Hormon Tanaman yang Dihasilkan Mikroba

1. Hormon Tanaman yang Dihasilkan Mikroba Oleh: Irda Safni

2. Pendahuluan • Istilah “hormon” pertama sekali diusulkan oleh ahli Fisiologi,BaylissdanStarling,pada tahun1904. • Definisi klasik Hormon adalah: pesan kimia yang meninggalkan satu daerah tubuh oleh aliran darah dan tiba pada bagian tubuh yang lain, yang menyebabkan perubahan perilaku. • Hormon tanaman adalah sinyal molekul yang berperan sebagai pengirim pesan yang mengendalikan pertumbuhan dan perkembangan tanaman.

3. Fitohormon adalah bahan organik yang disintesis di dalam organ tanaman tertentu yang dapat ditranslokasikan ke bagian yang lain, di mana dapat merangsang respon biokimia, fisiologi dan morfologi. • Tetapi fitohormon juga aktif di dalam jaringan tempat ia dihasilkan. • Selain itu, sejumlah bakteri dan jamur tanah juga menghasilkan fitohormon. • Fitohormon dapat dibagi 5 kelas, yaitu: • 1. Auksin • 2. Giberelin • 3. Sitokinin • 4. Asam Absisic (ABA) • 5. Etilen

4. Struktur Molekul berbagai Fitohormon

5. Soils are sources of diverse organisms, including fungi, bacteria,and plants. • Plant roots are heavily colonized with microorganisms (compared to soil and otherhabitats) because of the rich nutrient component of root exudates. • The rhizosphere is a relatively nutrient-rich environmentcontaining amino acids, sugars, fatty acids and other organiccompounds, which attract microbes that utilizethe various nutrients released by the root. • In turn, the microbes synthesize biologically active compounds, including phytohormones (auxins, cytokinins, gibberellins, and ABA), antifungal compounds, enzymes, and compatible solutes.

6. Several mechanisms of plantgrowth stimulation, plant protection and alleviation of saltstress by PGPR, including: • Nitrogen fixation • Synthesis of osmoprotectants, exopolysaccharides, 1-aminocyclopropane-1-carboxylate (ACC) deaminase • Cell wall degrading enzymes,and phytohormones • Modulation of antioxidant enzymes ornutrients • Solubilization of minerals, such as phosphorus,and potassium.

7. These microbial metabolites play a vital role in plant growth, nutrition and development. • They can stimulate plant growth development, provide resistance to various abioticand biotic stress factors, improve nutrient acquisition andprotect plants from various soil-borne pathogens.

8. The microbes mitigate stress responses byregulating the nutritional and hormonal balance in plants andinducing systemic tolerance to stress. • One of the mechanisms ofimprovement of plant growth and stress tolerance by microbesis their phytohormone synthesizing ability in the rhizosphere or root tissue. • Microbial phytohormones affect the metabolism of endogenous growth regulators in plant tissueand play a key role inchanging root morphology upon exposure to drought, salinity,extreme temperature and heavy metal toxicity.

9. The stress tolerance ability of bacterial strains provides important benefits to plants. • The ability of root-associated microbes to synthesize phytohormones is typically nothampered by high salt concentrations. • For example, phytohormone synthesis by endophytic actinobacteria Streptomyces coelicolor DE07 and StreptomycesgeysiriensisDE27 was not inhibited under water stress. • In a study, Pseudomonas putida, Pseudomonas extremorientalis, Pseudomonas chlororaphis, and P. aurantiaca were able to produce IAA in a 4% NaCl conditions (Egamberdieva and Kucharova, 2009). • Pseudomonas sp. and Bacillus sp. strains were able to produce IAA under high salt conditions (200–400 mMNaCl) and increased the plant biomass of Sulla carnosa under salt stress (Hidri et al., 2016).

10. The biosynthesis of phytohormones differs by bacterial strain. • For example, Bacillus and Pseudomonas strains synthesizedIAA concentrations up to 2.2 mg/mL, GA3 productionby A. xylosoxidans and B. halotolerans was between 36.5 and 75.5 mg/mL(Sgroy et al., 2009). • In another study, Bacillus amyloliquefaciensassociated with rice (Oryza sativa L.) synthesized gibberellins,and the quantities of GA differed, e.g., 17.8 ng/mL forGA20, 5.7 ng/mL for GA36, 5.6 ng/mLfor GA24,1.02 ng/mL for GA4, 0.7 ng/mL for GA53, 0.08 ng/mLfor GA5, and 0.01 ng/mL for GA8 (Shahzad et al., 2016).

11. Produksi dan Peran Fitohormon • Terdapat 2 sumber fitohormon secara alami, yaitu: • Produksi Endoginus dari jaringan tanaman • Produksi Eksoginus dari mikroorganisme, termasuk bakteri dan jamur tanah.

