## 4 Plant-Bacteria Interaction Regulates Plant Protein and Metabolites Production
### Bacterial Proteins and Metabolites in Plant Stress Mitigation (Direct Mechanism)  
#### 4.1.1 ACC Deaminase  
Ethylene is a gaseous hormone that is produced by plants and, in certain cases, microorganisms. At lower concentrations, ethylene promotes seed germination, root elongation, and flowering [19]. However, under stress conditions, ethylene synthesis in plants increases, which has a negative impact on plant growth. S-adenosyl-methionine (SAM), derived from the methionine amino acid, is the main substrate in the ethylene biosynthesis pathway. The ACC synthase enzyme catalyzes the synthesis of 1-aminocyclopropane-1-carboxylic acid (ACC) from SAM, and the ACC oxidase enzyme catalyzes the conversion of ACC to ethylene [20]. The production of ACC deaminase by bacteria aids in the reduction of stress ethylene by cleaving ACC into ammonia and alpha keto butyrate [21; 22] (Figure 2A). Another benefit of this ACC deaminase is that it eliminates IAA feedback inhibition produced by increased ethylene synthesis in plants. [PERSON] and colleagues investigated this enzyme and discovered that ACC deaminase is an inducible enzyme whose synthesis is encouraged by its substrate, ACC. This enzyme is found in the bacterial cytoplasm (it is not secreted), importantly, ACC deaminase-producing bacteria (rhizospheric, endophytic, or epiphytic) can modulate ACC concentrations in the rhizosphere and phyllosphere by consuming plant exuded ACC, or within plant tissues (e.g., the endosphere and root nodules), thereby directly limiting the actions of ACC and, as a result, limiting ethylene production [23]. [PERSON] et al. [24] developed a scenario for the operation of bacterial ACC deaminase in which a considerable amount of ACC is secreted from plant roots or seeds, taken up by soil microorganisms, and hydrolyzed to ammonia and alpha-ketobutyrate. ACC absorption and hydrolysis reduce the quantity of ACC found outside the plant roots. Furthermore, by exuding more ACC into the rhizosphere, the balance between the internal and external ACC levels is maintained (Figure 2A).  
Figure 2: Bacterial metabolite mediated mechanism for minimizing salt stress in order to boost plant growth and development. **(A)** The ACC deaminase enzyme, **(B)** EPS production, and **(C)** Phyto-hormones production. SAM (S-adenosyl-L-methionine); ACC (1-Aminocyclopropane-1-Carboxylic Acid); EPS (Extracellular Polymeric Substances). Top plane of **(B,C)** are without bacterial cell and bottom plane is with bacterial cells.  
In one of our studies, we have isolated halotolerant ACC-deaminase producing bacteria from the rhizosphere of halophytic plants near the Yellow Sea in saline media (850 mM NaCl) [25]. _Brevibacterium linens_ RS16, a promising isolate, alleviated salt stress in rice and red pepper plants [26; 27; 28]. Using _Pseudomonas_ sp. with ACC deaminase generating ability, we were also able to reduce salt stress ethylene in red pepper [29; 30]. [PERSON] et al. [31] demonstrated the role of ACC deaminase in lowering stress ethylene production in tomato plants by developing a _Pseudomonas azotoformans_ CHB 1107 ACC deaminase gene (_acdS_) positive mutant. The ACC deaminase producing halotolerant _Glutamicibacter_ sp. YD01 reduced stress ethylene production in rice seedlings grown up to 200 mM of salt stress [32]. Another evidence showed that ACC deaminase enzyme producing _Streptomyces_ sp. GMKU 336 down-regulated the ACC oxidase (_ACOI_) and the ethylene responsive element binding protein (_EREBP1_) genes in rice plants grown at 150 mM of salt stress [33].