Pak. J. _"1gri. sa, Vol. 30, No.3, 19?3

SUBSTRATE-DEPENDENT MICROBIAL Pi{ODUCTIONOF PHYTOHORMONES: A REVIEW

M.ArshadDepartment of Soil Science,

University of Agriculture, Faisalabaa

This mini-review focuses on the microbial production of phytohormones in-cluding auxins, cytokinins and ethylene derived from exogenously applied precur-sors. The L-tryptophan, adenine plus isopentyl alcohol and L-methionine have beenfound to promote the production of microbially-derived phytohormones in bothpure culture studies and in soils. We have evaluated the applications of theseprecursors as agrochemicals to promote microbially-derived phytohormones inmodifying plant growth and development. Soil amendments with these precursors tothe rhizosphere of young seedlings has affected plant growth, development and yieldof Douglas fir, radish peppers, melons, corn, tomatoes and black ciris.

INTRODUCTION

Numerous studies have demonstratedthat soil microorganisms are active in pro-duction of phytohormones. This has led sci-entists to speculate that phytohormonesreleased by various inocula may be an activemechanism in affecting plant growth, devel-opment and yields (Arshad and Franken-berger, 1991 a; 1992 a, b; Brown, 1982;Brown et al; 1968; Frankenberger and Ar-shad, 1991 c; Hubbell et al; 1979;Tien et at;1979). Several studies have shown similarplant responses upon treatment with phyto-hormones and inoculation with specific mi-croorganisms. Brown et al. (1968) found thata combination of indole-3-acetic acid (IAA)and gibberellic acid (GA3) applied to tomatoseedlings produced effects on plant growthsimilar to those of a pure culture of Azoto-bacter chroococcum. Azcon et al. (1978) re-ported that cell-free supernatants of Rhizo-bium, Azotobacter and rseuaomona« cul-tures affected plant growth similarly as didthe combined application of IAA, GA3 andkinetin. Combined applications of an auxin-gibberellin-kinetin preparation led to a

change in root morphology of sorghum andpearl millet similar to that produced by in-oculation with Azospirillum brasilense(Hubbell et al., 1979; Tien et al., 1979). Sim-ilarly, Kucey (1988) reported that many ofthe wheat shoot and root growth-alteringeffects of A. brasilense could be simulated bythe application of phytohormones, most no-tably 1M and GA3• The exogenous applica-tion of IAA elicited the same response asinoculation with A. brasilense in an assaywhere an increase in dry weight of intactwheat roots was demonstrated after inocu-lation within 10 days (Zimmer et al., 1988).These studies indicate that microbially-pro-duced phytohormones in the rhizospheremay have a significant ecological effect whensubjected to direct uptake by plant rootswith the intimate contact between microbialand plant cells.

Although microbial biosynthesis ofphytohormones can occur even in the ab-sence of exogenously supplied precursors,the addition of specific substrates can sig-nificantly simulate their production. Wehave used this approach to promote micro-bial production of phytohormones by adding

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Table 1. Influence of L-tryptophan (L-TRP) as an auxin precursor Oil growth ofDouglas fir (Source: Frankenberger & Poth, 19H7)

Douglas firTreatment ------------ ..------------------------ -----------------_ .•......-----_ ......-- -_ .......--- --- --_ ..

Shoot Root Shoot Root Stem dia-height (em) length (em) weight (g) weight (g) meter (mm)

Control 2.3 a 42.6 b 3.6 b 3.5 b 0.79 bPisolithus tinctorius (Pt) 2.3 a 4O.5b 3.6 b 3.7 b 0.80 bPt + 10-3M TRP 3.5 b 32.2 a 2.8 a 2.9 a 0.53 aPt + 10-4M TRP 3.4 b 38.8 ab 3.7 b 3.8 b 0.75 bPt + 10-5M TRP 4.5 be 40.6 b 3.9 be 4.9 b n.87 bPt + 10-6M TRP 3.8 b 53.0 e 4.3 be 5.1 be 1.02 ePt + 10-7M TRP 4.2 be 54.6 e 4.8 e 5.5 e 1.05 ePt + 10-8M TRP 5.1 e 54.3 e 5.1 e 5.9 e 1.12 e

Values followed by different letter(s) differ significantly at P<0.05.

Fig. 1. Established ectomycorrhizae formed by Pisolithus tinctoriusin the presence of 10.7M.

