Adaptive responses of legume plants to P deficiency involve changes in root morphology induced by dynamic remodeling of primary roots, lateral roots, root hairs and cluster roots, which maximize the acquisition of external Pi from soils. Low-P-enhanced elongation of primary roots is observed in many legume plants, such as soybean, common bean, stylo and crowtoe (Lotus corniculatus) (Wissuwa, 2003; Guo et al., 2011; Zhou et al., 2014; Luo et al., 2020; An et al., 2023). For example, the growth of primary root is stimulated in soybean and stylo during P deficiency, which may be beneficial for foraging P from soils (Li et al., 2015; Luo et al., 2020; Yang et al., 2021a; Xie et al., 2022). To date, a set of PSI genes have been demonstrated to be involved in the regulation of primary root growth in legumes (Guo et al., 2011; Li et al., 2022). It has been reported that more than 200 PSI genes have been identified in soybean roots. Among them, GmEXPB2, a β-expansin gene, is found to be induced by P deficiency in roots. GmEXPB2 is a secretory protein that localizes to the cell wall; overexpression of GmEXPB2 increases the growth of primary roots by enhancing the size and number of cortical cells in both the root meristematic and elongation zones, thereby increasing Pi uptake and P efficiency in soybean (Guo et al., 2008; Guo et al., 2011; Zhou et al., 2014; Li et al., 2015; Yang et al., 2021a). Furthermore, a low-P-induced expansin-like B gene, GmEXLB1, has been proved to participate in enhancing root elongation and modifying root architecture, which is contributed to increase plant PAE (Kong et al., 2019). Similar role of SgEXPB1 gene in regulating root growth has recently been characterized in stylo under P-deficient condition (Wang et al., 2023). A variety of expansin genes upregulated by P deficiency are also reported in other legumes, such as alfalfa (Li et al., 2022), suggesting the key role of expansin genes in legume root growth. In addition, a group of transcription factors are also reported to be involved in regulating the primary root growth during P deficiency, such as members belonging to the MYB transcription factor family. For example, GmWRKY46, belonging to the WRKY family localized in the nucleus, is induced by P deficiency in soybean roots; overexpression of GmWRKY46 promotes the growth of both primary root and lateral roots and increases Pi uptake in transgenic Arabidopsis probably through the regulation of downstream PSI gene (Li et al., 2021).

Lateral roots also play vital roles in efficient Pi acquisition by enhancing soil exploration (Li et al., 2020; Zhang et al., 2020; Li et al., 2021; Lynch, 2022). It has been shown that the growth and proliferation of lateral roots in legume plants are mediated by Pi availability. For example, elongation and density of lateral roots are increased by low P stress in alfalfa and common bean (Williamson et al., 2001; Linkohr et al., 2002; Reymond et al., 2006; Zhang et al., 2014). A group of genes involved in lateral root growth have been identified in alfalfa, soybean and white lupin (Cheng et al., 2011; Li et al., 2020; Zhang et al., 2020; Li et al., 2021). For example, Gm6PGDH1, encoding the 6-phosphogluconate dehydrogenase, is mainly expressed in the P-deficient soybean root; overexpression of Gm6PGDH1 increased lateral root length and Pi uptake in transgenic soybean plants under P-deficient conditions (Li et al., 2021). In addition, GmETO1 is an essential ethylene-biosynthesis regulator located in the cell nucleus; both hairy root length and number of lateral roots are significantly increased in transgenic soybean plants with GmETO1 overexpression (Zhang et al., 2020). Furthermore, it has been found that the overexpression of GmWRKY45 increases the adaptability of transgenic Arabidopsis to Pi starvation through an increase in lateral root growth, contributing to greater Pi uptake (Li et al., 2020). In white lupin, the NAC domain-containing LaNAC1 is also implicated in regulating the growth of lateral roots under P deficiency (Cheng et al., 2011).

