Pathogens that rely upon multiple hosts to complete their life cycles often modify behavior and development of these hosts to coerce them into improving pathogen fitness. However, few studies ...describe mechanisms underlying host coercion. In this study, we elucidate the mechanism by which an insect-transmitted pathogen of plants alters floral development to convert flowers into vegetative tissues. We find that phytoplasma produce a novel effector protein (SAP54) that interacts with members of the MADS-domain transcription factor (MTF) family, including key regulators SEPALLATA3 and APETALA1, that occupy central positions in the regulation of floral development. SAP54 mediates degradation of MTFs by interacting with proteins of the RADIATION SENSITIVE23 (RAD23) family, eukaryotic proteins that shuttle substrates to the proteasome. Arabidopsis rad23 mutants do not show conversion of flowers into leaf-like tissues in the presence of SAP54 and during phytoplasma infection, emphasizing the importance of RAD23 to the activity of SAP54. Remarkably, plants with SAP54-induced leaf-like flowers are more attractive for colonization by phytoplasma leafhopper vectors and this colonization preference is dependent on RAD23. An effector that targets and suppresses flowering while simultaneously promoting insect herbivore colonization is unprecedented. Moreover, RAD23 proteins have, to our knowledge, no known roles in flower development, nor plant defence mechanisms against insects. Thus SAP54 generates a short circuit between two key pathways of the host to alter development, resulting in sterile plants, and promotes attractiveness of these plants to leafhopper vectors helping the obligate phytoplasmas reproduce and propagate (zombie plants).
Somatic embryogenesis is an example of induced cellular totipotency, where embryos develop from vegetative cells rather than from gamete fusion. Somatic embryogenesis can be induced in vitro by ...exposing explants to growth regulators and/or stress treatments. The BABY BOOM (BBM) and LEAFY COTYLEDON1 (LEC1) and LEC2 transcription factors are key regulators of plant cell totipotency, as ectopic overexpression of either transcription factor induces somatic embryo formation from Arabidopsis (Arabidopsis thaliana) seedlings without exogenous growth regulators or stress treatments. Although LEC and BBM proteins regulate the same developmental process, it is not known whether they function in the same molecular pathway. We show that BBM transcriptionally regulates LEC1 and LEC2, as well as the two other LAFL genes, FUSCA3 (FUS3) and ABSCISIC ACID INSENSITIVE3 (ABI3). LEC2 and ABI3 quantitatively regulate BBM-mediated somatic embryogenesis, while FUS3 and LEC1 are essential for this process. BBM-mediated somatic embryogenesis is dose and context dependent, and the context-dependent phenotypes are associated with differential LAFL expression. We also uncover functional redundancy for somatic embryogenesis among other Arabidopsis BBM-like proteins and show that one of these proteins, PLETHORA2, also regulates LAFL gene expression. Our data place BBM upstream of other major regulators of plant embryo identity and totipotency.
Members of the MADS-box transcription factor family play essential roles in almost every developmental process in plants. Many MADS-box genes have conserved functions across the flowering plants, but ...some have acquired novel functions in specific species during evolution. The analyses of MADS-domain protein interactions and target genes have provided new insights into their molecular functions. Here, we review recent findings on MADS-box gene functions in Arabidopsis and discuss the evolutionary history and functional diversification of this gene family in plants. We also discuss possible mechanisms of action of MADS-domain proteins based on their interactions with chromatin-associated factors and other transcriptional regulators.
Growth and ripening of fleshy fruits are intricately regulated and yield an important part of the human diet. Here we review current knowledge of the regulation at the transcriptional level.
The study of transcriptional regulation of tomato ripening has been led by spontaneous mutations in transcription factor (TF) genes that completely inhibit normal ripening, suggesting that they are ...‘master regulators’. Studies using CRISPR/Cas9 mutagenesis to produce knockouts of the underlying genes indicate a different picture, suggesting that the regulation is more robust than previously thought. This requires us to revisit our model of the regulation of ripening and replace it with one involving a network of partially redundant components. At the same time, the fast rise of CRISPR/Cas mutagenesis, resulting in unexpectedly weak phenotypes, compared with knockdown technology, suggests that compensatory mechanisms may obscure protein functions. This emphasises the need for assessment of these mechanisms in plants and for the careful design of mutagenesis experiments.
A string of recent publications has put into question the function of TFs, which were previously considered master regulators in the control of tomato fruit ripening.RNA interference and CRISPR/Cas9 mutagenesis have enabled a re-evaluation of their proposed functions, which were previously thought to be mostly derived from the phenotype of spontaneous ripening mutations.Three spontaneous mutations in TF genes all appear to be gain-of-function mutations and, interestingly, probably represent three different mechanisms of this phenomenon.These observations raise interesting fundamental and practical questions related to the manipulation of fruit ripening and other processes.Although CRISPR/Cas9 mutagenesis has dramatically improved our options for addressing gene function, recent results suggest that compensatory mechanisms in CRISPR mutants also hide true gene functions.
Unlike animals, plants produce new organs throughout their life cycle using pools of stem cells that are organized in meristems. Although many key regulators of meristem and organ identities have ...been identified, it is still not well understood how they function at the molecular level and how they can switch an entire developmental programme in which thousands of genes are involved. Recent advances in the genome-wide identification of target genes controlled by key plant transcriptional regulators and their interactions with epigenetic factors provide new insights into general transcriptional regulatory mechanisms that control switches of developmental programmes and cell fates in complex organisms.
