In Nole-Wilson and collaborators proposed the connection between ANT and the hormone auxin on the base of the observation that the ant mutant is more sensitive than the wild type to alteration in PAT. Moreover, the expression of a subset of auxin-related genes was altered in the ant single and ant rev double mutant gynoecia, indicating that the morphological defects of the ant rev double mutants, at least in part, are due to an alteration in auxin homeostasis in these plants.
In the pistil of the mpS weak allele the CMM does not develop, and placenta and ovules are completely missing Cole et al. Moreover, ectopic expression of these MADS box genes results in ovule formation on sepals Favaro et al. It has been shown that also AG plays a role in ovule development by experiments in which the apetala2 ap2 single mutant was compared with the ap2 ag double mutant.
Thus, in the ap2 single mutant petals were mostly absent, while sepals were converted into carpel structures bearing ectopic ovules, some of which were transformed into carpelloid structures.
Interestingly, the sepals or first-whorl organs of the ap2 ag double mutant still presented carpel identity, and the number of ovules converted into carpel structures was significantly higher, indicating that AG activity also contributes to ovule identity establishment Bowman et al. Interestingly, Skinner et al. When new organ primordia are originated in the plant, two different regions, the boundaries and the zone of primordia outgrowth, need to be defined.
The organ boundary is defined as the region between the meristem and the developing organ, or, as in the case of ovules, as the region between two adjacent ovule primordia. Another important aspect is the arrangement of the plasmodesmata that regulates the movement of transcription factors between cells.
For example, the boundaries in the inflorescence meristem seem to restrict the passage of proteins into flower primordia Wu, The boundary-specific regulatory genes play a critical role in orchestrating several morphogenetic and patterning events and their spatial coordination. When this coordination is missing, fusion between organs is the most frequent observed phenotype Aida et al. The CUC gene family was the first discovered to have a fundamental role in organ boundary establishment.
In fact, in the cuc1cuc2 double mutant embryo the cotyledons do not separate Aida et al. The transcripts of CUC1 and CUC2 were detected by in situ hybridization in the anlagen placenta and in ovules at stage 1-II and later on, starting from stage 2-I, restricted to the boundary between two ovules Ishida et al. As we already mentioned, the study of the gynoecium phenotype of the cuc1 cuc2 double mutant was only possible on plants regenerated in vitro.
They showed defects in the formation of the septum and in ovule development; most of the gynoecia having less than 10 ovules Table 1. However, the cuc1 cuc2 double mutant plants never gave seeds Ishida et al. A further demonstration that CUC1 and CUC2 are directly linked to the determination of ovule number in a direct way came from the work of Galbiati et al. Despite the reduction in ovule number in the different mutant backgrounds, the size of the pistils was not reduced.
Therefore, the ovules were more distantly spaced compared to those in wild-type pistils Galbiati et al. All the information about these factors taken together indicates that they work in different ways: while ANT promotes ovule primordia growth, the CUCs play a role in the establishment of the ovule primordia boundaries Figure 2.
Figure 2. Proposed model for the control of the ovule primordia initiation. Once the primordia have formed, auxin accumulates at the edge of the developing ovule. An inhibitory loop of auxins on CUC1 and CUC2, as it is postulated for the leaf serration, could be happening at the ovule boundaries. Adapted from Galbiati et al. Several studies revealed that CUC expression is controlled and restricted to the boundaries in several ways.
Therefore, it would be interesting to study also the contribution of CUC3 in the regulation of defining ovule boundaries. LOB , the gene that names the family, is expressed at the base of all lateral organs. Interestingly, plants overexpressing LOB produced abnormal flowers with reduced floral organs and they were sterile even when fertilized with wild-type pollen Shuai et al.
Lee et al. The single mutant lof1 exhibits a novel fusion between the axillary stem and the cauline leaf. Additional fusions resulted when lof1 was combined with lof2, cuc2 or cuc3 , indicating the existence of overlapping roles for LOF1, CUC2 , and CUC3 to control organ separation during reproductive development.
