Nonpathogenic rhizobacteria can induce a systemic resistance in plants that is phenotypically similar to pathogen induced systemic acquired resistance Rhizobacteria mediated induced systemic resistance has been demonstrated against fungi,bacteria,and viruses in Arabidopis,bean,camation,cucumber,radish,tobacco and tomato under conditions in which the inducing bacteria and the challenging pathogen remained spatially separated Bacteria induce plants to produce the JA and ethylene to develop the ISR And ISR is effective under field conditions and offers a natural mechanism for biological control of plant disease

Systemic resistance induced by rhizosphere bacteria

The plant hormone auxin, which is predominantly represented by indole-3-acetic acid (IAA), is involved in the regulation of plant growth and development. Although IAA was the first plant hormone identified, the biosynthetic pathway at the genetic level has remained unclear. Two major pathways for IAA biosynthesis have been proposed: the tryptophan (Trp)-independent and Trp-dependent pathways. In Trp-dependent IAA biosynthesis, four pathways have been postulated in plants: (i) the indole-3-acetamide (IAM) pathway; (ii) the indole-3-pyruvic acid (IPA) pathway; (iii) the tryptamine (TAM) pathway; and (iv) the indole-3-acetaldoxime (IAOX) pathway. Although different plant species may have unique strategies and modifications to optimize their metabolic pathways, plants would be expected to share evolutionarily conserved core mechanisms for auxin biosynthesis because IAA is a fundamental substance in the plant life cycle. In this review, the genes now known to be involved in auxin biosynthesis are summarized and the major IAA biosynthetic pathway distributed widely in the plant kingdom is discussed on the basis of biochemical and molecular biological findings and bioinformatics studies. Based on evolutionarily conserved core mechanisms, it is thought that the pathway via IAM or IPA is the major route(s) to IAA in plants.

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The pathway of auxin biosynthesis in plants

The special conditions during in vitro culture result in the formation of plantlets of abnormal morphology, anatomy and physiology. After ex vitro transfer, these plantlets might easily be impaired by sudden changes in environmental conditions, and so need a period of acclimatization to correct the abnormalities. This review is focused upon contemporary information on the changes in leaf structure, water relations and photosynthesis during acclimatization of plantlets to ex vitro conditions. It also describes some ways of improving plant survival and for the speeding up of acclimatization.

Acclimatization of Micropropagated Plants to Ex Vitro Conditions

A substantial number of micropropagated plants do not survive transfer from in vitro conditions to greenhouse or field environment. The greenhouse and field have substantially lower relative humidity, higher light level and septic environment that are stressful to micropropagated plants compared to in vitro conditions. The benefit of any micropropagation system can, however, only be fully realized by the successful transfer of plantlets from tissue-culture vessels to the ambient conditions found ex vitro. Most species grown in vitro require an acclimatization process in order to ensure that sufficient number of plants survive and grow vigorously when transferred to soil. This article reviews current and developing methods for the acclimatization of micropropagated plantlets.

Acclimatization of tissue cultured plants

Functions of rol genes in plant secondary metabolism

Influence of rol genes in floriculture

Carnation plantlets (Dianthus caryophyllus L.) cultured in vitro often develop morphological and physiological anomalies, a phenomenon called hyperhydricity, which impairs their survival ex vitro. When the agar concentration of the growth medium was increased (from 0 to 12 g dm−3), thereby reducing water availability, the hyperhydricity of those adventitious shoots regenerated from carnation petals decreased. This was accompanied by a progressive fall in the water content of shoots (94.9 to 91.4 %), fresh mass (from 57.2 to 1.8 mg), number of leaf parenchyma cell layers (from 9.3 to 7.7), and the size of these cells (from 968 to 254 µm2). However, the number of regenerated shoots also decreased (17.7 in 2 g dm−3 agar to 4.3 in 12 g dm−3). Similarly, in ventilated tubes, which exhibit a lower relative humidity than tightly closed tubes, shoot organogenesis diminished up to 28 %, in tandem with shoot water content. Thus, relative humidity and water availability in culture vessels do not only influence shoot hyperhydricity in carnations, but also greatly affect adventitious shoot organogenesis.

