Silicon triggers sorghum root enzyme activities and inhibits the root cell colonization by Alternaria alternata.

MAIN CONCLUSION: Silicon inhibits the growth of Alternaria alternata into sorghum root cells by maintaining their integrity through stimulating biochemical defense reactions rather than by silica-based physical barrier creation. Although the ameliorating effect of silicon (Si) on plant resistance against fungal pathogens has been proven, the mechanism of its action needs to be better understood on a cellular level. The present study explores the effect of Si application in sorghum roots infected with fungus Alternaria alternata under controlled in vitro conditions. Detailed anatomical and cytological observations by both fluorescent and electron microscopy revealed that Si supplementation results in the inhibition of fungal hyphae growth into the protoplast of root cells. An approach of environmental scanning electron microscopy equipped with energy-dispersive X-ray spectroscopy enabling spatial detection of Si even at low concentrations showed that there is no continual solid layer of silica in the root cell walls of the rhizodermis, mesodermis and exodermis physically blocking the fungal growth into the protoplasts. Additionally, biochemical evidence suggests that Si speeds up the onset of activities of phenylpropanoid pathway enzymes phenylalanine ammonia lyase, peroxidases and polyphenol oxidases involved in phenolic compounds production and deposition to plant cell walls. In conclusion, Si alleviates the negative impact of A. alternata infection by limiting hyphae penetration through sorghum root cell walls into protoplasts, thus maintaining their structural and functional integrity. This might occur by triggering plant biochemical defense responses rather than by creating compact Si layer deposits.

Tobacco plants (Nicotiana benthamiana) were influenced by silicon and were not infected by dodder (Cuscuta europaea).

The effect of silicon (Si) on tobacco (Nicotiana benthamiana) development and dodder (Cuscuta europaea) - tobacco interaction were studied. Three Si application approaches were tested: tobacco seed priming (2.5 mM Si and 5 mM Si; 2.5S, 5S), watering tobacco plants with Si solution (2.5 mM Si and 5 mM Si; 2.5W, 5W) and foliar application (1 mM Si and 2.5 mM Si; 1F, 2.5F). Dodder was not able to infect the host plant in almost all Si treatments. Only in the control and 2.5W treatments was dodder able to infect its host. A significant increase in all observed antioxidant enzymes activities (POX, CAT and SOD) occurred in the plants of 2.5W treatment after infection in comparison with the uninfected 2.5W treatment and control plants, which indicated the importance of antioxidant enzymes activities in the plant parasite - host interaction. Resistance of Si treated plants to dodder could have been due to the changes in the cell wall properties of the epidermis and cortex where activity of POX was confirmed histochemically. The growth and development of tobacco shoots were evaluated after four and eight weeks of cultivation in the individual Si treatments. The development of shoots was enhanced after eight weeks of cultivation in the 2.5S, 5S, 2.5W and 5W treatments in comparison with the control treatment. However, a negative effect of Si was observed in 1F and 2.5F treatments. In the majority of cases, the plants treated with Si had decreased chlorophyll content when compared to control, except for chl a in 5W plants after 8 weeks of cultivation. Contrary to this, carotenoids increased in all Si treated plants after eight weeks cultivation in comparison with the control. The secondary xylem formation in tobacco was enhanced after 4 and 8 weeks cultivation in shoots of plants receiving the 2.5S, 5S, 2.5W and 5W treartments. The cambium was the most active in producing secondary xylem in the 2.5S treatment. Protein profile and antioxidant enzymes activities (POX, CAT and SOD) were altered by Si treatment. After 8 weeks of cultivation, activities of POX were significantly decreased in 2.5S, 5S, 2.5W and 5W in comparison with control. Catalase was decreased in 2.5S, 5S and 5W in comparison with the control, however, 1F and 2.5F treatments had significantly increased CAT and SOD activities. The specific activity of POX was confirmed histochemically in Si treated plants in the cell walls of several stem tissues like the epidermis, cortex and pith. A small amount of H2O2 was detected in leaves in the control and Si treated plants. The amount of O2- decreased in all treatments with time. The highest Si concentration in the plants (almost 800 mg . kg-1 d. w.) was detected in the 2.5W, 5W treatments.

Cuscuta europaea plastid apparatus in various developmental stages: localization of THF1 protein.

It was generally accepted that Cuscuta europaea is mostly adapted to a parasitic lifestyle with no detectable levels of chlorophylls. We found out relatively high level of chlorophylls (Chls a+b) in young developmental stages of dodder. Significant lowering of Chls (a+b) content and increase of carotenoid concentration was typical only for ontogenetically more developed stages. Lower content of photosynthesis-related proteins involved in Chls biosynthesis and in photosystem formation as well as low photochemical activity of PSII indicate that photosynthesis is not the main activity of C. europaea plastids. Previously, it has been shown in other species that the Thylakoid Formation Protein 1 (THF1) is involved in thylakoid membrane differentiation, plant-fungal and plant-bacterial interactions and in sugar signaling with its preferential localization to plastids. Our immunofluorescence localization studies and analyses of haustorial plasma membrane fractions revealed that in addition to plastids, the THF1 protein localizes also to the plasma membrane and plasmodesmata in developing C. europaea haustorium, most abundantly in the digitate cells of the endophyte primordium. These results are supported by western blot analysis, documenting the highest levels of the THF1 protein in "get together" tissues of dodder and tobacco. Based on the fact that photosynthesis is not a typical process in the C. europaea haustorium and on the extra-plastidial localization pattern of the THF1, our data support rather other functions of this protein in the complex relationship between C. europaea and its host.