Root hairs, deriving from root epidermal cells, increase the root surface area that can be in contact with the soil substrate (Williamson et al., 2001; Huang et al., 2017). For instance, the number and length of root hairs in alfalfa increase rapidly in the early stage of P deficiency, facilitating greater Pi uptake (Li et al., 2018). Many genes have been documented to be involved in root hair growth in common bean, soybean and white lupin (Cheng et al., 2011; Yao et al., 2014a; Yao et al., 2014b; Li et al., 2015). For example, two genes encoding glycerophosphodiester phosphodiesterase, LaGPX-PDE1/2, have been implicated in root hair growth and development in white lupin (Cheng et al., 2011). Both LaGPX-PDE1/2 are highly expressed in P-limited root hairs; knockdown of LaGPX-PDE1/2 in white lupin impairs root hair development and density, thereby decreasing P concentration (Cheng et al., 2011). Therefore, LaGPX-PDE1/2 are proposed to be involved in improving PAE by enhancing root hair development. In common bean, PvSPX1 is one of the SPX (SYG1, Pho81 and XPR1) domain-containing proteins that is localized in the nucleus and plays a central role in the P signaling network (Yao et al., 2014b). The expression of PvSPX1 is enhanced by P deficiency in both leaves and roots of common bean; overexpression of PvSPX1 leads to an increase root P concentration and an enlargement of root hair zone in transgenic bean hairy roots, suggesting that PvSPX1 can regulate the growth of root hairs (Yao et al., 2014b).

Cluster roots, also known as proteoid roots, are a specialized root structure consisting of closely spaced tertiary lateral roots, and it is the feature of the Proteaceae members and several other plant species (Shane and Lambers, 2005; Shu et al., 2005; Lambers et al., 2006). It has been established that Pi acquisition capacity within the cluster roots is greater than that of the normal roots, suggesting an important role for cluster roots in Pi acquisition (Keerthisinghe et al., 1998). White lupin is the representative plant used to study the formation and growth of cluster roots affected by P nutrition (Liu et al., 2005; Wang et al., 2015). The earliest response of white lupin to P deficiency is the formation of cluster roots (Neumann et al., 2000). Numerous PSI genes have been identified in cluster roots of white lupin (Zhou et al., 2019). Among them, a novel C terminally encoded peptide gene LaCEP1 is characterized to be negatively regulating cluster root development (Zhou et al., 2019). LaCEP1 is highly expressed in the pre-emergence zone of the cluster roots; overexpression of LaCEP1 results in the inhibition of cluster root formation (Zhou et al., 2019). Although the formation of cluster roots significantly increases root surface area, cluster roots increase P acquisition mainly through increasing root exudation rather than by strengthening P foraging (Gardner et al., 1982; Neumann and Römheld, 1999; Shen et al., 2003). It is therefore of interest to investigate the coordinated morpho-physiological and biochemical responses of cluster root to P deficiency.

Root architecture refers to the overall spatial configuration of the root system, and it has significant effects on nutrient acquisition (Zhao et al., 2004; Koevoets et al., 2016; Li et al., 2016; Lynch, 2022). Root architecture displays high plasticity under P deficiency (Rellán-Álvarez et al., 2016; Amtmann and Shahzad, 2017; Motte et al., 2019). Modification of root architecture is the key strategy for plants to cope with low P stress and maximize Pi acquisition under P-limited conditions (Zhao et al., 2004; Li et al., 2016; Liu, 2021). Root architecture determines the distribution range and expansion degree of roots in soils, which is highly correlated with the P efficiency of legumes. The development of shallow root architecture is generally considered as an effective strategy for the legumes adaptation to P deficiency, such as common bean, mungbean (Vigna radiata) and soybean (Liao et al., 2004; Reddy et al., 2020; Seck et al., 2020). For example, in soybean, shallow root architecture and high lateral rooting are helpful traits in increasing Pi uptake compared to the deep root architecture. Furthermore, among an applied core collection of soybean, the cultivated soybean displays a shallow root architecture and high P efficiency, while the wild, climbing soybean exhibits a deep root architecture and low P efficiency (Zhao et al., 2004).