L-tryptophan (L-TRP) (3.4 J.l.g kg-' of soil); A = With L-TRP; B = 3.4 J.l.g TRP kg" of soil.

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Table 2. Response of radish to L-TRP (3.0 mg kg-) soil) and various auxins (3.0 mg kg.1soil) applied to soil (Source: Frankenberger et al., 1990)

Treatment Root dry Shoot dryweight weight

........................(g plant·I) .

1.02a 0.60 a1.29b 0.56 a1.72d 1.16 be2.28 d 1.51ef1.88d 1.08b2.34 f 1.38 de1.49c 1.33cd2.11 e 1.68 f1.68cd 1.42 de

ControlL-tryptophanNutrient solutin (NS)L-tryptophan + NSIndole-3-acetic acid + NSIndole-3-acetamide + NSIndole-3-pyruvic acid + NSIndol-3-lactic acid + NSIndole-3-butyric acid + NS

Values followed by different letter(s) differ significantly at P<0.05.

Table 3. Response of watermelon and cantaloupe to L-TRP applied to soil (Source:Frankenberger and Arshad, 1991a)

Royal windsor Royal sweet Top scorewatermelon watermelon cantaloupeL-TRP ----------------------------- ------------------------- -------- -----_ ..------- ..---------------

(mg kg-I soil) Number Total Number Total Number Totalof fruit fresh of fruit fresh of fruit freshplant-) weight of planr'! weight of plant! weight of

fruit fruit fruitplane I plant·1 planr'!(kg) (kg) (kg)

Control 3.82* 20.5 3.69 26.9 11.94 11.860 4.64 28.2 5.16 37.6 14.93 15.16.0 4.07 31.2 4.53 34.8 12.12 14.26.0 X 10-) 4.43 30.2 4.81 36.2 13.68 15.06.0 X 10.2 5.31 33.7 3.81 33.2 13.62 16.66.0 X 10-3 5.00 31.5 4.41 40.6* 13.37 16.7*6.0x 10-4 5.06 34.5* 5.38 39.6* 12.25 16.5*6.0 X 10.5 5.21 36.9* 5.45* 42.6* 13.87 16.6*

"Means significantlydifferent from control at P<O.05.

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specific precursor(s) which is the main focusof this review. Although, numerous soil mi-croorganisms have been shown capable ofproducing gibberellins (Arshad andFrankenberger, 1992 a), this topic will notbe discussed in this review. Most of the lit-erature concerning microbial production ofphytohormones in soils is on auxins, cy-tokinins and ethylene.Auxins: Tryptophan (TRP) is a well-estab-lished precursor of auxins in higher plants. Italso serves as a precursor for microbially-derived auxins in pure culture and in soils(Arshad and Frankenberger, 1992 a). Invitro studies have demonstrated that somemicrobial cultures can produce smallamounts of lAA in the absence of a physio-logical precursor. However, in the presenceof L-TRP, the microbiota often releasemuch greater quantities of lAA and lAA de-rivatives. Frankenberger and Poth (1988)separated and purified a soluble o-ketogul-tarate-dependent L-TRP transaminaseenzyme from a cell free extract of a rhi-zobacterial isolate associated with Festucaocto flora Walt. This enzyme catalyzed theconversion of L-TRP to indole-3-pyruvicacid, an intermediate of lAA synthesis. Byusing HPLC, ELISA and GC-MS, they alsoconfirmed the ability of a mycorrhizal fun-gus, Pisolithus tinctorius to produce lAAfrom exogenously supplied L-TRP(Frankenberger and Poth, 1987). In micro-cosm studies with soil, Purushothaman et 01.(1973; 1974) reported active synthesis ofauxins when amended with TRP. Franken-berger and Brunner (1983) confirmed thisobservation and provided unequivocal proofthat lAA was produced in soil when incu-bated with L-TRP.