Despite the identification of a number of boundary-specific transcription factors, boundary formation and maintenance is still a poorly understood process, and only CUC1 and CUC2 have been demonstrated to have a role in ovule boundary establishment. The factors that have been described to regulate or interact with the CUCs in a different developmental context could also have a role during ovule initiation, and some of them, like AS1 and AS2 are already known to be expressed in the gynoecium and ovules Xu and Shen, In Arabidopsis many genes have been described to play roles in the different phases of ovule development, although most of them do not determine directly the number of ovules Schneitz et al.
However, the ANT transcription factor has been described to have a clear role in ovule primordia formation. In situ hybridization experiments showed that within the carpel it is expressed in the placenta and in the integuments of the developing ovules. In ant plants ovules do not develop integuments and megasporogenesis is blocked at the tetrad stage leading to complete female-sterility Elliott et al.
ANT is not only required for ovule development but it is also involved in ovule primordia formation. Indeed, in the ant-9 mutant the number of ovules per carpel is reduced by more than half in respect to the wild-type Table 1.
Given that the ant gynoecia have the same length as those of wild type, the ovules that do arise in ant are more distantly spaced than in wild-type plants Liu et al. Thus, plants presenting mutations in HLL display a phenotype similar to ant at the level of ovule integuments Schneitz et al.
The phenotype of the double mutant hll ant was more severe at the level of primordia outgrowth however, nothing was described regarding ovule number Schneitz et al.
A similar phenotype to hll was observed in the short integuments 2 sin2 mutants. Apart of an arrest in cell division in both ovule integuments, sin2 plants presented shorter pistils bearing less ovules than the wild type Table 1. Moreover, the authors describe an abnormal distribution of the ovules along the placenta, being the distance between ovules bigger than in wild-type plants Broadhvest et al.
Thus, in this particular case the shorter carpel might not be the only cause of reduced ovule numbers. The double mutant sin2 ant-5 was not different from ant-5 single mutant, indicating that ANT is epistatic to SIN2 with respect to ovule development.
On the contrary, sin2 hll-1 double mutant had a stronger effect on ovule development than sin2 or hll-1 single mutants Broadhvest et al. As we previously underlined, the boundary region and the primordia formation zone are highly interconnected. The directionality of auxin flux depends principally on the polar localization of the PIN proteins. PIN1 protein is localized at the membrane of placenta cells and later on, in the developing ovules, it is restricted to the lateral-apical membranes of nucellus cells.
PIN3 is also present in few cells at the tip of the developing nucellus shortly after ovule primordia emergence but, contrary to PIN1, it is not expressed in the placenta cells Ceccato et al. The weak pin mutant allele is able to develop some flowers in which the pistils have slightly reduced valves but normal styles and stigmas Sohlberg et al. The pistils of the pin weak allele have an average of 9 ovules per carpel Table 1 Bencivenga et al. In addition, Galbiati et al. In the same way, a CUC2-dependent regulatory pathway controlling PIN1-mediated auxin efflux has been described to explain leaf serrations Bilsborough et al.
Moreover, in the newly formed primordia of the SAM the auxin maxima, in a negative feed-back loop, repress CUC2 expression and restricts it to the boundaries Vernoux et al. A similar inhibitory loop could control CUC expression at the ovule boundaries Figure 2. These experiments evidence a convergence of two different plant hormones in the regulation of ovule primordia formation. In the next paragraph we will delve deeper into the role of CK in the formation and determination of ovule number.
CK is an essential hormone for plant growth and development as it has a central role in the regulation of cell division and differentiation. In the last 10 years, several studies have clearly proven that CK has also a significant role during ovule development. CK signaling, which has been recently summarized in a detailed review article Hwang et al.
These three genes are all expressed in inflorescences, carpels and developing ovules Higuchi et al. More precisely, AHK2 and AHK3 are expressed during all stages of ovule development, starting from early primordia stages to ovule maturity, whereas CRE1 expression remains restricted to the chalazal region and later to the integuments of ovules during all the developmental stages Bencivenga et al.
