GROWTH PROCESSES OF CUSCUTA JAPONICA IN THE EARLY STAGES OF DEVELOPMENT
GROWTH PROCESSES OF CUSCUTA JAPONICA IN THE EARLY STAGES OF DEVELOPMENT
Natalya Leusova
candidate of biological sciences, scientific secretary, Institute of Geology & Nature Management FEB RAS,
Russia, Blagoveshchensk
Dodder (Cuscuta L.) belongs to the group of parasitic angiosperms that feed entirely on the host plant, thus, they are practically devoid of photosynthesis in the process of evolution. The mechanism of adaptation of the growth and development of these stem parasites to environmental changes differs both from phototrophic plants and from other parasitic plants, most of which are root parasites.
Seedlings of the parasitic dodder plant, in our opinion, are an attractive model for studying the molecular mechanisms of growth processes in plants, since the development of other plants is accompanied by branching, so it is difficult to isolate tissues with division zones, elongation, and mature tissues in sufficient quantities. Coleoptiles and grass roots are very popular for studying growth processes. Dodder seedlings have an advantage over these models - the presence of unambiguous polarity (unipolarity). Growth occurs due to the apical meristem, as the seedling elongates, nutrients move from the lower part to the upper, the basal part dies off. As a result, for a sufficiently long time (up to 3 weeks), the constant length of the living stem of the dodder seedling is preserved (1, p.881). The high germination rates of dodder seeds require a very active mobilization of their own food resources.
The aim of the work is to study the growth processes of the Japanese dodder (Cuscuta japonica Choisy) seedling. To study the growth processes, dodder seedlings of different ages were used. Grown for 1-2 weeks at room temperature in the light. When examining the cells of different zones of the seedling, we preferred the method of crushed preparations with acetocarmine staining. The advantages of well-prepared squashed preparations are that all cells lie in a monolayer, and metaphase chromosomes lie almost without overlap in the same plane.
Results. According to our observations, the death of the basal pole of Japanese dodder seedlings occurs on the 4th day of the seedling's life. Therefore, for cytological studies, we used 4-day-old Japanese dodder seedlings (Fig.1).
Figure 1. Dodder (Cuscuta japonica) seedling, 4 days old
In the zone of the apex of the seedling (within 1500 µm from the apex), at low magnification (x56), a growth cone with leaf primordia is visible, although the adult plant is devoid of leaves (Fig.2). At higher magnification (x660) cells are visible at various phases of mitosis (Fig.3). They are small, isodiametric, with large nuclei, devoid of vacuoles, with indistinct intercellular boundaries. At a distance of 2 mm from the tip of the shoot, the cells are already elongated. The rudiments of the conducting system also appear here (Fig.4). With the exception of the apical meristem zone (100 µm), the cells of the entire apex contain chloroplasts, which is a characteristic feature, since an adult plant feeds only on the host plant. In addition, stomata of anomacytic type are found (Fig.5)
Figure 2. Apex of dodder shoot with leafy primordia (zoom x56)
Figure 3. Apex cells of dodder (Cuscuta japonica) seedling at different phases of mitosis (zoom x660)
Figure 4. Rudiments of the conducting system of dodder (Cuscuta japonica) seedling (zoom x660)
Figure 5. Stomata of the anomocytic type of dodder (Cuscuta japonica) seedling (zoom x660)
In the middle part of the seedling, cells of various shapes are observed, devoid of chloroplasts (Fig. 6); xylem vessels are well defined. The tracheal elements are annulated or spiral. Stomata do not meet.
Figure 6. Middle part of dodder (Cuscuta japonica) seedling (zoom x660)
The basal pole of a 4-day-old seedling begins to die: the color of the preparation changes, - as a result of destructive processes, the contents of the cells shrink, sections of the cytoplasm with organelles separate into the cell vacuole in the form of closed vesicles (Fig. 7). The nuclei are not stained with the dye due to the destruction of chromatin. It is possible that the death of the basal pole of the dodder seedling occurs according to the type of apoptosis - the programmed death of individual cells and tissues.
Figure 7. Basal pole of Japanese dodder seedling (zoom x660)
Apoptosis is a mandatory component of the development process, and without it, embryogenesis is impossible. Information about apoptosis in rats is still very fragmentary. Some authors even question the phenomena of apoptosis in plants (2). Therefore, in our opinion, the choice of another model for studying the phenomena of apoptosis can complete the picture that occurs in the plant cell during this process.
Thus, changes in cellular structures were found in various zones of the seedling; based on the structure of cells and tissues, the dodder seedling can be divided into clear zones: meristematic - division zone, middle - stretching zone, and basal - dying zone. It was concluded that the death of the basal pole occurs along the path of apoptosis, since its typical signs were observed - a decrease in the volume of the cell as a whole, the protoplast itself, the appearance of closed vesicles.
References:
- Carol C. Baskin, Jerry M. Baskin. Germination ecology of plants with specialized life cycles and/or habitats / Seeds. 2014. Chapter 11. p. 881-884.
- Doom W.G., Woltering E.J. Many ways to exit? Cell death categories in plants // Trends in Plant Science. 2005. Vol. 10, № 3. P. 117-122.