Since TRP is known to promote auxinsynthesis in soils, this biotransformation mayhave a physiological effect on plant growthvia microbially-derived auxins. We have car-ried out several experiments to evaluate this

Pak. J. Agri. s«. Vol. 30, No.3} 1993

hypothesis. After confirming L-TRP-depen-dent lAA synthesis by an cctomycorrhizalfungus, P. tinctorius, Frankenberger andPoth (1987) investigated the influence of L-TRP and the fungus as an inoculum onDouglas fir (Pseudotsuga menzicsii). It wasfound that P. tinctoriu stimulated the growthof potted seedlings of Douglas fir only whensupplied with low concentrations of L-TRP(ng to J.l.gquantities) (Table 1). There wasno difference in plant growth between inoc-ulated and uninoculated treatments in theabsence of L-TRP, however, the addition ofdilute solutions of L-TRP (l0-8 to 10-6 M,0.34 to 34 J.l.gkg-1 soil) with P. tinctoriuscaused a significant increase in growth indi-cating a physiological rather than nutritionaleffect. The higher concentration of L-TRPapplied as 10-3M caused reduction in dryweights of seedlings which is a typical auxinresponse. Root examination revealed thatthe mycelial inoculum of P. tinctorius washighly effective in forming ectomycorrhizaeon seedlings. However, the effectiveness ofthis inoculum on plant growth was not muchdifferent from that of the non-inoculatedstock seedlings unless L-TRP was added asa precursor of lAA (Fig. 1).. Considering the ability of soil indige-nous microbiota to produce lAA and its de-rivatives from L-TRP, the effects of L-TRPdirectly applied to soil on radish (Raphanussativus) (Frankenberger et al., 1990), wa-termelon (Citrullus lanatus) and muskmelon(Cucumis melo) (Frankenberger and Ar-shad, 1991 a) and pepper (Capsicum on-num] (Frankenberger and Arshad, 1991 b)were studied with optimum nutrition. Withthe radish experiments, selected auxins weretested along with L-TRP to assess theireffect on yield. A positive effect of L-TRPon growth parameters of radish was ob-served when low concentrations of L-TRP(3.0 mg kg-I) were applied at the seedlingstage, being comparable to that of selected

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Table 4. Response of bell pepper to L-TRP applied to soil (Source: Frankenberger &Arshad, 1991 b)

Gator bell pepper California Wonder pepperL-TRP ------------------------------------------ ---_.~-.--------------------~.- -----_ ....._-(mg kg-I soil) Number of Total fresh Number Total fresh

fruits plane I weight of fruit plant'! weight offruit plant'! fruit plant'!

(kg) (kg)

Control 5.31 203 6.88 37360 6.47 330* 8.11 515*6.0 7.23 326* 8.44 510·6.0 X 10-1 7.75 305* 8.18 4466.0 X 10-2 6.00 262 8.55 4706.0 X 10-3 7.80 250 8.29 4826.0 X 10...• 7.17 292 9.00 4756.0 X 10-5 9.60* 267 10.05* 483

*Means significantly different from control at P<0.05.

auxins like indole-3-acetamide and indolelactic-3-acid (Table 2). Higher concentra-tions of L-TRP had negative effects, aresponse usually observed in the presence ofhigh auxin levels. The response was verypronounced under high fertility status, whichexcluded any nutritional effect of L-TRP.Moreover, foliar application of L-TRP didnot have any significant effect on the growthparameters of radish indicating that the siteof entry of L-TRP and/or its microbialtransformed metabolites appeared to bethrough the rootzone (Frankenberger et 01.,1990). Similarly, a positive effect of L-TRPon melon yields (number and weight offruit) was apparent at low concentrations(ng to #Jogamounts) of L-TRP added at theseedling stage (Frankenberger and Arshad,1991 a). In testing the response of peppers,the cultivars reacted differently in terms ofnumber and weight of fruits upon exoge-nously applied L-TRP amendments

(Frankenberger and Arshad, 1991 b). Hus-sain et 01. (1989 b) reported a significantpositive effect of (l;lute concentrations of L-TRP applied to soil on the growth, nodula-tion and chemical composition of Albizialebbeck.

The possible mechanisms of action ofexogenously applied L-TRP to soil on plantgrowth include:1. substrate-dependent auxin production

in soil by the indigenous microflora;2. uptake directly by plant roots followed

by metabolism within their tissues;and/or

3. a change in the balance of rhizospheremicrobiota affecting plant growth(Frankenberger and Arshad, 1991 a, b;Frankenberger et 01., 1990).

