Transcription factors HB21/40/53 trigger inflorescence arrest through abscisic acid accumulation at the end of flowering

Flowers, and hence, fruits and seeds, are produced by the activity of the inflorescence meristem after the floral transition. In plants with indeterminate inflorescences the final number of flowers produced by the inflorescence meristem is determined by the length of the flowering period, which ends with inflorescence arrest. Inflorescence arrest depends on many different factors, such as the presence of seeds, the influence of the environment, or endogenous factors such as phytohormone levels and age, which modulate inflorescence meristem activity. The FRUITFULL-APETALA2 (FUL-AP2) pathway plays a major role in regulating the end of flowering, likely integrating both endogenous cues and those related to seed formation. Among AP2 targets, HOMEOBOX PROTEIN21 (HB21) has been identified as a putative mediator of AP2 function in the control of inflorescence arrest. HB21 is a homeodomain leucine zipper transcription factor involved in establishing axillary bud dormancy. Here we characterized the role of HB21 in the control of the inflorescence arrest at the end of flowering in Arabidopsis (Arabidopsis thaliana). HB21, together with HB40 and HB53, are upregulated in the inflorescence apex at the end of flowering, promoting floral bud arrest. We also show that abscisic acid (ABA) accumulation occurs in the inflorescence apex in an HB-dependent manner. Our work suggests a physiological role of ABA in floral bud arrest at the end of flowering, pointing to ABA as a regulator of inflorescence arrest downstream of the HB21/40/53 genes.

Analysis of pea mutants reveals the conserved role of FRUITFULL controlling the end of flowering and its potential to boost yield

Monocarpic plants have a single reproductive phase in their life. Therefore, flower and fruit production are restricted to the length of this period. This reproductive strategy involves the regulation of flowering cessation by a coordinated arrest of the growth of the inflorescence meristems, optimizing resource allocation to ensure seed filling. Flowering cessation appears to be a regulated phenomenon in all monocarpic plants. Early studies in several species identified seed production as a major factor triggering inflorescence proliferative arrest. Recently, genetic factors controlling inflorescence arrest, in parallel to the putative signals elicited by seed production, have started to be uncovered in Arabidopsis, with the MADS-box gene FRUITFULL (FUL) playing a central role in the process. However, whether the genetic network regulating arrest is also at play in other species is completely unknown. Here, we show that this role of FUL is not restricted to Arabidopsis but is conserved in another monocarpic species with a different inflorescence structure, field pea, strongly suggesting that the network controlling the end of flowering is common to other plants. Moreover, field trials with lines carrying mutations in pea FUL genes show that they could be used to boost crop yield.

Genetic and Phenotypic Analyses of Carpel Development in Arabidopsis

Carpels are the female reproductive organs of the flower, organized in a gynoecium, which is likely the most complex organ of the plant. The gynoecium provides protection for the ovules, helps to discriminate between male gametophytes, and facilitates successful pollination. After fertilization, it develops into a fruit, a specialized organ for seed protection and dispersal. To carry out all these functions, coordinated patterning and tissue specification within the developing gynoecium has to be achieved. In this chapter, we provide different methods to characterize defects in carpel morphogenesis and patterning associated with developmental mutations, as well as a list of reporter lines that can be used to facilitate genetic analyses.

A cryptic variation in a member of the Ovate Family Proteins is underlying the melon fruit shape QTL fsqs8.1

The gene underlying the melon fruit shape QTL fsqs8.1 is a member of the Ovate Family Proteins. Variation in fruit morphology is caused by changes in gene expression likely due to a cryptic structural variation in this locus. Melon cultivars have a wide range of fruit morphologies. Quantitative trait loci (QTL) have been identified underlying such diversity. This research focuses on the fruit shape QTL fsqs8.1, previously detected in a cross between the accession PI 124112 (CALC, producing elongated fruit) and the cultivar 'Piel de Sapo' (PS, producing oval fruit). The CALC fsqs8.1 allele induced round fruit shape, being responsible for the transgressive segregation for this trait observed in that population. In fact, the introgression line CALC8-1, carrying the fsqs8.1 locus from CALC into the PS genetic background, produced perfect round fruit. Following a map-based cloning approach, we found that the gene underlying fsqs8.1 is a member of the Ovate Family Proteins (OFP), CmOFP13, likely a homologue of AtOFP1 and SlOFP20 from Arabidopsis thaliana and tomato, respectively. The induction of the round shape was due to the higher expression of the CALC allele at the early ovary development stage. The fsqs8.1 locus showed an important structural variation, being CmOFP13 surrounded by two deletions in the CALC genome. The deletions are present at very low frequency in melon germplasm. Deletions and single nucleotide polymorphisms in the fsqs8.1 locus could not be not associated with variation in fruit shape among different melon accessions, what indicates that other genetic factors should be involved to induce the CALC fsqs8.1 allele effects. Therefore, fsqs8.1 is an example of a cryptic variation that alters gene expression, likely due to structural variation, resulting in phenotypic changes in melon fruit morphology.