A range of quantitative trait loci (QTL) are reported to be related to root architecture and Pi acquisition (Liao et al., 2004). In legumes, for example, according to QTL analysis of basal root growth angle (BRGA) in bean recombinant inbred lines (RILs), the QTL for BRGA co-segregates with gain in yield under low P stress, and thus the BRGA has a major effect on PAE and yield under low-P conditions (Lynch and Brown, 2001; Liao et al., 2004). Furthermore, QTL analysis using bean RILs shows that some of the identified root traits, including those for BRGA, shallow basal root length and relative shallow basal root length, are associated with QTL for PAE (Liao et al., 2004). In addition, the QTL for basal root growth is also linked to the QTL for PAE in common bean (Beebe et al., 2006). Similarly, various QTL controlling root traits and P efficiency have been identified using soybean RILs; and the authors proposed that some of the identified QTL have great potential for genetic improvement of soybean with high P efficiency through a selection of root traits (Liang et al., 2010b).

Although key genes responsible for the QTL controlling root architecture in legumes have not been well characterized by forward genetic approaches, several genes possibly involved in modifying root architecture in response to Pi starvation have been identified by reverse genetics (Li et al., 2014; Yang et al., 2021a). For example, in soybean, GmEXPB2 is intrinsically involved in root system architecture responses to abiotic stresses, and overexpression of GmEXPB2 modified soybean root architecture through expanding root hair density and size of the root hair zone (Guo et al., 2011; Li et al., 2015).

Phosphate transporters (PHTs) are known to control Pi uptake and transport in plants (Versaw and Garcia, 2017; Dai et al., 2022). PHTs have been identified in many plants and can be divided into four subfamilies (PHT1, PHT2, PHT3 and PHT4) according to their functional differences and subcellular localization. Among them, the PHT1 family is a multiprotein family that is localized in the plasma membrane for transporting Pi from apoplast to cytoplasm (Nussaume et al., 2011). Most PHT1 genes are induced by Pi starvation in plant roots and are implicated in Pi uptake from soil or Pi translocation within plant tissues or cells (Liu et al., 2011; Remy et al., 2012). The role of PHT1 homologues in Pi uptake has been documented in legumes, such as M. truncatula, soybean and white lupin (Liu et al., 2001; Liu et al., 2008; Guo et al., 2022). Of the eleven PHT1 homologues in M. truncatula, only MtPT1, MtPT2, MtPT3 and MtPT5 are induced by low P stress (Chiou et al., 2001; Liu et al., 2008). MtPT1 is mainly expressed in the root and is likely involved in the Pi uptake (Chiou et al., 2001). MtPT5 encodes a plasma membrane-localized Pi transporter (Liu et al., 2008). Ectopic expression of MtPT5 complements the Pi uptake capability in the Arabidopsis pht1;1pht1;4 double mutant, confirming its function in Pi uptake (Wang et al., 2022). In soybean, GmPT4 is a plasma membrane-localized Pi transporter; and overexpression of GmPT4 increases the growth and Pi uptake in soybean, improving PAE (Guo et al., 2022). Pi-starvation-enhanced expression of PHT1 genes are also found in white lupin and stylo (Liu et al., 2001; An et al., 2023). Overexpression of SgPT1 from stylo can increase Pi uptake and enhance root growth in transgenic plants (An et al., 2023).

Insoluble P in soil can be divided into inorganic P and organic P. Inorganic P can be further classified into calcium phosphate (Ca-P), aluminum phosphate (Al-P) and iron phosphate (Fe-P), all of which is not easily available for the plant (Ao et al., 2014), while organic P mainly exists in the form of organic P esters and anhydrides, such as phytate, phospholipids, nucleotide and its derivatives (Simpson et al., 2011; Lorenzo-Orts et al., 2020).

