The represented studies concern the development of the technology for turnip growth in soil-free systems with the purpose to recover glucosinolates from plants and their exudates. The main approaches consist in the enhancement of glucosinolate content in plant tissues as well as intensification of glucosinolate exudation from roots.
The key conclusions, received in the course of the investigations can be summarised as following.
1. Turnip can be cultivated as soil-free culture for the production of glucosinolates from plants and their exudates. The type of growing system influences on glucosinolate synthesis, plant growth, particularly that of the secondary (fine) roots, and intensity of exudation. The optimal system for turnip growth under the Hoagland solution is AD and to the lesser extends H and AS.
However, after the modification of nutrient supply H shows to be more optimal according to the aims of the researches.
2. Nutrient supply and type of system influence on individual and consequently total glucosinolate content in plants as well as exudates of turnip. Moreover, it differs between parts of plants and changes during plant growth. Secondary roots have higher concentration of glucosinolates as other plant parts, which means, they could be used as a rich source of glucosinolates.
a) Increase of Hoagland solution concentration as well as sulfur supply leads to enhance of total glucosinolate content, particularly aliphatic glucosinolates in plants, mostly by costs of secondary roots.
b) Sulfur application leads to disappearing of aromatic glucosinolate gluconasturtiin, while for H2H its content as well as that of indole glucosinolates increases.
c) H2H intensifies the exudation: the amount of exuded during 30 days glucosinolates reaches for this system 48% of that in secondary roots, while for H1H it reaches 18% and for H2H2S 39%. This can be explained by 2H stimulation of root growth and correspondingly increase of secondary root growth.
d) Rhizosecretion of glucosinolates is selective process and not leaching, which could be explained by ecological role of exuded glucosinolates. Plants exude preferably aliphatic glucosinolates on the beginning of their growth, while later morstly indole glucosinolates. In addition, plants excrete more intensively butenyl as pentenyl glucosinolates and hydroxyalkenyl as alkenyl.
e) Increased sulfur application leads to enhance of aliphatic glucosinolates in exudates, while under the influence 2H the excretion of indole glucosinolates is more intensive.
f) Slowing down the intensity of glucosinolate exudation for plants after 30days of growth in systems could be explained by altering of roots, and expediently by decrease of their metabolic activity.
Conclusions
3. Plant treatment with elicitors, particularly SA and MJ, leads to increase of glucosinolates in plants and exudates.
a) The response of plants on root treatment with elicitors is inducible. The root induction by SA and MJ has an impact on glucosinolate content also in leaves and primary roots of turnip as well as in exudates. However, the kinetics of induced accumulation of glucosinolates differs between plant parts, the effect of elicitor application is greater at the site of treatment, namely in secondary roots.
b) Not all classes of glucosinolates respond equally to elicitor treatment. This could be explained by the independent regulation of glucosinolate syntheses in different plant organs.
Application of MJ decreases aliphatic, while increases indole glucosinolates in all plant parts and prevents the disappearance of aromatic gluconasturtiin in leaves, which can be explained by synthesis intensification or by slowing down the intensity of glucosinolate transport to the roots.
Application of SA increases aliphatic glucosinolate content in secondary roots, and indole glucosinolates in all plant parts. Different effects of elicitors on the individual glucosinolates could be exounded by the defense mechanisms, they are involved in.
c) SA induces secondary root growth, which could be explained by elicitor influence auxin status of plants. However, the concentration of glucosinolates for SA treated plants is nearly the same as for the plants without treatment.
d) Application of MJ results in suppressing of plant growth. Right after the treatment the fresh weight of HMJ0 leaves and secondary roots is 2 and 3.2-fold lower as for H, however, the glucosinolate concentration for MJ treated leaves and secondary roots is 10 and 3-folds higher as that for H. This could be explained by stimulation of glucosinolate synthesis by MJ, but on the same time by suppressing of plant and especially secondary root growth, as it was explained by Hansen and Halkier (2005) by the influencing on the expression of gene entconding γ-glutamatcysteinsynthase, responsible for cell division in roots meristem. It is possible to assume that if MJ causes the same increase in production of defense compound, it become not economical for plants to grow intensively, because it deprives too many resources, and this could be the reason of decrease of plant growth.
e) Chemical diversity and content of exudates could be dramatically enhanced by elicitation process. Both of elicitors stimulate rhizosecretion of glucosinolates, especially during the first days after treatment, and then the effect declines gradually. In the course of post-treatment period, the intensity of glucosinolate exudation decreases, but their content increases in plant tissues. According to Hugot et al. (2004) the balance between the synthesis and secretion of defense-related substances is a critical point of the establishment of plant resistance. For turnip plants it takes 10-20 days, because from this time the glucosinolate content in plant tissues starts to increase strongly, while the intensity of exudation decreases.
f) Elicitor-induced rhizosecretion is based on de novo synthesis of secondary metabolites and not on leakage from root tissues. This is substituted by the fact that the profile of glucosinolates in exudates differs of those in plants. Elicitors decrease the content of aliphatic glucosinolates in exudates for all systems, which in case with MJ has stronger effect as with SA. After the application of both of elicitors, the content of indole glucosinolates increases essentially right after the treatment: on 10th day for HSA0 and HMJ0 it exceeds 6 and 6.5-folds of that for H. In general, during 30 days H plants exude 1.6 mg plant-1 of indole glucosinolates, HSA0 5.0 mg plant-1 (2.6-folds more), and HMJ0 5.9 mg plant-1 (3.1-folds more).
4. Plant response on SA treatment changes during the growth period. Inductivity of glucosinolate accumulation depends on stage of plant development.
a) The highest total glucosinolate accumulation occurs in plants and exudates of turnip when SA is applied on 15th day, in the period of the most intensive plant growth. During 30 days H plants exude 4.8 mg plant-1 of glucosinolates, HSA25 6.4 mg plant-1, while HSA15 8.6 mg plant-1.
b) The induction of glucosinolate synthesis in plants after SA application is maximally expressed 10-20 days after the treatment. However, glucosinolate content in exudates increases during the first 5-10 days after the application and afterwards decreases. Therefore, if the task is to receive the glucosinolates from exudates, it should be done until the 20th day after the treatment with elicitor, but if the source of glucosinolates is plant tissues, it is more reasonable to collect the phytochemicals after the 20th day after the treatment.
c) The highest increase of glucosinolates in exudates right after the treatment is measured for HSA25 plants, but the total yield of glucosinolates is lower that from earlier treated plants, which is connected with short post-treatment period of plant growth in system. Probably, it could be possible to slow down the plant growth intensity and hence prolong the period of their growing in systems, which can prolong the time for exudates collecting.
This study could contribute to the understanding of the biology of the root exudation process and to the development of approaches for designing of novel strategies for the production and isolation of glucosinolates. In addition, the knowledge of the regulation of glucosinolate synthesis in plants as well as their rhizosecretion would be a major advance in the production of low-cost nutraceuticals.
References