**1. Introduction**

Dwarf phenotypes have been widely used to improve lodging resistance and enhance harvest index in crops. For this reason, the proper modulation of plant height has always been a priority for breeders. Although many factors regulate plant height, gibberellin (GA) plays a leading role and is also known as the "green revolution phytohormone" because of its grea<sup>t</sup> contribution to the cultivation of high yields and lodging resistant crop varieties. The "green revolution" gene *semi-dwarf 1* (*sd1*) encoding GA biosynthesis enzyme GA 20-oxidase (GA20ox) is always important in rice breeding from the 1960s [1]. The *reduced height 1* (*rht-B1b* and *rht-D1b*) mutants showed a semi-dwarfing phenotype due to insensitivity to GA and were also used to breed for lodging resistance and yield increase in wheat [2].

GA is involved in various processes of plant growth and development, including leaf expansion, seed germination, induction of flowering and stem elongation [3–7]. With the extensive characterization of dwarf mutants related to GA, numerous genes encoding GA biosynthetic enzymes have been identified [8,9]. Bioactive GA biosynthesis is divided into three stages. In the first stage, geranylgeranyl diphosphate (GGDP), the precursor of GA, is catalyzed by the copalyl diphosphate synthase (CPS) and *ent*-kaurene synthase (KS) to form *ent*-kaurene, and this process takes place in the plastid [10,11].

Then, in the second stage, *ent*-kaurene is converted to GA12 by *ent*-kaurene oxidase (KO) and *ent*-kaurenoic acid oxidase (KAO), both of which are cytochrome P450 enzymes [11–13]. In the final stage, GA12 is catalyzed by GA20ox and converted to GA9 via GA15 and GA24, and then GA9 is converted to GA4 by GA 3β-hydroxylase (GA3ox) [14–17]. GA12 is also converted to GA1 through the 13-hydroxylation pathway [9]. The biosynthesis of bioactive GA1 and GA4 occurs in the cytoplasm. Because the early-step genes of GA biosynthesis, *CPS1*/*GA1*, *KS* and *KO*, are single copy in *Arabidopsis*, mutations of these genes usually induce severely dwarf phenotype with greatly impaired fertility [12,18]. In contrast, the loss-of-function mutants of *GA20ox* and *GA3ox* (the late-step genes of GA biosynthesis) show a semi-dwarf phenotype due to the functional redundancy of multiple copies of genes [19,20]. In addition, the mutants altered in GA degradation and signal transduction pathway also show various degrees of dwarf phenotype and are valuable in molecular breeding [21–25].

Previous studies have suggested that, in addition to GAs, other plant hormones, such as brassinosteroids (BRs) [26,27] and strigolactones (SLs) [28,29], also play important roles in plant height development. The dwarf mutants related to these hormones can be divided into two types, hormone-sensitive and hormone-insensitive. The plant height of those hormone-sensitive mutants could be restored by exogenous hormones because their hormone content is reduced due to the disorder of hormone metabolic pathways [26,29–32]. The hormone-insensitive mutants are not sensitive to the hormone due to the abnormal signaling pathway [27,33–36]. So far, the molecular genetic pathways underlying the plant height regulation are well-characterized in *Arabidopsis* and rice, but only a few studies have been conducted on other species.

Legumes are the second most important economic crops after cereals and provide the major sources of plant proteins and oils for humans and animals [37]. Investigations on dwarf mutants in peas and soybeans have strongly suggested that the GA pathway plays a conserved role in determining the plant height of legumes [38–40]. *Medicago truncatula*, a diploid model legume plant, has been sequenced [41,42], but little attention was given to the basis of its height and the involved regulatory mechanisms of the GA pathway.

In this study, we characterized the severely dwarf mutant *mnp1* with two alleles isolated from the *Tnt1* retrotransposon-tagged mutant population of *M. truncatula*. Through forward genetic screening and the subsequent whole-genome resequencing approach, we cloned the *MNP1*/*Medtr7g011663* gene and found that it was well-clustered with the homologous genes encoding *Pisum sativum* LS, *Solanum lycopersicum* GIB-1, *Arabidopsis thaliana* CPS1/GA1, *Oryza sativa* OsCPS1 and *Zea mays* An1, all of which are the enzymes involved in the first step of GA biosynthesis. Because the dwarf phenotype of *mnp1* was significantly restored by exogenous application of GA3, and the mini-plant phenotype of the *Arabidopsis cps1* mutant was partially rescued by the expression of *MNP1*, we proposed a conserved function of MNP1 in GA biosynthesis. Given the evidence that both the *mnp1* and the pea *ls* mutants are fertile and there are multiple possible copies of *MNP1*/*LS* in *M. truncatula*, peas and soybeans, it is reasonable to hypothesize that the duplication of *CPS* genes and the subsequent functional divergence may have occurred in legumes during evolution [43]. The result has significant implications for the legume breeding programs and provides a good model to further study the regulatory mechanism of height regulation in *M. truncatula*.