Previous work indicates that the regulation of root organic acid exudation is an important process for inorganic P acquisition from the soil by chelating metal ions of insoluble phosphate, thereby increasing Pi concentration in soil solution (Peng et al., 2018; Lambers, 2022). Various types of organic acids secreted from roots are observed in legumes, such as white lupin (Lambers et al., 2013), soybean (Ryan et al., 2009), pigeon pea (Cajanus cajan) (Ishikawa et al., 2002) and common bean (Shen et al., 2002). For example, citrate and malate are the major organic acids secreted from cluster roots of white lupin under low P stress, which can help to increase Pi concentration in the rhizosphere (Vance, 2010; Lambers et al., 2013). Furthermore, organic acid exudation from a low-P-tolerant soybean genotype was higher than that of a low-P-sensitive soybean genotype (Zhang et al., 2011).

Organic acid synthesis and exudation are controlled by a variety of genes in legumes (Ryan et al., 2001; Wang et al., 2013; López-Arredondo et al., 2014). For example, organic acid synthesis is significantly increased in roots of transgenic alfalfa overexpressing the malate dehydrogenase gene, MDH, which is beneficial for increasing Pi uptake (Tesfaye et al., 2001; Tesfaye et al., 2003). Similar roles are observed in GmMDH12 from soybean and SgMDH1 from stylo (Chen et al., 2015; Zhu et al., 2021; Song et al., 2022). In addition, it has been reported that overexpression of citrate synthase (DcCs) from Daucus carota increases the synthesis and exudation of citrate in transgenic pigeonpea; and transgenic lines display enhancement of root growth compared to the wild type (Hussain et al., 2016). To date, the well-characterized malate and citrate transporters are members belonging to Aluminium Active Malate Transporter (ALMT) and Multidrug and Toxic Compound Extrusion (MATE) families, respectively (Sasaki et al., 2004; Hoekenga et al., 2006). For example, a group of GmALMT genes are regulated by P deficiency in soybean. Among them, GmALMT5, localized to the plasma membrane, is enhanced by Pi starvation in soybean roots; both root growth and P content of transgenic Arabidopsis overexpressing GmALMT5 are increased when grown in a medium supplied with Ca-P as the external P source (Peng et al., 2018). Similarly, LaALMT1 is also characterized to be the plasma membrane-localized malate transporter in white lupin (Zhou et al., 2020). In addition, several genes encoding citrate transporters have also been reported in legumes, such as VuMATE in rice bean (Vigna umbellata) and LaMATE1/3 in white lupin, all of which are induced by P deficiency (Wang et al., 2013; López-Arredondo et al., 2014; Zhou et al., 2021). For instance, LaMATE1/3 exhibit the highest expression in mature cluster root under low-P conditions; mediating citrate transport when LaMATE1/3 are expressed in oocytes, suggesting the role for LaMATE1/3 in regulating citrate transport during low P stress (Zhou et al., 2021). In addition, secreting protons by legume root can acidify the rhizosphere soil, thereby improving the bioavailability of insoluble P (Kouas et al., 2009). For example, a plasma-membrane transporter GmVP2 has recently been reported to mediate H⁺ exudation from root of soybean exposed to low P treatment; overexpression of GmVP2 in Arabidopsis can increase H⁺ exudation, promote root growth and increase Pi availability (Xie et al., 2022).

As mentioned before, about 30-65% of insoluble P in soil exists in the form of organic P, which can only be utilized by plants via the participation of various phosphoesterases, such as phosphatases, phosphodiesterases and nucleotidases (Matange et al., 2015; Tian and Liao, 2015; Yang et al., 2017). Purple acid phosphatases (PAPs) are among the most identified phosphoesterases in plants, which belong to the hydrolases that hydrolyze organic P to release inorganic Pi for plant uptake (Tian and Liao, 2015; Wu et al., 2018). It has been demonstrated that the root-associated/secreted PAPs are either associated with root surfaces or secreted into the rhizosphere, scavenging Pi from external organic P and increasing Pi availability (López-Arredondo et al., 2014; Tian and Liao, 2015; Wang and Liu, 2018). The Pi-starvation-increased activity of root-associated/secreted acid phosphatase is observed in common bean, stylo and peanut (Arachis hypogaea) (Liang et al., 2010a; Wei et al., 2022; Liu et al., 2016, 2018).

