Flowers are reproductive organs organized in whorls, that play an essential role in reproduction and yield (
Pawar and Rana 2019;
Lyngdoh et al., 2018). The genetic study of
Arabidopsis thaliana and
Antirrhinum majus mutants led to the proposition of the ABC model of flower development
(Alvarez-Buylla et al., 2010), which explains the identity of floral whorls being controlled by three classes of genes (A, B and C). Function A only specifies the identity of the sepals; the identity of the petals is controlled by functions A and B; the identity of the stamens is controlled by functions B and C and function C, specifies the identity of the carpels
(Soltis et al., 2007; Fig 1). Thus, ABC model and its variations apply to a wide range of gymnosperm
(Soltis et al., 2007) and angiosperm species (
Irish, 2017). Amongst them, the Fabaceae comprise an affordable source of protein
(Jukanti et al., 2017) and minerals for a large proportion of rural populations in the world
(Jayalaxmi et al., 2016) and under a context of climate change and a need for food security, their contribution is recognized (
FAO, 2021). No data have been published on Fabaceae flower evolution but their diversification started approximately 60 million years ago and the most important clades separated some 50 million years ago (
Lavin, 2005). The Fabaceae family was reorganized into six subfamilies, comprising c. 19 000 species (
LPWG, 2017). Among them,
Medicago truncatula and
Cicer arietinum belong to the Trifoleae and Cicereae Tribes, respectively and the ABC model in these species has been studied in detail (
Weller and Macknight 2018).
The aim of this review is to scrutinize the current state of knowledge on “ABC” model at Sophoreae, Phaseoleae, Dalbergieae and Genisteae tribes. Moreover, we examine the role of non-coding RNAs on floral development at Cajaninae subtribe, Dalbergieae and Genisteae tribes.
Sophoreae tribe
Sophoreae includes approximately 122 species in 14 genera. It is an early-branched Papilionid, often regarded as “primitive” or “basal”, alongside the Swartzieae tribe
(Cardoso et al., 2013). Within this tribe, the genus
Sophora comprises approximately 50 species
(Song et al., 2008). This genus has a variation in the order of development of floral organs, differing from the standard sequential pattern of sepals, petals, stamens and carpels (
Tucker 2006). In
Sophora tetraptera and within the subfamily Papilionoideae, the variations include a precocious carpel initiation, delayed petal development and late development with interrupted floral organ development
(Song et al., 2008). Also, the developmental order is acropetal among whorls and unidirectional from the abaxial side in the whorls of sepals, petals and stamens
(Benlloch et al., 2009). This unusual pattern of initiation and development of the floral organs in
S. tetraptera is interesting because it suggests that the expression of ABC genes varies according to the initiation, differentiation and development of different types of floral organs
(Song et al., 2008). Indeed, the sequence analysis of the putative floral identity genes of
S. tetraptera suggests that
StLFY,
StAP1,
StPI and
StAG are homologues to
LFY/FLO,
AP1/SQUA,
PI/GLO and
AG/PLE for
Arabidopsis and
Antirrhinum, respectively
(Chanderbali et al., 2010). In addition, Southern blot analyses have revealed that there is a single copy of
StAP1,
StPI and
StAG genes in the
S. tetraptera genome and two copies of
StLFY (Song et al., 2008).
In
S. tetraptera, the expression of class A gene
StAP1 is strictly limited to floral primordia during floral development and to sepals and petals after floral differentiation
(Song et al., 2008). Meanwhile, the expression of
StPI, a class B homolog gene, is limited to the petals and stamens; with the homolog of the class C gene,
StAG, being expressed in stamens, carpels and seeds
(Song et al., 2008). These expression patterns are completely consistent with the defined functions of these genes in
Arabidopsis and
Antirrhinum (Song et al., 2008). Even though the same genes appear to regulate the flowering process in Sophoreae, the sequential pattern of gene expression has been demonstrated to differ from that of
Arabidopsis and
Antirrhinum on which the ABC model is based
(Song et al., 2008).
Song et al., (2008) have suggested that the ABC genes could also be required during the later stages of flower development. This is supported by the findings in
Arabidopsis, in which
AG appears to be functionally redundant with SHATTERPROOF (
SHP) and SEEDSTICK (
STK)
(Song et al., 2008). However, it has to be considered that the role of
AG in the development of Arabidopsis fruit has not yet been fully described. In species such as
Impatiens balsamina, the homologous gene
IbAG, is expressed in the floral meristem until the ovule production, not being expressed afterwards
(Ordidge et al., 2005). Thus, in
S. tetraptera, in which the floral identity genes:
StAP1,
StPI and
StAG show low initial expression levels that increase only after the onset of differentiation of floral organs, it has been proposed that these genes could work as the main barrier for the differentiation of floral organs
(Song et al., 2008).
