Triazoles as Plant Growth Regulators and Stress Protectants

Plant growth regulators are chemical substances which govern all the factors of development and growth within plants. The application of plant growth regulators to crops modifies hormonal balance and growth leading to increased yield, enhanced crop tolerance against abiotic stress and improved physiological trait of crops. Paclobutrazol (PBZ) [(2RS, 3RS)-1-(4-chlorophenyl)- 4, 4-dimethyl-2-(1H-1, 2, 4-trizol-1-yl)-pentan-3-ol], is one of the members of triazole family having growth regulating property. The growth regulating properties of PBZ are mediated by changes in the levels of important plant hormones including the gibberellins (GAs), abscisic acid (ABA) and cytokinins (CK). PBZ affects the isoprenoid pathway, and alters the levels of plant hormones by inhibiting gibberellin synthesis and increasing cytokinins level and consequent reduction in stem elongation. When gibberellins synthesis is inhibited, more precursors in the terpenoid pathway accumulate and that resulted in the production of abscisic acid. PBZ is more effective when applied to the growing media and application on the growing medium would give longer absorption time and more absorption of active ingredient than foliar spray. The application of PBZ to crops is important in reducing plant height to prevent lodging and in increasing number and weight of fruits per tree, in improving the fruit quality in terms of increases in carbohydrates, TSS, TSS/TA and decreases acidity. It further reduces evapo-transpiration and decreases plant moisture stress by enhancing the relative water content of leaf area and develops resistance in the plants against biotic and abiotic stresses. In addition, it acts as highly active systemic fungicide and used against several economically important fungal diseases. In this review, the current knowledge and possible applications of PBZ, which can be used to improve the growth, yield and quality of crops, have been reviewed and discussed. The role of PBZ to mitigate the harmful effects of environmental stresses in crops is also examined. Moreover, various biochemical and physiological processes leading to improved crop production under the effect of PBZ are discoursed in detail.

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Paclobutrazol as a plant growth regulator

Effect of exogenous GA3 and its inhibitor paclobutrazol on floral formation, endogenous hormones, and flowering-associated genes in 'Fuji' apple (Malus domestica Borkh.).

The role of gibberellins (GAs) in the regulation of shoot elongation is well established but the phytohormonal control of dry-matter production is poorly understood. In the present study, shoot elongation and dry-matter production were resolved by growing Brassica napus L. seedlings under five light intensities (photon flux densities) ranging from 25 to 500 μmol m−2 s−1. Under low light, plants were tall but produced little dry weight; as light intensity was increased, plants were progressively shorter but had increasing dry weights. Endogenous GAs in stems of 16- and 17-d-old plants were analyzed by gas chromatography-selected ion monitoring with [2H2] internal standards. The contents of GAs increased dramatically with decreasing light intensity: GA1, GA3, GA8 and GA20 were 62, 15, 16 and 32 times higher, respectively, under the lowest versus highest light intensities. Gibberellin A19 was not measured at 25 μmol m−2 s−1 but was 9␣times greater in the 75 compared to 500 μmol m−2 s−1 treatment. Shoot and hypocotyl lengths were closely positively correlated with (log) GA concentration (for example: r2 = 0.93 for GA1 and hypocotyl length) but shoot dry matter was negatively correlated with GA concentration. The application of gibberellic acid (GA3) produced elongation of plants grown under high light, indication that their low level of endogenous GA was limiting shoot elongation. Although endogenous GA20 showed the greatest influence of light treatment, metabolism of [3H]GA20 and of [3H]GA1 was only slightly influenced by light intensity, suggesting that neither 2β- nor 3β-hydroxylation were points of metabolic regulation. The results of this study indicate that GAs control shoot elongation but are not directly involved in the regulation of shoot dry weight in Brassica. The study also suggests a role of GAs in photomorphogenesis, serving as an intermediate between light condition and shoot elongation response.

Light intensity, gibberellin content and the resolution of shoot growth in Brassica

The role of gibberellins (GAs) during early embryodevelopment was examined using microspore-derived embryos (MDEs) of Brassica napus. At theglobular stage of development, 10 d after initialculture (DAC) when endogenous GA 1 levels areincreasing rapidly, a triazole, uniconazole, was usedat 1, 33 and 100 mM to inhibit GA biosynthesis.Within this dose range there was no apparent effectof the inhibitor on embryo growth through to theearly torpedo stage. However, by 25 DAC unicona-zole-treated MDEs showed signiﬁcantly reduced(50%) axis elongation. Addition of GA 1 at 33 mMon14 DAC to embryos pretreated with 1 mM uniconazoleon 10 DAC prevented this reduction in axis length,giving axis elongation equivalent to untreated MDEs.Application of GA 1 alone, however, did not signiﬁ-cantly increase axis elongation. The reduced axisgrowth seen with uniconazole treatment was due toreduced cell elongation, but not cell number, and theco-applied GA 1 thus prevented the uniconazole-induced reduction in cell length. The elongating axisof MDEs may thus be a useful tool for examining therole of GAs in cell elongation.Key words: Brassica napus, embryo development,gibberellins, microspore-derived embryos (MDEs),uniconazole.

The role of gibberellins in embryo axis development

Endogenous gibberellins (GAs) were extracted from safflower (Carthamus tinctorius L.) stems and detected by capillary gas chromatography-mass spectrometry from which GA1, GA3, GA19,, GA20, GA29, and probably, GA44 were detected. The detection of these GAs suggests that the early 13-OH biosynthetic pathway is prevalent in safflower shoots. Deuterated GAs were used as internal standards and GA concentrations were determined in stems harvested at weekly intervals. GA1 and GA19 levels per stem increased but concentrations per gram dry weight decreased over time. GA20 was only detected in young stem tissue. Gibberellic acid (GA3) was also applied in field trials and both GA3 and the GA biosynthetic inhibitor, paclobutrazol, were applied in growth chamber tests. GA3 increased epidermal cell size, internode length, and increased internode cell number causing stem elongation. Conversely, paclobutrazol reduced stem height, internode and cell size, cell number and overall shoot weight. In field tests, GA3 increased total stem weight, but decreased leaf weight, flower bud number and seed yield. Thus, GA3 promoted vegetative growth at the expense of reproductive commitment. These studies collectively indicate a promotory role of GAs in the control of shoot growth in safflower, and are generally consistent with gibberellin studies of related crop plants.

T. I. Potter

Papers

Gibberellin physiology of safflower: endogenous gibberellins and response to gibberellic acid