A set of PSI-secreted PAPs has been identified in legume plants, for example, LaSAP1/2 from white lupin (Wasaki et al., 2000; Wasaki et al., 2009), PvPAP3 from common bean (Liang et al., 2010a; Liang et al., 2012), and GmPAP7a/7b from soybean (Zhu et al., 2020). GmPAP14 is a secreted PAP in soybean. GmPAP14 overexpression increases secreted APase and phytase activities, contributing to external phytate utilization and growth enhancement (Kong et al., 2018). Similar roles for GmPAP7a and GmPAP7b have been found in soybean and these two PAP members exhibit high activities against adenosine triphosphate (ATP) in vitro. Overexpression of GmPAP7a and GmPAP7b is able to increase root-associated APase activities, thereby improving utilization of organic P in soybean (Zhu et al., 2020). In common bean, PvPAP3 is found to localize to the plasma membrane and apoplast. The transcript of PvPAP3 is induced by Pi starvation in roots of common bean, especially in the P-efficient genotype; and overexpression of PvPAP3 increases the growth and P content in bean hairy roots and Arabidopsis when ATP and dNTPs are supplied as the sole external P source, respectively (Liang et al., 2010a; Liang et al., 2012). In addition, overexpression of MtPHY1 or MtPAP1 increases the ability of organic P utilization in both white clover (Trifolium repens) and alfalfa (Ma et al., 2009; Ma et al., 2012). The roles of other PAP homologues, such as SgPAP7/10/23/26 from stylo and CaPAP7 from chickpea in organic P utilization have also been elucidated (Liu et al., 2016; Bhadouria et al., 2017; Liu et al., 2018). These studies suggest that PSI-secreted PAPs play an important role in the utilization of external organic P for improved P acquisition.

In soils, plant roots can interact with many rhizosphere microorganisms, such as mycorrhizal fungi and phospho-relieving bacteria, regulating Pi uptake and its utilization (Smith et al., 2003; Smith et al., 2004; Liang et al., 2014). Most of the legumes can be infected by mycorrhizal fungi to form a symbiotic system. The formation of plant-mycorrhizal fungal symbiosis is one of the vital mechanisms of plant adaption to low P stress (Smith et al., 2003; Smith et al., 2004), which can improve plant P efficiency ( Figure 2 ). In plant-mycorrhizal fungi symbiosis, mycorrhizal association generates a large number of extra-root hyphae which can extend beyond the P-deficient roots, but also enter into the small soil particle gaps to improve the spatial utilization of P in soils (Bago, 2000; Wang et al., 2011). Mycorrhizal fungi can promote Pi uptake and transport by regulating the expression of PT genes in the extracorporeal filaments and roots of the host plant. A set of PT homologues are induced by mycorrhizal fungi inoculation in host plants, such as MtPT4/6 in M. truncatula, AsPT4 in Astragalus sinicu and GmPT8/9/10/11 in soybean (Harrison et al., 2002; Tamura et al., 2012; Xie et al., 2013; Wang et al., 2016; Cui et al., 2019). For example, MtPT4 encoding a low-affinity Pi transporter is expressed in mycorrhizal roots of M. truncatula. Complemental analysis shows that MtPT4 can enhance Pi uptake in yeast (Saccharomyces cerevisiae) cells, suggesting that MtPT4 is involved in Pi acquisition from arbuscules in M. truncatula (Harrison et al., 2002; Volpe et al., 2016). In soybean, GmPT10 and GmPT11 are two mycorrhiza-inducible Pi transporters that can complement Pi transport in the yeast mutant PAM2, which is lacked PHO84 and PHO89, two high-affinity Pi transporters (Tamura et al., 2012). In addition, PAP genes have also been characterized to involve in plant-mycorrhizal fungi symbiosis (Li et al., 2017; Li et al., 2019; Wang et al., 2020). The plasma membrane-localized GmPAP33 is found to be mainly expressed in arbuscule-containing cells; overexpression of GmPAP33 significantly increases the percentages of large arbuscules and P content of transgenic soybean when inoculated with AM fungi (Li et al., 2019).