Phaseoleae tribe
Cajaninae subtribe
Pigeonpea [
Cajanus cajan (L.) Millsp.] belongs to Cajaninae subtribe and it was domesticated in India no earlier than 3,500 years ago
(Kassa et al., 2012). Moreover, it has many wild relatives such as
C. scarabaeoides,
C. sericeus,
C. acutifolius and
C. albicans (Khoury et al., 2015) (Table 1). Summarizes the 13 homologous genes for the ABC model in
C. cajan. Related to the ABCDE model proposed by
Theißen et al., (2016),
STK has not been identified. On the other hand,
Kumar et al., (2021) were able to report only one copy of the
FLC and
SVP genes, while three paralogs of
SOC1were found in the
C. cajan genome.
CcMADS1.5 is one paralog of
SOC1 and it was found to be missing in
C. scarabaeoides,
C. platycarpus and
C. cajanifolius. Finally,
Das et al., (2020) were able to report long non-coding RNAs (lncRNAs) and miRNAs during the process of flower development in
C. scarabaeoides.
Glyciniae subtribe
Glycine max and
Glycine soja belong to the Glyciniae subtribe. In regards to the ABCDE model proposed by
Theißen et al., (2016), a total of 12 MADS box genes were identified as homologs for
G. max and
G. soja (Table 2). As in other species
AP1 is in charge of the development of petals and sepals, while
AP3 and
PI are involved in that of petals and stamens and Class C AG is involved in the formation of stamens, carpels and ovules. Regarding Class D,
SHP1 and
SHP2 are involved in the development of ovules, While Class E, homologs
SEP1, SEP2, SEP3 and
SEP4 have also been (
Vasquez-Regalado, 2021). In addition,
AP2 and some genes of the floral transition process were identified (Table 2). This data is in agreement with the study of
Jung et al., (2012), who also identified
WUS,
LFY and
SPL3 genes. On the other hand,
Chi et al., (2011) were able to isolate and characterize
GmAP1, which is specifically expressed in sepals and petals. In addition,
Machado et al., (2020) identified two genes similar to
AG and three genes similar to
PI. According to
Chi et al., (2017), GmAGL1 is expressed in carpel and its overexpression causes carpel loss. On the other hand,
GsLFY showed a high expression in sepals and stamens but having a weak expression in petals and carpels
(Guo et al., 2017). On the other hand,
Huang et al., (2009), state that
GmSEP1 is possibly involved in the development of petals. Finally,
GmMADS28, a class E homolog, can control the number of floral organs and petal identity, producing stamen sterility when it is ectopically expressed
(Huang et al., 2014). Furthermore,
chicken toes-like leaf and petalody flower is a novel and critical pleiotropic regulator of leaf and flower development
(Zhao et al., 2017).
Phaseolinae subtribe
The information on the genetic mechanism that explains the identity of the floral organs is scarce in
Vigna and
Phaseolus. However, 13 and 11 homologs were found for classes A, B, C, D and E, respectively
(Theiben et al., 2016) as is detailed in Table 3.
Lin et al., (2020) also described
VrSE1 in
V. radiata, which belongs to class C, modulating cell division in petals and cell expansion in style. On the other hand, integrative genes such as
SOC1 and
FUL that work as promoters flower transition (
Torti and Fornara, 2012). Furthermore,
Squamosa Promoter Binding protien-like 8 and
SPL9 are able to modulate the expression of
SEP3 and
MADS 32 (Gou et al., 2019).
Dalbergieae tribe
Dalbergieae is regarded as basal Papilionoideae subfamily of Fabaceae. Flower development in this taxon shows a deviation from that of other Papilionoideae. Organ inception is principally acropetal, with a precocious carpel inception. The whorls develop in different manners, with a helicoidal initiation in
Dalbergia brasiliensis sepals and a lateral stamen development in
Pterocarpus rotundifolius. These patterns appear at initiation and late stage in ontogenesis, rather than at mid-stage, which is opposed to the unidirectional order usually seen in Papilionoideae (
Klitgaard, 1999). These variations seem to be controlled at a genetic level by genes of the ABC model. Indeed, the genetic studies in
Arachis hypogaea have focused on the regulation of pod formation by MADS-box genes
(Li et al., 2016). Some of these genes have been cloned by
Mei et al., (2005), who states that they are related to flower morphogenesis in
A. hypogaea.
Regarding Class A,
Alyr et al., (2020) have characterized a genomic region involved in pod and seed size reduction, providing insights into the flowering regulation in
A. hypogaea. They studied
Aradu.DN3DB, a gene that codes for the transcriptional regulator STERILE APETALA-like (
SAP), demonstrating that its expression can negatively regulate
AG expression in the perianth whorls. In
A. thaliana,
SAP is required for the maintenance of floral identity acting in a similar manner to
AP1, with severe aberrations in inflorescences being caused by its loss of function, leading to sterile flowers with small petals
(Byzova et al., 1999). On the other hand, the expression of
AP2 has been proved to be significantly increased in response to drought
(Dang et al., 2012). Additionally, the expression profile of
A. hypogaea studied by
Li et al., (2016) showed that
SPL proteins are capable of interacting with different coding genes belonging to the MADS-box family. This gene has also been reported to have floral homeotic A function (
Theißen et al., 2016).
