Seed Endophytic Bacteria: Origin, Transmission and Functional roles
Author: Gaurav Pal
Numerous microorganisms, often bacteria and fungi have been reported to be present inside the plant tissues. Many of these microorganisms are pathogenic causing disease to their host plants while others may be present in a latent form causing no harm to their host. Those microbes that invade and reside for all or part of their life cycle inside the living tissues of plants and do not cause any negative effects to the plants are called endophytes. These endophytes have been reported from all parts of the plants like root, stem, leaves, flowers, seeds, meristematic tissues, etc. The endophytes present inside the seeds are referred to as seed endophytes. Bacterial endophytes have been isolated from all the plants examined so far by the research community. These bacterial endophytes are of particular importance to plants as they play various important roles such as nutrient acquisition facilitation, plant growth hormone modulation, phosphate solubilisation, siderophore formation, providing resistance against biotic as well as abiotic stresses, etc.
In a general sense, it is believed that the endophytic bacteria have been originated from the epiphytic bacteria living in the rhizosphere of plant and gained entrance inside the plant through wounds or plant openings such as hydathodes and lenticels. This led to the establishment of an endo-symbiotic relationship between the plant and the bacterial species in which the host plant provided necessary nutrients along with conditions required for the growth and survival of bacteria while the bacteria help the plants is acquisition of essential mineral nutrients from the soil thereby helping in growth promotion of the plant.
The bacterial seed endophytes are known to be transmitted to the next generation through vertical mode of transmission while the transmission among others take place through horizontal means. Vertical transmission of seed endophytic bacteria take place via gametes or during transformation of shoot meristems into reproductive meristems. On the other hand, horizontal transmission of seed endophytic bacteria can take place through the vegetative parts of the plant to the seed.
Seed endophytic bacteria are known to play different important functional roles inside the plant. The major functions include plant growth promotion, biocontrol activity and bioremediation. Plant growth promotion by seed endophytic bacteria is done by various means such as nutrient acquisition facilitation, modulation of hormonal levels and ACC deaminase activity. Phosphate solubilisation and siderophore formation are the major ways by which seed endophytic bacteria facilitates nutrient acquisition. The biocontrol activity of seed endophytic bacteria is mainly based on the synthesis of allelochemicals such as antibiotics or secretion of lytic enzymes. Induced systemic resistance is also one of the possible mechanisms employed by seed endophytic bacteria for biocontrol against various pathogens and pests. It is also believed that these endophytic bacteria have the genetic machinery which can synthesize compounds capable of degrading toxic molecules and compounds present in the rhizosphere and thus can be very useful in the process of phytoremediation.
1. Hallmann, J., Quadt-Hallmann, A., Mahaffee, W. F., & Kloepper, J. W. (1997). Bacterial endophytes in agricultural crops. Canadian Journal of Microbiology, 43(10), 895-914.
2. Truyens, S., Weyens, N., Cuypers, A., & Vangronsveld, J. (2015). Bacterial seed endophytes: genera, vertical transmission and interaction with plants. Environmental Microbiology Reports, 7(1), 40-50.
About Author / Additional Info:
I am a research scholar (with CSIR-JRF and GATE) in Banaras Hindu University