## 1 Introduction
### Secondary Metabolites Recruit Beneficial Rhizospheric Microbes  
Root exudates are crucial in modulating the composition and functional diversity of rhizosphere microbes. Mucilage is actively released from the roots, while diffusates are passively released due to osmotic differences between the soil solution and the cell. Both types include organic compounds, which are classified into low and high molecular weight compounds. Heavy molecular weight compounds (i.e., mucilage, cellulose) are not easily used by microbes and make up the majority of C released from roots. Low molecular weight compounds are highly diverse and have a wide array of functions. These consist of organic acids, amino acids, proteins, sugar, phenolics, and other secondary metabolites (including benzoxazinoids, coumarins, flavonoids, indole compounds, and terpenes) that shape the rhizosphere microbiome [58].  
Plant secondary metabolites have differential effects on soil microbiota. Two indole metabolites, benzoxazolinone and gramine, produced by different Graminae species, and quercetin, a flavonoid synthesized by many dicot species, revealed significant effects on soil bacteria, with benzoxazolinone showing a predominantly inhibitory effect preventing the accumulation of many predominantly harmful taxa, while gramine and quercetin mostly exert their function by attracting beneficial bacteria [58]. Oleanolic acid possesses chemoattractant properties and is the principal constituent of _Salicornia_ root exudates  
Figure 1: Mechanism of halobacteria in ameliorating salt stress and improving growth and yield of halophytes under saline conditions.  
that have the potential to expand the colonization of salt-tolerant _Salicornia hispanica_ by halophilic siderophore-producing bacteria _Halomonas anticriensis_ FP35\({}^{\text{T}}\)[6].  
Terpene is a highly diverse group of secondary metabolites that play an important role in plant microbe interaction. _Arabidopsis thaliana_ root exudates contain triterpenes, namely thalianin, thalianyl medium-chain fatty acid esters (three steps), and arabidin, that are involved in belowground communication and able to directly modulate _A. thaliana_-specific root bacterial communities in a very selective manner [59]. [PERSON] et al. [60] reported that organic acids (2-methylbutyric acid, stearic acid, palmitic acid, palmitoleic acid, and oleic acid) found in the root exudates of _Limonium sinensis_ promoted the chemotaxis of the rhizosphere PGPR strain _Bacillus flexus_ KLBMP 4941. Strigolactone is a new class of phytohormone that acts as a signaling molecule in symbiosis to recruit root- and rhizosphere-associated microbiomes. [PERSON] et al. [61] analyzed the bacterial and fungal microbial communities of 16 rice genotypes differing in exudation of root strigolactone. The results showed that structural differences in the exuded strigolactones affected different sets of microbes, i.e., the relative abundance of phosphate solubilizing microbes; _Burkholderia_, _Caballeronia_, _Paraburkholderia_ and _Acidobacteria_ were linked to orobanchol strigolactone, whereas 4-deoxyorobanchol was associated with genera _Dyella_ and _Umhelopsis_[61].