Here, we review progress on understanding the development and function of the FG, starting with the functional megaspore, which represents the haploid founder cell of the FG. We highlight recent achievements that have greatly advanced our understanding of pollen tube attraction strategies and the mechanisms that regulate plant hybridization and gamete fusion. In addition, we discuss novel insights into plant polyploidization strategies that expand current concepts on the evolution of flowering plants.

MADS-Box and bHLH Transcription Factors Coordinate Transmitting Tract Development in Arabidopsis thaliana

The MADS-domain transcription factor SEEDSTICK (STK) controls several aspects of plant reproduction. STK is co-expressed with CESTA (CES), a basic Helix-Loop-Helix (bHLH) transcription factor-encoding gene. CES was reported to control redundantly with the brassinosteroid positive signaling factors BRASSINOSTEROID ENHANCED EXPRESSION1 (BEE1) and BEE3 the development of the transmitting tract. Combining the stk ces-4 mutants led to a reduction in ovule fertilization due to a defect in carpel fusion which, caused the formation of holes at the center of the septum where the transmitting tract differentiates. Combining the stk mutant with the bee1 bee3 ces-4 triple mutant showed an increased number of unfertilized ovules and septum defects. The transcriptome profile of this quadruple mutant revealed a small subset of differentially expressed genes which are mainly involved in cell death, extracellular matrix and cell wall development. Our data evidence a regulatory gene network controlling transmitting tract development regulated directly or indirectly by a STK-CES containing complex and reveal new insights in the regulation of transmitting tract development by bHLH and MADS-domain transcription factors.

Peptide/receptor-like kinase-mediated signaling involved in male-female interactions

In flowering plants, extensive male-female interactions during pollen germination on the stigma, pollen tube growth and guidance in the transmitting tract, and pollen tube reception by the female gametophyte are required for successful double fertilization in which various signaling cascades are involved. Peptide/receptor-like kinase-mediated signaling has been found playing important roles in these male-female interactions. Here, we mainly summarized the progress made on the regulatory roles of peptide/receptor-like kinase-mediated signaling pathways in four critical stages during reproduction in higher plants.

Insights into secrets along the pollen tube pathway in need to be discovered

The process of plant fertilization provides an outstanding example of refined control of gene expression. During this elegant process, subtle communication occurs between neighboring cells, based on chemical signals, that induces cellular mechanisms of patterning and growth. Having faced an immediate issue of self-incompatibility responses, the pathway to fertilization starts once the stigmatic cells recognize a compatible pollen grain, and it continues with numerous players interacting to affect pollen tube growth and the puzzling process of navigation along the transmitting tract. The pollen tube goes through a guidance process that begins with a preovular stage (i.e. prior to the influence of the target ovule), with interactions with factors from the transmitting tissue. In the subsequent ovular-guidance stage a specific relationship develops between the pollen tube and its target ovule. This stage is divided into the funicular and micropylar guidance steps, with numerous receptors working in signalling cascades. Finally, just after the pollen tube has passed beyond the synergids, fusion of the gametes occurs and the developing seed-the ultimate aim of the process-will start to mature. In this paper, we review the existing knowledge of the crucial biological processes involved in pollen-pistil interactions that give rise to the new seed.

New roles of NO TRANSMITTING TRACT and SEEDSTICK during medial domain development in Arabidopsis fruits

The gynoecium, the female reproductive part of the flower, is key for plant sexual reproduction. During its development, inner tissues such as the septum and the transmitting tract tissue, important for pollen germination and guidance, are formed. In Arabidopsis, several transcription factors are known to be involved in the development of these tissues. One of them is NO TRANSMITTING TRACT (NTT), essential for transmitting tract formation. We found that the NTT protein can interact with several gynoecium-related transcription factors, including several MADS-box proteins, such as SEEDSTICK (STK), known to specify ovule identity. Evidence suggests that NTT and STK control enzyme and transporter-encoding genes involved in cell wall polysaccharide and lipid distribution in gynoecial medial domain cells. The results indicate that the simultaneous loss of NTT and STK activity affects polysaccharide and lipid deposition and septum fusion, and delays entry of septum cells to their normal degradation program. Furthermore, we identified KAWAK, a direct target of NTT and STK, which is required for the correct formation of fruits in Arabidopsis These findings position NTT and STK as important factors in determining reproductive competence.

REM34 and REM35 Control Female and Male Gametophyte Development in Arabidopsis thaliana

The REproductive Meristem (REM) gene family encodes for transcription factors belonging to the B3 DNA binding domain superfamily. In Arabidopsis thaliana, the REM gene family is composed of 45 members, preferentially expressed during flower, ovule, and seed developments. Only a few members of this family have been functionally characterized: VERNALIZATION1 (VRN1) and, most recently, TARGET OF FLC AND SVP1 (TFS1) regulate flowering time and VERDANDI (VDD), together with VALKYRIE (VAL) that control the death of the receptive synergid cell in the female gametophyte. We investigated the role of REM34, REM35, and REM36, three closely related and linked genes similarly expressed in both female and male gametophytes. Simultaneous silencing by RNA interference (RNAi) caused about 50% of the ovules to remain unfertilized. Careful evaluation of both ovule and pollen developments showed that this partial sterility of the transgenic RNAi lines was due to a postmeiotic block in both female and male gametophytes. Furthermore, protein interaction assays revealed that REM34 and REM35 interact, which suggests that they work together during the first stages of gametogenesis.