The molecular mechanisms by which floral homeotic genes act as major developmental switches to specify the identity of floral organs are still largely unknown. Floral homeotic genes encode ...transcription factors of the MADS-box family, which are supposed to assemble in a combinatorial fashion into organ-specific multimeric protein complexes. Major mediators of protein interactions are MADS-domain proteins of the SEPALLATA subfamily, which play a crucial role in the development of all types of floral organs. In order to characterize the roles of the SEPALLATA3 transcription factor complexes at the molecular level, we analyzed genome-wide the direct targets of SEPALLATA3. We used chromatin immunoprecipitation followed by ultrahigh-throughput sequencing or hybridization to whole-genome tiling arrays to obtain genome-wide DNA-binding patterns of SEPALLATA3. The results demonstrate that SEPALLATA3 binds to thousands of sites in the genome. Most potential target sites that were strongly bound in wild-type inflorescences are also bound in the floral homeotic agamous mutant, which displays only the perianth organs, sepals, and petals. Characterization of the target genes shows that SEPALLATA3 integrates and modulates different growth-related and hormonal pathways in a combinatorial fashion with other MADS-box proteins and possibly with non-MADS transcription factors. In particular, the results suggest multiple links between SEPALLATA3 and auxin signaling pathways. Our gene expression analyses link the genomic binding site data with the phenotype of plants expressing a dominant repressor version of SEPALLATA3, suggesting that it modulates auxin response to facilitate floral organ outgrowth and morphogenesis. Furthermore, the binding of the SEPALLATA3 protein to cis-regulatory elements of other MADS-box genes and expression analyses reveal that this protein is a key component in the regulatory transcriptional network underlying the formation of floral organs.
Monocarpic plants have a single reproductive cycle in their lives, where life span is determined by the coordinated arrest of all meristems, or global proliferative arrest (GPA). The molecular bases ...for GPA and the signaling mechanisms involved are poorly understood, other than systemic cues from developing seeds of unknown nature. Here we uncover a genetic pathway regulating GPA in Arabidopsis that responds to age-dependent factors and acts in parallel to seed-derived signals. We show that FRUITFULL (FUL), a MADS-box gene involved in flowering and fruit development, has a key role in promoting meristem arrest, as GPA is delayed and fruit production is increased in ful mutants. FUL directly and negatively regulates APETALA2 expression in the shoot apical meristem and maintains the temporal expression of WUSCHEL which is an essential factor for meristem maintenance.
MicroRNAs (miRNAs) play important roles in plant development through regulation of gene expression by mRNA degradation or translational inhibition. Despite the fact that tomato (Solanum lycopersicum) ...is the model system for studying fleshy fruit development and ripening, only a few experimentally proven miRNA targets are known, and the role of miRNA action in these processes remains largely unknown. Here, by using parallel analysis of RNA ends (PARE) for global identification of miRNA targets and comparing four different stages of tomato fruit development, a total of 119 target genes of miRNAs were identified. Of these, 106 appeared to be new targets. A large part of the identified targets (56) coded for transcription factors. Auxin response factors, as well as two known ripening regulators, COLORLESS NON-RIPENING (CNR) and APETALA2a (SlAP2a), with developmentally regulated degradation patterns were identified. The levels of the intact messenger of both CNR and AP2a are actively modulated during ripening, by miR156/157 and miR172, respectively. Additionally, two TAS3-mRNA loci were identified as targets of miR390. Other targets such as ARGONAUTE 1 (AGO1), shown to be involved in miRNA biogenesis in other plant species, were identified, which suggests a feedback loop regulation of this process. In this study, it is shown that miRNA-guided cleavage of mRNAs is likely to play an important role in tomato fruit development and ripening.
The BABY BOOM (BBM) AINTEGUMENTA-LIKE (AIL) AP2/ERF domain transcription factor is a major regulator of plant cell totipotency, as it induces asexual embryo formation when ectopically expressed. ...Surprisingly, only limited information is available on the role of
BBM
during zygotic embryogenesis. Here we reexamined
BBM
expression and function in the model plant
Arabidopsis thaliana
(
Arabidopsis
) using reporter analysis and newly developed CRISPR mutants.
BBM
was expressed in the embryo from the zygote stage and also in the maternal (nucellus) and filial (endosperm) seed tissues. Analysis of CRISPR mutant alleles for
BBM
(
bbm-cr
) and the redundantly acting
AIL
gene
PLETHORA2
(
PLT2
) (
plt2-cr
) uncovered individual roles for these genes in the timing of embryo progression. We also identified redundant roles for
BBM
and
PLT2
in endosperm proliferation and cellularization and the maintenance of zygotic embryo development. Finally, we show that ectopic
BBM
expression in the egg cell of
Arabidopsis
and the dicot crops
Brassica napus
and
Solanum lycopersicon
is sufficient to bypass the fertilization requirement for embryo development. Together these results highlight roles for
BBM
and
PLT2
in seed development and demonstrate the utility of
BBM
genes for engineering asexual embryo development in dicot species.
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