**1. Introduction**

Strigolactones (SLs) are a class of carotenoid derivates. They were first discovered in root exudates of cotton and found germinating to be potent stimulants for seed germi‐ nation of the parasitic plant *Striga lutea* Lour. (witchweed) [1], for review, see [2]. Why plants should produce and secrete a signal molecule that is recognized by its parasites was revealed much later when SLs were identified as signal molecules involved in the establishment and maintenance of interactions with arbuscular mycorrhizal fungi (AMF) [3] and N‐fixing bacteria [4]. This also marked the time when SLs were classified as a new class of phytohormones, based on the semi‐dwarf and shoot‐branched mutants of

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*Arabidopsis thaliana* L. and *Oryza sativa* L. [5, 6]. In the following years, it was discov‐ ered that SLs are involved in additional aspects of plant development (**Figure 1**), that is, in the regulation of root system development by promotion of primary roots (PRs) elongation and inhibition of adventitious roots (ARs) formation. The SL effect on lateral roots (LRs) development was found to depend on the availability of nutrients, especially phosphorous (P) and nitrogen (N). Under optimal growth conditions, SLs will inhibit the elongation of LRs, but when plants are exposed to starvation stress SLs induce LR growth, for review see [7]. It thus became clear that SLs play a role in the complex plant response to nutrient stresses. Under both P and N deficiency conditions, the synthesis of SLs is increased and larger amounts of this hormone are secreted into the soil, prob‐ ably to promote the symbiotic relations with AMF and bacteria. The elevated levels of SLs also influence the plant phenotype by suppressing shoot growth and stimulating root development, thus adapting the plant to starvation conditions, for review see [8, 9]. The contribution of SLs in plant adaptation to the other stresses such as drought is

**Figure 1.** SLs regulate plant development by the promotion of internode elongation, leaf senescence, elongation of primary root (PR) and lateral root (LR)1 or inhibition of shoot branching, shoot gravitropism, and formation of adventitious root (AR) and LR2 . Additionally, SLs promote the symbiosis with arbuscular mycorrhizal fungi (AMF) and N‐fixing bacteria, and play a role in plant adaptation to drought and nutrient stresses. <sup>1</sup> SLs promote LR elongation under starvation stress and 2 inhibit LR elongation under optimal growth conditions.

unclear. According to some recent studies, SL mutants of *A. thaliana* are more sensitive to drought stress in comparison to the wild‐type plants [10, 11]. The results were not conclusive, however. In one study, the SL‐signaling mutants, *max2‐1* and *max2‐2*, were found to be hypersensitive to drought, whereas the SL biosynthesis mutants, *max1, max3* and *max4*, were not [10]. In a second study, both groups of mutants, defective in SL‐bio‐ synthesis (*max3‐11, max4‐7*) or SL signaling (*max2‐3*), were more sensitive to drought [11]. Additional studies on *Lotus japonicas* L. [12] and *Solanum lycopersicum* L. [13, 14] confirmed that SLs together with abscisic acid (ABA) play a role in plant adaptation to the limited water conditions. In response to drought, tomato plants show decreased SL biosynthesis in the roots but increased biosynthesis in the shoots [14]. Results like these feed the impression that SLs may well present a broad‐spectrum class of phytohormones (**Figure 1**). Based on an *in silico* analysis of the genes involved in SL biosynthesis in *A. thaliana* and rice, it has been postulated that SLs may also participate in pathogen defense mechanisms and plant responses to wounding, cold stress or flooding [15]. Until now, the experimental evidences have confirmed the role of SLs in plant resistance to bacterial and fungal pathogens, reviewed by Marzec [16].