Identification of Players Controlling Meristem Arrest Downstream of the FRUITFULL-APETALA2 Pathway

The end of the reproductive phase in monocarpic plants is determined by a coordinated arrest of all active meristems, a process known as global proliferative arrest (GPA). GPA is linked to the correlative control exerted by developing seeds and, possibly, the establishment of strong source-sink relationships. It has been proposed that the meristems that undergo arrest at the end of the reproductive phase behave at the transcriptomic level as dormant meristems, with low mitotic activity and high expression of abscisic acid response genes. Meristem arrest is also controlled genetically. In Arabidopsis (Arabidopsis thaliana), the MADS-box transcription factor FRUITFULL induces GPA by directly repressing genes of the APETALA2 (AP2) clade. The AP2 genes maintain shoot apical meristem (SAM) activity in part by keeping WUSCHEL expression active, but the mechanisms downstream of this pathway remain elusive. To identify target genes, we performed a transcriptomic analysis, inducing AP2 activity in meristems close to arrest. Our results suggest that AP2 controls meristem arrest by repressing genes related to axillary bud dormancy in the SAM and negative regulators of cytokinin signaling. In addition, our analysis indicates that genes involved in the response to environmental signals also respond to AP2, suggesting that it could modulate the end of flowering by controlling responses to both endogenous and exogenous signals. Our results support the previous observation that at the end of the reproductive phase the arrested SAM behaves as a dormant meristem, and they strongly support AP2 as a master regulator of this process.

Inflorescence Meristem Fate Is Dependent on Seed Development and FRUITFULL in Arabidopsis thaliana

After a vegetative phase, plants initiate the floral transition in response to both environmental and endogenous cues to optimize reproductive success. During this process, the vegetative shoot apical meristem (SAM), which was producing leaves and branches, becomes an inflorescence SAM and starts producing flowers. Inflorescences can be classified in two main categories, depending on the fate of the inflorescence meristem: determinate or indeterminate. In determinate inflorescences, the SAM differentiates directly, or after the production of a certain number of flowers, into a flower, while in indeterminate inflorescences the SAM remains indeterminate and produces continuously new flowers. Even though indeterminate inflorescences have an undifferentiated SAM, the number of flowers produced by a plant is not indefinite and is characteristic of each species, indicating that it is under genetic control. In Arabidopsis thaliana and other species with indeterminate inflorescences, the end of flower production occurs by a regulated proliferative arrest of inflorescence meristems on all reproductive branches that is reminiscent of a state of induced dormancy and does not involve the determination of the SAM. This process is controlled genetically by the FRUITFULL-APETALA2 (FUL-AP2) pathway and by a correlative control exerted by the seeds through a mechanism not well understood yet. In the absence of seeds, meristem proliferative arrest does not occur, and the SAM remains actively producing flowers until it becomes determinate, differentiating into a terminal floral structure. Here we show that the indeterminate growth habit of Arabidopsis inflorescences is a facultative condition imposed by the meristematic arrest directed by FUL and the correlative signal of seeds. The terminal differentiation of the SAM when seed production is absent correlates with the induction of AGAMOUS expression in the SAM. Moreover, terminal flower formation is strictly dependent on the activity of FUL, as it was never observed in ful mutants, regardless of the fertility of the plant or the presence/absence of the AG repression exerted by APETALA2 related factors.

Genetic control of meristem arrest and life span in Arabidopsis by a FRUITFULL-APETALA2 pathway

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.

Genetic control of meristem arrest and life span in Arabidopsis by a FRUITFULL-APETALA2 pathway

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.

The lifespan of monocarpic plants is determined by coordinated arrest of shoot meristems. Here the authors show that this process is promoted by the FRUITFULL transcription factor which negatively regulates expression of the meristem maintenance factor WUSCHEL in an AP2-dependent manner.

The CRC orthologue from Pisum sativum shows conserved functions in carpel morphogenesis and vascular development

BACKGROUND AND AIMS

CRABS CLAW (CRC) is a member of the YABBY family of transcription factors involved in carpel morphogenesis, floral determinacy and nectary specification in arabidopsis. CRC orthologues have been functionally characterized across angiosperms, revealing additional roles in leaf vascular development and carpel identity specification in Poaceae. These studies support an ancestral role of CRC orthologues in carpel development, while roles in vascular development and nectary specification appear to be derived. This study aimed to expand research on CRC functional conservation to the legume family in order to better understand the evolutionary history of CRC orthologues in angiosperms.