Effects of agar concentration and vessel closure on the organogenesis and hyperhydricity of adventitious carnation shoots

The soil/substrate that supports plant roots and provides water and nutrients to plants is often considered a hostile environment that harbours plant pathogens. Moreover, the most common strategy used to control risk of disease from the soil is the eradication or minimization of soil pathogens regardless of the presence of other organisms. Consequently, the extensive use of physical/chemical biocides generates a soil/substrate microbiological vacuum which makes it more susceptible to reinfestation by pathogens, increases disease incidence and in some cases enhances fungal resistance. In Europe, legislation on plant protection products has been re-evaluated since 1991 (Directives 1991/414 and 2009/128), leading to a drastic reduction in the use of chemical compounds. A reduction in fungicides that effectively combat disease is achieved with the use of suppressive composts in pot plants. Suppressive composts similar to suppressive soils are examples of natural biological control of disease as the result or a more or less complex three-way interaction between microorganisms in the composts, plant pathogens, and plants. The roles of the physical properties and chemical composition of composts are also important in the suppressive effect, not only because they are responsible for the type and quantity of microorganisms established, but also because of their effects on pathogens, plant root health and leaf nutrient status. The two classic modes of action of microorganisms involved in the suppression of soil-borne plant pathogens (general and specific suppression) can be defined in composts. In specific suppression, only one or a few species are involved; while in contrast, a great diversity of microorganisms working together as a nutrient sink contributes to general suppression. A third mode of action is also induced by microorganisms in composts: systemic resistance, similar to challenge inoculation with a pathogen or beneficial organism. _____________________________________________________________________________________________________________

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Review on compost as an inducer of disease suppression in plants grown in soilless culture.

Levels and immunolocalization of endogenous cytokinins in thidiazuron-induced shoot organogenesis in carnation.

This paper reports on the fast fluorescence responses of Gardenia jasminoides Ellis plantlets, at two successive stages (shoot multiplication and root induction) of culture in vitro. We test whether plantlets in vitro suffer photoinhibition during culture and whether the degree of photoautotrophy of these mixotrophic plantlets has any effect on the extent of photoinhibitory impairment. In this regard the effects of different sucrose levels in the medium and PPFD during growth on the development of photoautotrophy and the extent of photoinhibition were evaluated. Plantlets were grown under low, intermediate, and high (50, 100, and 300 μmol m-2 s-1) PPFD, and at 3 different sucrose concentrations (0.5, 1.5, and 3.0%, w/v) in the medium, during shoot multiplication. During root induction the same growth conditions were assayed except for the high PPFD. The development of photoautotrophy was assessed via the difference between the stable carbon isotope composition of sucrose used as heterotrophic carbon source and that of leaflets grown in vitro. Plantlets from root induction showed more developed photoautotrophy than those from shoot multiplication. For both stages the low-sucrose medium stimulated the photoautotrophy of plantlets in vitro. In addition, intermediate PPFD induced photoautotrophy during shoot multiplication. For plantlets of both culture stages at the lowest PPFD no photoinhibition occurred irrespective of the sucrose concentration in media. However, during the shoot multiplication stage chlorophyll fluorescence measurements showed a decrease in Fv/Fm and in t1/2 as growing PPFD increased, indicating photoinhibitory damage. The decline of Fv/Fm was caused mostly by an increase in Fo, indicating the inactivation of PSII reaction centers. However plantlets growing under low sucrose showed reduced susceptibility to photoinhibition. During root induction, only plantlets cultured with high sucrose showed a decrease in Fv/Fm as PPFD increased, although t1/2 remained unchanged. In this case, the decline of Fv/Fm was mostly due to a decrease in Fm, which indicates increased photoprotection rather than occurrence of photodamage. Therefore, growth in low-sucrose media had a protective effect on the resistance of PSII to light stress. In addition, plantlets were more resistant to photoinhibition during root induction than during shoot multiplication. Results suggest that increased photoautotrophy of plantlets reduces susceptibility to photoinhibition during gardenia culture in vitro.

M.I. Trillas

Papers

Influence of rol genes in floriculture

Effects of agar concentration and vessel closure on the organogenesis and hyperhydricity of adventitious carnation shoots

Review on compost as an inducer of disease suppression in plants grown in soilless culture.

Levels and immunolocalization of endogenous cytokinins in thidiazuron-induced shoot organogenesis in carnation.

Development of photoautotrophy and photoinhibition of Gardenia jasminoides plantlets during micropropagation