On the other hand, legumes are able to develop symbioses with rhizobia to generate a special organ, nodule, which can release an abundance of protons to acidify the rhizosphere, thereby increasing Pi availability in soils (Qin et al., 2011; Ding et al., 2012). It has been reported that the growth and N and P contents are increased in soybean after inoculating with rhizobia in low-P acid soils (Cheng et al., 2009). A variety of PSI genes and proteins have been identified in nodules exposed to P deficiency ( Figure 2 ; Hernández et al., 2009; Chen et al., 2011; Cabeza et al., 2014; Xue et al., 2018; Zhong et al., 2023). For example, 1140 and 2055 genes have been found to be regulated by P deficiency in nodules of M. truncatula and soybean, respectively (Cabeza et al., 2014; Xue et al., 2018). Among them, a group of Pi transporter genes have been reported to be involved in improving Pi uptake in nodules. For example, GmPT7, a nodule-localized Pi transporter, is induced by low P stress in soybean nodules. GmPT7 is found to be mainly expressed in the outer cortex and N fixing zones of the nodules; overexpression of GmPT7 in soybean increased nodulation, P content and soybean yield, suggesting that GmPT7 is responsible for direct Pi entry to nodules (Chen et al., 2019). Unlike GmPT7, GmPT5 is proposed to function in transporting Pi from roots to nodules, thereby regulating nodulation and soybean growth (Qin et al., 2012). In addition, GmPT1/4/11 are also implicated in Pi homeostasis and nodulation in soybean (Qin et al., 2012; Chen et al., 2019; Lu et al., 2020). In M. truncatula, MtPT6 is preferentially expressed in vascular bundles, cortical cells, and fixation zone cells of nodules. Functional analysis confirms that MtPT6 is a typical Pi transporter and can increase PAE in plants (Cao et al., 2021). Furthermore, MtPHO1.1 and MtPHO1.2 are expressed not only across the various nodule zones but also the root vascular system, participating in transporting Pi from infected nodule cells to bacteroids in M. truncatula (Müller, 2021; Nguyen et al., 2021). In addition, the transcript of GmPAP12 is gradually increased during soybean nodule growth. Subsequent analysis shows that overexpression of GmPAP12 leads to increase nodule number, shoot dry weight, and N and P content of transgenic composite soybean plant under low P stress, suggesting the roles of GmPAP12 involved in nodulation and SNF in soybean (Wang et al., 2020). Among the nine β-expansin members, the expression of GmINS1 is found to be enhanced by Pi starvation in soybean nodule; overexpression of GmINS1 increases nodule size and N₂ fixation capacity, ultimately enhancing P content, plant biomass and soybean yield (Li et al., 2018; Yang et al., 2021b). Although only a few genes involved in symbiosis have been identified, their function highlights the importance of symbiosis for improving legume adaption to low P stress.