12. Keragaman Penghasil Fitohormon Tabel 1. Fitohormon yang dihasilkan tanaman dan mikroorganisme, dan pengaruhnya terhadap perkembangan dan morfologi tanaman

16. MICROBIAL PHYTOHORMONES IN PLANT STRESS TOLERANCE • Microbes synthesize low amounts of phytohormones andimprove stress tolerance and plant growth under various stressconditions, including salinity, heat, drought and metal toxicity. • The beneficial effect of phytohormone-producing microbes on alleviating abiotic stressin plants was reported in numerous studies (Figure 1).

17. Fig 1. An overview of mechanisms in microbial phytohormone-mediated plant stress tolerance. Several root associated microbes produce cytokinin (CK),gibberellin (GB), indole-3-acetic acid (IAA), salicylic acid (SA) and abscisic acid (ABA), which help plants to withstand stress by enhancing its antioxidant potential, byup-regulation of the antioxidant system and by accumulation of compatible osmolytes thus reducing oxidative stress-induced damage; improving photosyntheticcapacity and membrane stability; promoting cell division and stomatal regulation; stimulating growth of root system, andacquisition of water and nutrients.

18. Some examples of phytohormone-producing bacteria andtheir ability to mitigate abiotic stress (Table 1). • Many studies have reported the positive effects of bacteria associatedwith plants and IAA production on plant growth stimulation under abiotic stress conditions. • For example, bacterial strains Curtobacterium flaccumfaciens E108 and Ensifer garamanticusE110 isolated from Hordeum secalinum stimulated plant biomass and salt stress resistance in barley (Cardinale et al., 2015). • The root-colonizing halotolerant bacterium B. licheniformis • HSW-16 was able to mitigate salt stress-induced damage and • stimulate the growth of wheat through the production of IAA • under saline soil conditions (Singh and Jha, 2016).

19. Salt-tolerant bacterial strains B. subtilis and Arthrobacter sp. • increased wheat biomass and total soluble sugars and reducedsodium concentration in plant tissue (Upadhyay et al., 2012). • In another study, Pseudomonas spp.isolated from extreme environments (close to the sites of volcanos) synthesized IAA under salt stress (500 mM NaCl) and high temperature (40C), and they were able to stimulateincreases in the root and shoot biomass of maize (Mishraet al., 2017).

20. According to Bianco and Defez (2009), protectionof plants from negative effects of abiotic stress by IAA isrelated to enhanced cellular defense systems. • Several salt tolerant strains synthesizing IAA in culture medium, namely, Serratia plymuthica RR-2-5-10, Stenotrophomonas rhizophila e-p10, P. fluorescens SPB2145, P. extremorientalis TSAU20, andP. fluorescens PCL1751, improved cucumber biomass and yield in greenhouse conditions (9–24%) (Egamberdieva et al., 2011). • Root-associated IAA-producing bacteria were found to improve drought stress in plants. • IAA-producing bacteria were also found to improve plantgrowth and development under nutrient-poor soil conditions.

22. The tripartite interaction of root-associated microbes withsymbiotic microbes and the host plant is also a mutualisticinteraction that improves plant growth under stress throughthe induction of osmoregulation, hormonal balance, biochemicalprocesses and changes in metabolic interfaces among partners. • IAA-producing B. subtilis NUU4 in combination with Mesorhizobium ciceriIC53 stimulated root and shoot biomass and improved noduleformation in chickpea (Cicer arietinum L.) under salt stress, ascompared to uninoculated plants and plants inoculated withMesorhizobium ciceri IC53 alone (Figure 2).

23. Figure 2. Growth of chickpea in salinated soil after inoculation with Mesorhizobium ciceri IC53 alone or with the combination of Mesorhizobium ciceri IC53 and IAA-producing Bacillus subtilis NUU4 in pots (A) and under field condition (B)

24. Progress of Phytohormones in Industrialization • For the industrial production of plant hormones such as GAs and ABA, several processes using fungal fermentation were successfully set up in China. • As early as 1950s–1960s, the solid state fermentation of GAs has been applied. • However, due to the low yield, tedious operation and other shortcomings, the solid state fermentation was gradually replaced by liquid submerged fermentation, the titer of the process is generally at about 2.0 g/L. • At present, China is continuing to explore new fermentation technologies and have made some achievements, which greatly promotes the further applications of GAs in the agricultural field.

25. Compared with GAs, the industrial microbial fermentation of ABA started relatively late. • The world’s first industrial production line of ABA was formally established through the cooperation with the company Sichuan Lomon Bio Co. Ltd, which greatly reduced the cost and price of ABA and further promoted the application of ABA in agriculture.

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