METHODS

CRC orthologues from Pisum sativum and Medicago truncatula were identified. RNA in situ hybridization experiments determined the corresponding expression patterns throughout flower development. The phenotypic effects of reduced CRC activity were investigated in P. sativum using virus-induced gene silencing.

KEY RESULTS

CRC orthologues from P. sativum and M. truncatula showed similar expression patterns, mainly restricted to carpels and nectaries. However, these expression patterns differed from those of other core eudicots, most importantly in a lack of abaxial expression in the carpel and in atypical expression associated with the medial vein of the ovary. CRC downregulation in pea caused defects in carpel fusion and style/stigma development, both typically associated with CRC function in eudicots, but also affected vascular development in the carpel.

CONCLUSIONS

The data support the conserved roles of CRC orthologues in carpel fusion, style/stigma development and nectary development. In addition, an intriguing new aspect of CRC function in legumes was the unexpected role in vascular development, which could be shared by other species from widely diverged clades within the angiosperms, suggesting that this role could be ancestral rather than derived, as so far generally accepted.

Sequential action of FRUITFULL as a modulator of the activity of the floral regulators SVP and SOC1

The role in flowering time of the MADS-box transcription factor fruitfulL (FUL) has been proposed in many works. FUL has been connected to several flowering pathways as a target of the photoperiod, ambient temperature, and age pathways and it is has been shown to promote flowering in a partially redundant manner with suppressor of overexpression of constans 1 (SOC1). However, the position of FUL in these genetic networks, as well as the functional output of FUL activity during floral transition, remains unclear. In this work, a genetic approach has been undertaken to understand better the functional hierarchies involving FUL and other MADS-box factors with well established roles as floral integrators such as SOC1, short vegetative phase (svp) or flowering locus C (FLC). Our results suggest a prominent role of FUL in promoting reproductive transition when photoinductive signalling is suppressed by short-day conditions or by high levels of FLC expression, as in non-vernalized winter ecotypes. A model is proposed where the sequential formation of FUL-SVP and FUL-SOC1 heterodimers may mediate the vegetative and meristem identity transitions, counteracting the repressive effect of FLC and SVP on flowering.

The effect of NGATHA altered activity on auxin signaling pathways within the Arabidopsis gynoecium

The four NGATHA genes (NGA) form a small subfamily within the large family of B3-domain transcription factors of Arabidopsis thaliana. NGA genes act redundantly to direct the development of the apical tissues of the gynoecium, the style, and the stigma. Previous studies indicate that NGA genes could exert this function at least partially by directing the synthesis of auxin at the distal end of the developing gynoecium through the upregulation of two different YUCCA genes, which encode flavin monooxygenases involved in auxin biosynthesis. We have compared three developing pistil transcriptome data sets from wildtype, nga quadruple mutants, and a 35S::NGA3 line. The differentially expressed genes showed a significant enrichment for auxin-related genes, supporting the idea of NGA genes as major regulators of auxin accumulation and distribution within the developing gynoecium. We have introduced reporter lines for several of these differentially expressed genes involved in synthesis, transport and response to auxin in NGA gain- and loss-of-function backgrounds. We present here a detailed map of the response of these reporters to NGA misregulation that could help to clarify the role of NGA in auxin-mediated gynoecium morphogenesis. Our data point to a very reduced auxin synthesis in the developing apical gynoecium of nga mutants, likely responsible for the lack of DR5rev::GFP reporter activity observed in these mutants. In addition, NGA altered activity affects the expression of protein kinases that regulate the cellular localization of auxin efflux regulators, and thus likely impact auxin transport. Finally, protein accumulation in pistils of several ARFs was differentially affected by nga mutations or NGA overexpression, suggesting that these accumulation patterns depend not only on auxin distribution but could be also regulated by transcriptional networks involving NGA factors.

Genetic and phenotypic analyses of carpel development in Arabidopsis

Carpels are the female reproductive organs of the flower, organized in a gynoecium, which is arguably the most complex organ of a plant. The gynoecium provides protection for the ovules, helps to discriminate between male gametophytes, and facilitates successful pollination. After fertilization, it develops into a fruit, a specialized organ for seed protection and dispersal. To carry out all these functions, coordinated patterning and tissue specification within the developing gynoecium have to be achieved. In this chapter, we describe different methods to characterize defects in carpel patterning and morphogenesis associated with developmental mutations as well as a list of reporter lines that can be used to facilitate genetic analyses.