Physiological and biochemical changes of plants in response to low P stress are mediated by a complex signaling network, including local and systemic sensing and signaling. A set of important components shown in Figure 1 , such as transcription factors and regulators, have been demonstrated to be involved in the Pi signaling network. Among them, an R2R3 MYB member, Phosphate Starvation Response 1 (PHR1) is the most well-characterized transcription factor involved in Pi starvation responses in the Pi signaling network (Chiou and Lin, 2011). PHR1 or PHR1-like is considered as the central transcription factor of Pi signaling network through the regulation of a lot of downstream PSI genes (Wang et al., 2009; Bustos et al., 2010; Wang et al., 2014; Zhong et al., 2018). To date, several PHR1 genes have been identified in legumes, such as GmPHR1/4/25 from soybean (Xue et al., 2017; Lu et al., 2020) and PvPHR1 from common bean (Valdés-López et al., 2008). For example, in soybean, the Pi starvation up-regulated gene GmPHR25 is an important regulator in the Pi signaling network that controls Pi homeostasis in soybean; and overexpressing GmPHR25 results in increasing Pi concentration in transgenic soybean hairy roots, probably through enhancing the expression of several high-affinity Pi transporters (GmPTs) and GmPAP14/21 (Xue et al., 2017). In common bean, PvPHR1 is a positive regulator of the Pi signaling network. The transcript of PvPHR1 is increased by Pi starvation in both leaves and roots; and the knockdown of PvPHR1 leads to a decreased expression of a set of PSI genes, such as PvPHT1, PvPHO1, Pv4, PvRNS and PvAPC5, thereby decreasing Pi concentrations in composite common bean plants (Valdés-López et al., 2008).

In addition, SPX domain-containing proteins also have been characterized as key sensors and regulators of Pi homeostasis and signaling in soybean, common bean and M. truncatula (Yao et al., 2014a; Yao et al., 2014b; Wang et al., 2021; Zhuang et al., 2021). For instance, in M. truncatula, two SPX domain-containing proteins, MtSPX1 and MtSPX3, are found to be localized in the cytoplasm and the nucleus and can also interact with MtPHR2. The expressions of MtSPX1 and MtSPX3 are increased by Pi starvation in arbuscule-containing cells. Lost function of MtSPX1 and MtSPX3 results in decreased root colonization and arbuscule abundance as well as the expression of Rhizophagus Irregularis Elongation Factor (RiEF) and MtPT4, suggesting that MtSPX1 and MtSPX3 play important roles in maintaining arbuscular mycorrhizal symbiosis in M. truncatula (Wang et al., 2021). In common bean, PvSPX1 is a positive regulator in the Pi signaling network. PvSPX1 can regulate a set of downstream of PSI genes that are involved in Pi transport, translocation and homeostasis, such as PvPAP1/2/3/4/5, PvPS2:1, Pv4 and PvPHR1 (Yao et al., 2014b). A similar role of GmSPX3 has been implicated in soybean (Yao et al., 2014a). In addition to interaction with PHR, one of the GmSPX members in soybean, GmSPX5, has been reported to interact with the transcription factor GmNF-YC4 to activate GmASL6 expression, mediating nodule development through regulating asparagine metabolic processes (Zhuang et al., 2021).

Other transcription factors belonging to bHLH, WRKY, SPL, EIN3, ZAT and GARP families are also involved in the regulation of PSI genes in the Pi signaling network (Zhang et al., 2014; Scheible and Rojas-Triana, 2015; Song and Liu, 2015). For example, in soybean, GmPTF1 is an HLH transcription factor responsible for activation of GmEXPB2, modifying root architecture during P deficiency (Li et al., 2014; Yang et al., 2021a). In addition, other regulators, such as miRNAs and PHO2, have also been reported to be involved in the Pi-starvation response in many legume plants, such as common bean (Valdés-López et al., 2010), white lupin (Zhu et al., 2010), soybean (Zeng et al., 2010), M. truncatula (Boualem et al., 2008) and alfalfa (Li et al., 2018). For example, PvPHO2, encoding E2 ubiquitin ligase, is a target that is negatively regulated by miR399 in common bean; PvPHO2 is suppressed by P deficiency and it negatively regulates PSI genes, including PvAPC5, PvAP and PvPHT1 (Valdés-López et al., 2008; Ramírez et al., 2013).