*Arabidopsis thaliana* L. and *Oryza sativa* L. [5, 6]. In the following years, it was discov‐ ered that SLs are involved in additional aspects of plant development (**Figure 1**), that is, in the regulation of root system development by promotion of primary roots (PRs) elongation and inhibition of adventitious roots (ARs) formation. The SL effect on lateral roots (LRs) development was found to depend on the availability of nutrients, especially phosphorous (P) and nitrogen (N). Under optimal growth conditions, SLs will inhibit the elongation of LRs, but when plants are exposed to starvation stress SLs induce LR growth, for review see [7]. It thus became clear that SLs play a role in the complex plant response to nutrient stresses. Under both P and N deficiency conditions, the synthesis of SLs is increased and larger amounts of this hormone are secreted into the soil, prob‐ ably to promote the symbiotic relations with AMF and bacteria. The elevated levels of SLs also influence the plant phenotype by suppressing shoot growth and stimulating root development, thus adapting the plant to starvation conditions, for review see [8, 9]. The contribution of SLs in plant adaptation to the other stresses such as drought is

102 Phytohormones - Signaling Mechanisms and Crosstalk in Plant Development and Stress Responses

**Figure 1.** SLs regulate plant development by the promotion of internode elongation, leaf senescence, elongation

inhibit LR elongation under optimal growth conditions.

and N‐fixing bacteria, and play a role in plant adaptation to drought and nutrient stresses. <sup>1</sup>

or inhibition of shoot branching, shoot gravitropism, and formation of

SLs promote LR elongation

. Additionally, SLs promote the symbiosis with arbuscular mycorrhizal fungi (AMF)

of primary root (PR) and lateral root (LR)1

adventitious root (AR) and LR2

under starvation stress and 2

Up to now, more than 20 naturally occurring SLs, synthetized from the carlactone precur‐ sor, have been identified in the plant kingdom [17]. They share a similar structure, com‐ posed of a tricyclic lactone (ABC rings) connected to a butenolide group (D ring) by an enol‐ether bond (**Figure 2**). SLs are divided into two groups based on the stereochemical differences at the junctions between B and C rings: the orobanchol group with an α‐ori‐ ented C ring and the strigol group with a β‐oriented C ring (**Figure 2**) [18]. SLs are mainly produced in roots and transported to the shoot *via* specific transporters [19]. Alternatively, they might be also produced in the above‐ground parts of plants [20]. SLs biosynthesis started in plastids with the conversion of all‐*trans*‐β‐carotene into 9‐*cis*‐β‐carotene by the carotenoid isomerase Dwarf27 (D27), an iron‐containing protein [21–23]. The following stages of SLs biosynthesis are conducted by the carotenoid cleavage dioxygenases (CCDs) CCD7 [24, 25] and CCD8 [26, 27] and first involve the transformation of 9‐*cis*‐β‐carotene into 9‐*cis*‐β‐apo‐10′‐carotenal which is subsequently converted into carlactone [23]. Carlactone, a precursor for all known SLs, has no activity attributed to SLs, until it is converted into carlatonic acid by more axillary growth (MAX1) that belongs to the cytochrome P450 fam‐ ily [23]. The final methylation of carlatonic acid is mediated by an as‐yet unknown enzyme [28] (**Figure 3**).

Whereas the *A. thaliana* genome contains only one MAX1 gene, different rice varieties were characterized from two to five MAX1 homologs, which are involved in the synthesis of differ‐ ent SLs [29]. Still the open question remains if *A. thaliana* MAX1 mediates in the production of all SLs or only in the specific ones. The enzyme involved in the subsequent steps of SLs specialization, lateral branching oxidoreductase (LBO), converts methylated carlatonic acid into an as‐yet unidentified strigolactone‐like compound [30]. Characterization of the product of LBO activity and identification of additional enzymes involved in the last stages of SLs biosynthesis will be essential to compare the production pathways of this hormone in mono‐ and dicot species.

**Figure 2.** Structures of SL precursor carlactone and SLs represent two main stereochemical groups: strigol‐type SL with a β‐oriented C ring – 5‐deoxystrigol and orobanchol type with an α‐oriented C ring – orobanchol. Differences are present at the 8b and 3a positions between B and C rings.


**Figure 3.** Scheme of SLs biosynthesis and a list of enzymes involved in this process. Descriptions are given in the text.