Cytokinins: Several studies have reportedthe biosynthesis of cytokinins and cytokinin-like substances by soil microorganisms par-ticularly by microsymbionts forming

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Table 5. Influence of adenine (ADE) and isopentyl alcohol (IA) as cytokinin precursorson plant growth (From: Nieto & Frankenberger, 1990, 1991)

Axenic/inoculatedexperiment

Glasshouse-soil experiment

Fieldexperiment

Radish glass-house fieldexperiment

TreatmentShoot

dry massRoot

dry massShoot

dry massRoot

dry massShoot

dry massRoot

dry massShoot

dry massRoot

dry mas.s

................................................................................plant·I .

1.07aOo 0.63 a 0.45 a 0.43 a 0.46 a 0.36 a 1.54 ab 1.39 a1.16 ab 0.64 ab 0.61 b 0.44 a 0.36 a 0.36 a 1.39 a 1.40 a1.74 c 0.71 bc 0.71 c 0.49 a 0.46 a 0.34 a 1.60b 1.41 a

ControlIAIA + AzolObacterchroococcum (A.c)

ADE + IAADE + IA + A.c.

1.90 cd2.37e

0.78 c

0.97dO.86d1.13 e

O.70b

O.79c

0.52 b0.93c

0.52 b0.56 b

1.82 c1.98d

1.56 b1.62b

Corn

Root Shoot Shoot Internodal Internodal Stem leafdry mass dry mass mass distance width width

(g) (g) (cm) (cm) (mm) (mm)

Control 1.82aU 3.8 a 61.3 a 5.3 a 17.1 a 3.08 aA.c. un« 6.5 b 77.5b 7.2 b 16.7 a 3.55 bIA + A.c. 1.91ab 8.8c 85.1 b 8.3 c 17.4 a 8.83bADE 6.02d 10.3cd 83.4b 9.8 d 16.8 a 6.10 cdADE + A.c. 6.99d 13.3d 97.5 c 13.6 e 20.0b 5.73 cADE + IA 4.39 d 11.4cd 100.9c 12.9 e zf.2b 5.80 cADE + IA + A.c. 10.10e 18.8e 127.1d 14.9 f 25.0 c 6.50 d

• In the radish experiment, ADE and IA were applied at 0.2 mg kg-1 soil and 13 mg kg-Isoil, respectively. In the corn experiment, ADE and IA were added at 2.0 mg kg-I and 13mg kg-I soil, respectively.

•• Values sharing same letter(s) do not differ significant at P<0.05 .

symbiosis with plants (legumes and non-legume nodulation and mycorrhiza), phy-topathogenic parasites (pathogens) and freeliving diazotrophs (Arshad and Franken-berger, 1992 a; Greene, 1980; Mahadevan,1984; Nieto and Frankenberger, 1990 a).

However, precursors that might stimulatemicrobial biosynthesis of cytokinins has notbeen thoroughly investigated until recentlyby Nieto and Frankenberger 1989 a, b; 1990b; 1991). They screened a number of com-pounds including adenine, adenosine-5-

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Table 6. Influence of L-methionine-derived ezM. produced by indigenous soil microf1oraon etiolated pea seedlings (Source: Arshad and Frakenberger, 1988)

Treatment Seedlingdiameter (mm)

ControlAgNOJ (240 mg 1-1)L-methionine (5 mM)L-methionine (5 mM) + AgNOJ (240 mg 1-1)L-methionine (10 mM)L-methionine (10 mM) + AgNOJ (240 mg 1-1)

Seedlinglength (cm)

6.56 b13.50 d5.14 ab

11.10 c3.90 a

10.10 c

1.87 a1.93 ab2.49 c2.06 ab2.75 d2.11 b

Values followed by the same letters are not significantly different at P <0.05.

monophosphate, hypoxanthine, inosine, ino-sine-5-monophosphate, isopentyl alcohol,isoprene, 3-methyl-butane-l-ol, and 3-methyl-I-2-butane-l-ol, as possible sub-strates for biosynthesis of cytokinins by threespecies of Azotobacter (A. chroococcum, A.vineiandii, A. beijerickii) and two Pseu-domonas spp. (P. fluorescens, P. putida)(Nieto and Frankenberger, 1989 a). The ad-dition of adenine (ADE) plus isopentyl al-cohol (IA) enhanced cytokinin bioactivity tothe greatest degree in the culture filtrates ofthe bacteria tested. Among the biosyntheticpathways which have been proposed, ADEis considered the common precursor for cy-tokinin production in both plant tissues andmicroorganisms. Azotobacter chrOOCOCCll11listhe most profilic producer of cytokininsamong soil bacteria as of this date. Nietoand Frankenberger (1989 a) utilized HLPCand UV spectrometry for identification andquantification of the cytokinins produced byA. chroococcum and found that zeatin ribo-side and t-zeatin were the major metabo-lites. In another study, Nieto and Franken-berger (1989 b) investigated the effect ofprecursors (adenine and isopentyl alcohol)with the inoculation of A. chroococcum on