The single and double mutants of AHKs do not present any phenotype at the level of the ovules Higuchi et al. However, mutants lacking all three receptors exhibit no perception of CK and present a strong slowdown of shoot and root growth. The resulting miniature plants also show delayed flower induction and impaired fertility Higuchi et al.
Thus, the triple mutant cre ahk ahk do not produce seeds Higuchi et al. Moreover, a severe reduction in the ovule number, an average of 5 ovule per pistil, was noticed in these triple mutant plants Table 1 Bencivenga et al. A similar sterile phenotype was also observed for another allelic combination: the ahk ahk ahk triple mutant Nishimura et al.
Differently, Riefler et al. Attention has also been given to the importance of CK catabolism. Werner and colleagues engineered transgenic Arabidopsis plants that individually overexpressed six different CKXs in order to enhance CK degradation.
As expected, these plants manifested phenotypes linked to CK deficiency, like delayed vegetative growth and leaf expansion, diminished activity and size of the SAM but increased overall root system.
The reproductive development of CK-deficient plants was also altered. In 35S::CKX1 and 35S::CKX3 plants, flowering was strongly delayed and furthermore the fertility of flowers was heavily reduced, partially due to the lack of pollen.
Although the number of ovules formed in these plants was not reported in this work, the expression patterns together with the phenotypes in the flowers and fruits indicate once more that CK play a role during reproduction. Moreover, the authors suggest a role for ANT in the observed reduced cell division in the leaves of ckx plants.
Considering the documented role of ANT in ovule primordia initiation already introduced in this article, it will be very interesting to analyze also its role in the reproductive tissues of these plants.
With an opposite experimental approach, the simultaneous mutations of two CKX genes, it was demonstrated that plants with an increased level of CK had an enhanced activity of the reproductive meristem Bartrina et al. Indeed, the ckx ckx double mutant produced more flowers due to a larger inflorescence meristem with more cells than the wild type. Moreover, flowers were bigger and so were the gynoecia. Besides, double mutant gynoecia contained twice as many ovules as wild-type ones, indicating an increased activity of their placental tissue.
The ckx and ckx single mutants already developed more ovules than the wild-type, and the flower size and the number of ovules was reflected into the length of the fruits siliques of ckx3 ckx5 were 20 mm long compared with the 17 mm of the wild type and the seed number seeds in the ckx3 ckx5 mutant siliques compared with an average of 65 seeds in wild-type siliques, Table 1 The authors suggested that CKX3 and CKX5 may regulate the activity of meristematic cells in the placenta thus affecting organogenic capacity and ovule primordia formation.
A conclusive evidence about the relationship between the levels of CK and the initiation of ovule formation was obtained from experiments in which inflorescences were treated with synthetic CK 6-Benzylaminopurine, BAP. These results point out the importance of the cross-talk between CK and auxin during ovule primordia formation. However, the hormonal cross-talk is not limited to auxin and CK since very recently it has been demonstrated that also brassinosteroids BR play a crucial role in ovule and seed formation by regulating the expression of genes that control ovule development Huang et al.
BRs are hormones known to control general plant development. More specifically, they have been described as involved in the control of the initiation and formation of reproductive organs Szekeres et al. Huang et al. The analysis of the number of ovules and seeds and the morphological analysis of the siliques of det-2 a BR-deficient mutant involved in BR biosynthesis , bri the mutant for the BR receptor , heterozygous plants for bin a gain of function mutant deficient in BR signaling and bzrD a BR signal-enhanced mutant leaded to the conclusion that BR signaling positively regulates ovule number Table 1 Huang et al.
By treating plants with BR it was shown that BR influences ovule development through regulating the transcription of genes such as HLL and ANT , which are redundant in the control of ovule primordia growth as already introduced in this review Schneitz et al. A model for ovule primordia formation that integrates the molecular and hormonal networks has been proposed by Galbiati et al.
This model can be easily extended with the recently discovered role of the plant hormones BR, which positively regulate the number of ovule primordia, in part by the direct regulation of ANT by BZR1 Figure 2. Interestingly, in different Arabidopsis ecotypes diploid accessions a variation in ovule numbers can be observed. Alonso-Blanco et al. Recently a considerable genetic variation in ovule number was described in selfed F1 triploids of different A.