Friend or foe: Signaling mechanisms during double fertilization in flowering seed plants

Since the first description of double fertilization 120 years ago, the processes of pollen tube growth and guidance, sperm cell release inside the receptive synergid cell, as well as fusion of two sperm cells to the female gametes (egg and central cell) have been well documented in many flowering plants. Especially microscopic techniques, including live cell imaging, were used to visualize these processes. Molecular as well as genetic methods were applied to identify key players involved. However, compared to the first 11 decades since its discovery, the past decade has seen a tremendous advancement in our understanding of the molecular mechanisms regulating angiosperm fertilization. Whole signaling networks were elucidated including secreted ligands, corresponding receptors, intracellular interaction partners, and further downstream signaling events involved in the cross-talk between pollen tubes and their cargo with female reproductive cells. Biochemical and structural biological approaches are now increasingly contributing to our understanding of the different signaling processes required to distinguish between compatible and incompatible interaction partners. Here, we review the current knowledge about signaling mechanisms during above processes with a focus on the model plants Arabidopsis thaliana and Zea mays (maize). The analogy that many of the identified "reproductive signaling mechanisms" also act partly or fully in defense responses and/or cell death is also discussed.

Transcriptional networks orchestrating programmed cell death during plant development

Transcriptional gene regulation is a fundamental biological principle in the development of eukaryotes. It does control not only cell proliferation, specification, and differentiation, but also cell death processes as an integral feature of an organism's developmental program. As in animals, developmentally regulated cell death in plants occurs in numerous contexts and is of vital importance for plant vegetative and reproductive development. In comparison with the information available on the molecular regulation of programmed cell death (PCD) in animals, however, our knowledge on plant PCD still remains scarce. Here, we discuss the functions of different classes of transcription factors that have been implicated in the control of developmentally regulated cell death. Though doubtlessly representing but a first layer of PCD regulation, information on PCD-regulating transcription factors and their targets represents a promising strategy to understand the complex machinery that ensures the precise and failsafe execution of PCD processes in plant development.

Contribution of cellular autolysis to tissular functions during plant development

Plant development requires specific cells to be eliminated in a predictable and genetically regulated manner referred to as programmed cell death (PCD). However, the target cells do not merely die but they also undergo autolysis to degrade their cellular corpses. Recent progress in understanding developmental cell elimination suggests that distinct proteins execute PCD sensu stricto and autolysis. In addition, cell death alone and cell dismantlement can fulfill different functions. Hence, it appears biologically meaningful to distinguish between the modules of PCD and autolysis during plant development.

Dying two deaths - programmed cell death regulation in development and disease

Programmed cell death (PCD) is a fundamental cellular process that has adopted a plethora of vital functions in multicellular organisms. In plants, PCD processes are elicited as an inherent part of regular development in specific cell types or tissues, but can also be triggered by biotic and abiotic stresses. Although over the last years we have seen progress in our understanding of the molecular regulation of different plant PCD processes, it is still unclear whether a common core machinery exists that controls cell death in development and disease. In this review, we discuss recent advances in the field, comparing some aspects of the molecular regulation controlling developmental and pathogen-triggered PCD in plants.

Dying to live: cell elimination as a developmental strategy in angiosperm seeds

The complete elimination of unwanted cells during development is a repeated theme in both multicellular animals and in plants. In plants, such events have been extensively studied and reviewed in terms of their molecular regulation, of marker genes and proteins expressed, and in terms of cellular changes associated with their progression. This review will take a slightly different view of developmental cell elimination and will concentrate specifically on the numerous elimination events that occur during ovule and seed development (here grouped together as seed development). It asks why this cell elimination occurs in specific seed tissues, in order to understand something about the commonalities underlying how seemingly disparate events are triggered and regulated. Finally, by placing the seed in its broader evolutionary context, the question of why cell elimination may have emerged as such a key component of the seed developmental toolbox will be considered.

Arabinogalactan Proteins as Interactors along the Crosstalk between the Pollen Tube and the Female Tissues

Arabinogalactan proteins (AGPs) have long been considered to be implicated in several steps of the reproductive process of flowering plants. Pollen tube growth along the pistil tissues requires a multiplicity of signaling pathways to be activated and turned off precisely, at crucial timepoints, to guarantee successful fertilization and seed production. In the recent years, an outstanding effort has been made by the plant reproduction scientific community in order to better understand this process. This resulted in the discovery of a fairly substantial number of new players essential for reproduction, as well as their modes of action and interactions. Besides all the indications of AGPs involvement in reproduction, there were no convincing evidences about it. Recently, several studies came out to prove what had long been suggested about this complex family of glycoproteins. AGPs consist of a large family of hydroxyproline-rich proteins, predicted to be anchored to the plasma membrane and extremely rich in sugars. These two last characteristics always made them perfect candidates to be involved in signaling mechanisms, in several plant developmental processes. New findings finally relate AGPs to concrete functions in plant reproduction. In this review, it is intended not only to describe how different molecules and signaling pathways are functioning to achieve fertilization, but also to integrate the recent discoveries about AGPs along this process.