cytokinin biosynthesis in soil. It was reportedthat an application of ADE plus IA and A.chroococcum enhanced production of zeatinriboside and t-zeatin in soil up to 1.5 and1.39-fold, respectively in comparison withcontrols without the added precursors. Theproduction of zeatin riboside and t-zeatin inthe presence of ADE and fA was 1.35- and2.44-fold greater with inoculation by A.chroococcum, when compared with the in-digenous micriobiota alone. This study is thefirst to reveal that microbial biosynthesis ofcytokinins can be enhanced by applyingphysiological precursors to soil. Later, theeffects of ADE and fA on the growth ofradish [Raphanus sativus) and corn (Zeamays) were evaluated (Nieto and Franken-berger, 1991), both in the presence and abosence of the inoculum, A. chrOOCOCCIlI1l.Thegreatest enhancement in plant growth wasobserved from the combined application ofADE, IA and A. chroococcum (Table 5)which may be attributed primarily to an in-crease in microbial production of cytokininsin the rhizosphere. However, under axenicconditions, plant growth was slightly in-creased when the precursors (ADE and IA)were added alone, compared with the

324

.-I-f.I~I

II!i=!II11

=:;~!j].'0

I'i-

Pak. J. Agri. sa; Vol. 30, No.3, /993

control (without precursors and inoculation)indicating the plant's ability to assimilateand utilize ADE and IA for growth andmetabolism. The precursor-inoculum in-teraction caused an increase in dry weight ofroots and shoot tissues of corn up to 5.57-and 5.00-fold, respectively over the control.These two studies demonstrate the possibil-ity of improving plant growth and develop-ment through increased microbial produc-tion of cytokinins in response to an exoge-nous application of cytokinin precursors.Ethylene: Among the phytohormones, mi-crobial production of ethylene (C2H4) hasbeen investigated more than any other classof hormones. A diverse group of microbiotaincluding both pathogens and non-pathogensare active in producing C2~ (Arshad andFrankenberger, 1992 a, b). A wide range ofdifficult compounds have been reported aspossible precursors for C2~ generation bydifferent microbial isolates. However, L-methionine (L-MET), the sole precursor forC2~ biosynthesis in higher plants (Adamsand Yang, 1979), has been thoroughly evalu-ated as an C2~ precursor in pure cultureand soil studies. In many cases, bacteria andfungi only produce C2~ in the presence ofL-MET (Arshad and Frankenberger, 1989;Billington et al., 1979; Fukuda et al., 1989;Inee and Knowles, 1985, 1986; Lynch, 1972;Primrose, 1976). Arshad and Frankenberger(1989) observed that corn rhizosphere con-tained an appreciable number of microfloracapable of producing C2~ from L-MET,most likely via pathway different from thatof higher plants. After screening 63 com-pounds, Arshad and Frankenberger (1990 b)reported that many amino acids (includingL-MET), organic acids and carbohydrates,typically found in root exudates, stimulatedC2~ biosynthesis in soil under field capac-ity. This indicates that an excellent site formicrobial biosynthesis of C2~ is the rhizo-sphere where substrate levels and

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Table 8. Inflnence of L-ethionine (L-ETH) as an ethylene precursor 1111 tomato growth(Source: Arshad and Frankenberger, 1990 a)

L-ETH Fresh fruit Average Total Epinastic(mg kg-I soil) yield weight of number of movement (0)

fresh fruit ripe fruit 72 h after(g) (g) treatment

Control 261 a 37.3 a 11a 4.8 a0.2 477 b 55.0 ab 15 ab 9.0 b2.0 445b 62.1 b 16 ab 9.8 b

