Parasitic Plant - Seed Germination

Seed Germination

Seed germination of parasitic plants occurs in a variety of ways. These means can either be chemical or mechanical and the means used by seeds often depends on whether or not the parasites are root parasites or stem parasites. Both root and stem parasitic plants have evolved to use one or more means of finding their hosts in order to germinate. Most parasitic plants need to germinate in close proximity to their host plants because their seeds are limited in the amount of resources necessary to survive without nutrients from their host plants. Resources are limited due in part to the fact that most parasitic plants are not able to use autotrophic nutrition to establish the early stages of seeding.

Root parasitic plant seeds tend to use chemical cues for germination. In order for germination to occur, seeds need to be fairly close to their host plant. For example, the seeds of the parasitic plant Witchweed (Striga asiatica) need to be within 3 to 4 millimeters (mm) of its host in order to pick up chemical signals in the soil to signal germination. This range is important because Striga asiatica will only grow about 4 mm after germination. Chemical compound cues sensed by parasitic plant seeds are from host plant root exudates that are leached in close proximity from the host’s root system into the surrounding soil. These chemical cues are a variety of compounds that are unstable and rapidly degraded in soil and are present within a radius of a few meters of the plant exuding them. Parasitic plants germinate and follow a concentration gradient of these compounds in the soil toward the host plants if close enough. These compounds are called strigolactones. Strigolactone stimulates ethylene biosynthesis in seeds causing them to germinate.

There are a variety of chemical germination stimulants. Strigol was the first of the germination stimulants to be isolated. It was isolated from a non-host cotton plant and has been found in true host plants such as corn and millets. The stimulants are usually plant specific, examples of other germination stimulants include sorgolactone from sorghum, orobanchol and alectrol from red clover, and 5-deoxystrigol from Lotus japonicas. Strigolactones are apocarotenoids that are produced via the carotenoid pathway of plants. Strigolactones and mycorrhizal fungi have a relationship in which Strigolactone also cues the growth of mycorrhizal fungus.

Stem parasitic plants unlike most root plants germinate using the resources inside its endosperm and are able to survive for a small amount of time. An example, Dodder (Cuscuta spp.) is a parasitic plant whose seed falls to the ground and may remain dormant for up to five years before it is able to sense a host plants nearby. Using the resources in the seed endosperm, Dodder is able to germinate. Once germinated, the plant has 6 days to find and establish a connection with its host plant before its resources run out.

Dodder seeds germinate above ground and then the plant sends out stems in search of its host plant reaching up to 6 cm before it dies. It is believed that the plant uses two methods of finding a host. The stem is able to pick up its host plant’s scent whereby it then is able to orient itself in the direction of its host. Scientists used volatiles from tomato plants (α-pinene, β-myrcene, and β-phellandrene) to test the reaction of C. pentagona and found that the stem will oriented itself in the direction of the odor. Some studies suggest that by using light reflecting from nearby plants dodders are able to select host with higher sugar because of the levels of chlorophyll in the leaves. Once Dodder finds its host, it wraps itself around the host plants stem. Using adventitious roots, Dodder taps into the host plant’s stem and creates a haustorium, which is a special connection into the host plant vascular tissue. Dodder makes several of these connections with the host as it moves up the plant.