Triploids were obtained by crossing a tetraploid L er -0 line used as a male or female parent with different diploid accessions. Interestingly, it was observed an effect of the parental genome excess 2m:1p vs. These were the first parent-of-origin effects on ovule number in reciprocal triploids of plants. Indeed, in Arabidopsis the ASH1 class of proteins, that can methylate lysine residues on histone tails, maintains an active transcriptional state during development.
These data altogether indicate that epigenetics may also play a role in the control of ovule number, and they open up a new interesting field of research. Nevertheless, most of their targets, which might be the genes that determine the correct development of the ovule, remain to be uncovered. Another quite unknown process is the regulation of the ovule primordia initiation. The majority of them are transcription factors, and the transcriptional cascades triggered by them, that will determine the regulation of the morphogenetic parameters such as cell division and expansion, or expression patterns of identity genes of particular organs, are also largely unknown.
It is worth to highlight that these regulators are not exclusively transcription factors, but also mitochondrial proteins or chromatin remodeling factors, indicating that a correct ovule initiation depends on a complex genetic and molecular network.
One of the difficulties of the genetic dissection of ovule initiation and development is that many mutations that affect ovule initiation have already pleiotropic effects on earlier stages of the development of the reproductive tissues, causing floral aberrations that may mask their effects on ovules. Thus, many genes that control ovule development are also involved in primordium initiation and growth of other floral organs Elliott et al.
Moreover, it is difficult to establish if a mutation in a gene causes a reduction in ovule number if this mutant already has an altered gynoecium phenotype Alvarez and Smyth, ; Western and Haughn, ; Broadhvest et al. The reanalysis of these carpel mutants, measuring the space between ovules, or expressing the ovule number as the ovule number per millimeter of gynoecium, as some authors already presented Huang et al.
Besides, a reverse-genetic strategy using RNA interference or insertional mutants can be used to identify new regulators of ovule numbers determination. Ovule boundary establishment is still a poorly understood process, and only CUC1 and CUC2 have been demonstrated to play a role Galbiati et al. The contribution to the determination of ovule boundaries of the genes that have been described to regulate or interact with the CUCs in other organ boundaries would be worth to be analyzed, by means of the study of their patterns of expression and how these are accurately determined.
The identification and characterization of single and multiple mutants, as has been done for the CUC genes Aida et al. Moreover, the analysis of their incidence at the cellular level will help to define the effects on cell behavior i. It has also been widely demonstrated that hormones play a role in the regulation of ovule primordia initiation, being auxin, CK and more recently also BR identified as the important hormonal players in this process. The crosstalk between these hormones, as Bencivenga et al.
How to Make Nesting Structures. Certification Programs. What is Pollination? Cross Pollination vs. Self Pollination. What is Fertilization. How Does the Ovule Develop? How Does the Pollen Grain Develop?
Apidae — Bumblebees. Apidae — Carpenter bees, Squash bees, Blueberry bees, Cuckoo bees. Megachilidae — Leafcutter and Mason bees. Halictidae — Sweat bees. Butterflies and Moths. Habitat Change. Pesticide Use.
Climate Change. Genetically Modified Organisms. GAPP Buzz. At the seed-filling stage, other ovules have collapsed and the seed gradually comes to occupy the total volume of the pod. The fruit-to-seed length ratio decreases considerably during seed ripening.
At fertilization, ovary length is four times greater than ovule length. In the mature state, the fruit and seed lengths are approximately equal.
Seed size and weight diminish with an increase in seed number within a pod, although pod size remains constant. It is assumed that nonrandom abortion of young seeds in M. We suppose that evolution of this species may have proceeded in the direction of a decrease in seed number and an increase in its sizes, which may play an important role in seed dispersal and seedling establishment.
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J Exp Bot — Bot Acta 64— Grossniklaus U, Schneitz K The molecular and genetic basis of ovule and megagametophyte development. Cell Dev Biol 9: — Science — Haig D Conflicts among megaspores. J Theor Biol — Annu Rev Evol Syst 1: —
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