20.0 351 ab 50.1 ab 19 b 12.3 c

microbiota populations are comparativelyvery high. In another study, the productionand stability of C2~ in amended (L-METand D-glucose) and non-amended soilsunder waterlogged and unsaturatedconditions (field capacity) was investigatedby Arshad and Frankenberger (1990 c). Itwas concluded that the effectiveness of theamendments to stimulate C2~ generationvaried under these conditions and wascontrolled by the nature of amendments aswell as environmental factors and soilconditions. The stability of C2H4 underwaterlogged conditions was higher thanunder aerobic conditions. This implies thatunder waterlogged conditions, C2t4 persistslonger and because of its high solubility inwater, C2~ is trapped within thesurrounding roots. It is most likely that thephysiological action of C2~ is metabolizedat a faster rate. Arshad and Frankenberger(1991 b) also reported that the presence oftrace elements [Ag (I), Cu (II), Fe (II, III),Mn (II), Ni (II), Zn (II), Co (II), Hg (II), Al(Ill), Mo (VI)] had a concentration-dependent effect on C2~ production in L-MET -amended soil. Each of these studiesindicates that precursor-dependent C2~accumulation in soil may be controlled byvarying environmental conditions.

Values sharing same letter(s) do not differ significantly at P<0.05.Accumulation of C2~ in soils is of

great ecological significance because veryminute amounts (ppb range) of C2H4 in thevicinity of plant roots can have tremendouseffects on plant growth and development(Arshad and Frankenberger, 1991 a; 1992 a,b; Primrose, 1979; Smith, 1976; Smith andRussell, 1969). However, the ecological sig-nificance of microbially produced C2H4 un-der non-waterlogged conditions has notbeen evaluated until recently, By using anC2t4 bioassay, the classical "triple" responsein etiolated pea seedlings, Arshad andFrankenberger (1988) demonstrated thatC2t4 of microbial origin can effect plantgrowth. They showed that L-MET-depen-dent C2t4 produced by an inoculum, Acre-11lo11iu11l [alciforme or by soil indigenous mi-croflora (Table 6) caused elongation,swelling of the hypocotyl and change in thedirection of growth (horizontal). A similarresponse was observed by direct applicationof the C2t4 gas. The specific anti-Cyl-l,properties of Ag (I) confirmed that C2~was the fungal metabolite responsible forthe observed effects. In another study, Ar-shad and Frankenberger (1990 a) found thatsoil application of L-MET affected the veg-etative growth and resistance to stembreaking (lodging) of two corn cultivars(Table 7). Similarly, soil treatment with

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ethionine (as a precursor of C2H4) resultedin a significant epinastric response of tomatoplants (a typical response of C2H4) (Arshadand Frankenberger, 1990 a). At varying con-centrations of the soil amendment (L-ETH),we noted enhanced fruit yield with earlyfruit formation and ripening of tomatoes(Table 8). Hussain et 01. (1989 a) in-vestigated the effect of soil applied L-METon the growth, nodulation and chemicalcomposition of Albizia lebbeck (Black ciris),They found that plant height, girth, dryweight of shoots and roots, number andweight of nodules and N, P, K, Ca and Mgconcentrations and their uptake were signifi-cantly increased in response to soil appliedL-MET at low concentrations (J..Lgto mg kg-1 range). One proposed mechanism of ac-tion in this study was substrate-dependentC2~ production by the indigenous soil mi-croflora.

DISCUSSION

There is now ample evidence that soilmicrobiota are active in producing phyto-hormones (Arshad and Frankenberger, 1992a, b). Numerous studies have shown an im-provement in plant growth and developmentin response to seed or root inoculation withvarious microbial inoculants and it has beenproposed that phytohormones released assecondary metabolites may contribute tothese growth-altering effects. Strong ev-idence to support this hypothesis has beenprovided by many other scientists (Azcon etal., 1978; Barbieri et 01., 1986, 1988; Bareaand Brown, 1974; Brown et 01., 1%8;Grapelli and Rossi, 1981; Grayston et 01.,1991; Holl et al; 1988; Hussain et 01., 1987;Inbal and Feldman, 1982; Kucey et 01., 1988;Loper and Schorth, 1986; Mahmoud et 01.,1984; Muller et al., 1989; Tien et al., 1979;Triplett et 01., 1981; William and deMal-lorca, 1982; Zimmer et 01., 1988). Despite