Recently,
Vasquez-Regalado (2021) has performed a study in comparative genomics to find orthologs in the flowering pathway among 13 different species of legumes, where the data for ABC model in
Arachis is summarized in (Table 4). On the other hand, the AG expression seems to be highly affected by environmental factors. In a study about Ca
2+ regulation in
A. hypogaea, the expression of AG related gene called MADS transcription factor family was downregulated in plants grown in free-calcium-sufficient treatment
(Yang et al., 2017). In
A. hypogaea,
Kumar and Reddy (1997) identified a single copy of AG homologous that was tissue specific during flower bud formation. These findings support the hypothesis that the AG locus is related to environmental response. In fact, some of its introns are capable of producing non-coding RNAs that are capable of interacting with different targets at a molecular level
(Wu et al., 2018).
Genisteae tribe
In a
Lupinus albus population derived from Kiev Mutant and P27174 is found homologs to class A (
Lup009441,
Lup020693), homologs to class C (
AGL8, AGL21, AGL24, AGL38, AGL42, AGL65, AGL80) and homologs to class D (
SEP2, SEP3, SEP4); which are related to ABC model
(Rychel-Bielska et al., 2021). Moreover, genes from the ABC model have also been found in the genome of
L. angustifolius Tanjil cultivar
(Hane et al., 2017). With this information,
Taylor et al., (2019) have studied insertion and deletion mutations in the regulatory regions of the major flowering gene LanFTc1, a homolog of Arabidopsis
FT. They have further identified transcription factor binding site motifs of
AGL9 and
PI that are expressed during early vegetative growth in
L. angustifolius (Taylor et al., 2019). Genes of the ABC model have also been related to the vernalization response in
L. angustifolius, including
SEP3, SEP4 and
AGL15 (Nelson et al., 2017). In addition, Fig 2 shows
Arabidopsis homolog genes for class A, B and C. All of these genes, with the exception of
AP2, are classified as MADS-box type genes (
Theißen et al., 2016) and possibly encode or interact with transcription factors that contain highly conserved DNA domains in
Lupinus (Delgado-Benarroch
et al., 2009). In addition,
L. angustifolius ABC model (Fig 2) is conceptualized under the floral development mechanism proposed by
Bouche et al., (2016).
Other genes affecting the flowering process in the genus
Lupinus have been published. Thus,
Rychel-Bielska et al., (2021) reported the presence of homologs for
CO,
FT,
MFT,
PIF4,
SKIP1 and
VIP3 in
L. albus. The same
FT gene has a homolog (
FTc1) responsible for vernalization and it is located at locus
Ku in
L. luteus (Lichtin et al., 2020). Meanwhile, in
L. angustifolius, there is evidence of similar roles for the homolog
LanFTc1, which is associated with vernalization response and flowering time
(Taylor et al., 2019).
Genisteae non coding RNA controlling floral development
The ABC genes can interact with plant hormones (Chandler, 2011), transcription factors
(Krishnamurthy et al., 2015) and noncoding RNAs (ncRNAs) such as microRNAs (miRNAs)
(Luo et al., 2013). These noncoding sequences are capable of regulating processes involved in floral development
(Budak et al., 2020). The ncRNAs include two major groups: lncRNAs and small RNAs (sRNAs).
The sRNAs include several different types of RNA, including miRNAs and small interference RNA (siRNA). Some miRNA, such as
miR172, have been shown to repress the expression of genes belonging to class A of the ABC model of flower development, including
AP2, in sepals and petals (
Chen, 2004). Besides, it has been demonstrated that some miRNAs and siRNAs can interact with auxin response factor (ARF) in
Arabidopsis thaliana (
Marín et al., 2010).
Glazinska et al., (2019) reported miRNA and siRNA that are involved in flower development in
L. luteus. They also reported 46 differential expressed miRNAs that were found while comparing the upper and lower flowers. Furthermore, two lncRNAs are involved in the regulation of
FLC (
Yamaguchi and Abe 2012).
In
L. angustifolius, the transcript rna-XR_002106613.1 reported in PLncDB V2.0
(Jingjing et al., 2021) is homologous to lncRNA EL0144 (AT2G46192) (Fig 3). According to TAIR
(Berardini et al., 2015), this transcript is expressed at different levels, in particular sepals, stamens and carpels, which suggests that the expression of this gene could affect functional genes belonging to the ABC model in
Lupinus (Fig 3
). AT2G46192 has been demonstrated to have differential expression in response to several conditions.
Mergner et al., (2020) have reported that this transcript is upregulated in flower pedicels and flowers and is found in carpels and petals
Arabidopsis. On the contrary, it is downregulated in
Arabidopsis under drought, heat and salinity stress
(Di et al., 2014). In particular, its repression is notable in the pollen of
Arabidopsis knockout cyclic nucleotide-gated cation channel 16 mutants exposed to heat stress
(Rahmati et al., 2018). These findings suggest that besides regulating flower development, lncRNA EL0144 could also be involved in abiotic stress responses.