Pak. J. Agri. sa, Vol. 30, No.3, 1993

the fact that plants are capable of synthe-sizing phytohormones, they may not havethe capacity to synthesize sufficient amountsfor optimal growth and development undersuboptimal climatic and environmental con-ditions. Hence, plants may respond to exo-genously supplied phytohormones duringcertain growth phases and under certaincultivation conditions. This view is supportedby the plants responses to exogenoussources of phytohormones applied to theroots. The exogenous application of phyto-hormones may affect the endogenous hor-monal pattern of plants, either by supple-mentation of suboptimal levels or interac-tion with the synthesis, translocation and/orinactivation of the plant hormones. Theplants responses to phytohormones withinthe rhizosphere may be governed by the rateof hormone uptake, the active concentrationin the rhizosphere and modification of theplant's own pool of hormones as a result ofthe exogenous supply (Frankenberger et 01.,1984).

The rhizosphere is an ideal site for mi-crobial biosynthesis of phytohormones be-cause of the dense microbial population. Be-cause of the intimate contact between plantroots and the microbiota in the rhizosphere,the phytohormones released as secondarymicrobial metabolites are more likely to besubjected to direct assimilation by plantroots. Production of phytohormones as mi-crobial metabolites is often directly relatedto substrate(s) availability. The studies con-ducted by our group have provided unequiv-ocal proof that microbial production ofphytohormones could be increased several-fold by providing suitable precursor(s). Wehave also demonstrated that by addingprecursor(s) of specific hormones, plantgrowth and development can be modifiedmost likely by the hormones released by theinocula or by the soil indigenous microflora.

327

Although various phytohormone pre-parations are commercially available, theyare either expensive, unstable or requirelabour intensive practice to deliver thesepreparations to plants. The high cost of ap-plying naturally occurring phytohormones tosoil precludes their use in agriculture. Thestimulation in microbial biosynthesis ofphytohormone(s) within the rhizospherewith low cost precursor may be an effectiveapproach in modifying plant growth. A mi-crobial source of phytohormone(s) is notonly the most economical but also con-venient in a practical sense. Moreover, phy-tohormone(s) released as microbialmetabolites from the added substrate(s)provide a continuous release of active sub-stances for plant uptake compared to a one-time application of synthetic compounds.Since precursor-inoculum interactions haveoften been found to be much better in cre-ating favourable plant responses than theapplication of precursor alone, treatmentwith both precursors and a particular in-oculum may be the most effective approachto increase yield. This work has provided anew innovative approach in soil science forimproving crop yields. However, moreresearch is needed to determine the practi-cal significance of this approach in today'sagriculture management systems.Conclusions: Many soil microorganismswhich significantly affect plant growth arealso capable of producing phytohormone(s).Moreover, the biosynthesis of phytohor-mones in soils have been unequivocally de-monstrated. These studies reveal that phyto-hormones of microbial origin could be ofgreat ecological significance in the agricul-ture industry. To use - microbially-derivedphytohormones to improve crop yields, fu-ture research should be focused on the fol-lowing projects:

Pak. J Agri. sa, Vol. 30, No.3, 1993

I. isolation of prolific microbial pro-ducers of phytohormone(s) withint he rhizosphere,

ii. determination of substrate-depen-dent biosynthesis of phytohormonesby specific inocula and the soil in-digenous microflora,

III. evaluation of environmental factorsaffecting substrate-dcpcndent phy-tohormone production by the rhizo-sphere microflora and inocula,

iv. monitoring the stability andbioavailability of various phytohor-mones in soils,

v. determination of the concentrationoptima of various phytohormonesto promote growth of differentcrops,

VI. studying the effects of phytohor-mones on nutritional aspects(availability and uptake of nutri-ents) of the plants,

vii. studying the effects of various agri-cultural practices (e.g. fertilizationand pesticides) on phytohormonesbiosynthesis in soil,

viii. evaluation of the effects of root ex-udates of various plants in providingsubstrates for microbially-derivedphytohormones, and

ix. investigating the role of phytohor-mones on plant-microbe associa-tions (nodulation and mycorrhizaldevelopment) beneficial for plantgrowth.

ACKNOWLEDGEMENTThe The author is thankful to Mr.

Muhammad Javed, Ph.D student in the De-partment of Soil Science, University of Agri-culture, Faisalabad, Pakistan for his per-sonal assistance in preparation of this arti-cle.

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