HorticultureKeymasterOctober 14, 2016 at 11:15 amPost count: 98
Healthy soil is alive with beneficial microorganisms and we usually think of their symbioses with plants as occurring between bacteria and fungi, but yeasts too have an important role in soil microbiology.
Yeast are “eukaryotes” since they are more complex microbes than bacteria, and possess a true nucleus and other cellular organelles. Yeasts are similar to fungi in this regard, but unlike fungi they have adapted to grow in environments with higher osmotic pressure, such as niches with high amounts of dissolved salts and sugary substances.
That’s why yeasts have a smaller, spherical shape to their cells; it reduces the surface area of the cell compared to it’s volume. With this small shape and specialized enzymes, yeast cells can withstand higher osmotic pressure and not shrivel up in high-solute niches.
A very unique niche for yeast cells has been discovered inside the root of plants, and the term “rhizophagy” has been coined to describe the mechanism used by plant root cells to engulf and absorb these tiny eukaryotes. The suffix “phagy” means to eat, and that is what roots do; they “eat” yeast cells, absorb them into their tissues and then slowly digest them over the course of a few weeks.
To enter a root it takes enzymes to digest through the cellulose of the root cell wall, and this is an energy demanding process. Plants release cellulase enzymes to enable this; the much smaller yeast cells are allowed into the root cells to live awhile, secrete enzymes and hormones, before being digested.
This article describes experiments done on sugarcane and tomato plants using brewer’s yeast, showing how yeast were “eaten” by the roots and then allowed to live in the root’s epidermal cells and in the apoplastic space between root cells before being digested.
These two types of plant were used to compare monocots (sugar cane) to dicots (tomato) for their ability to conduct rhizophagy, as they are distantly related evolutionary. Since both monocots and dicots can absorb bacteria and yeast this way, it suggests that rhizophagy is an ancient evolutionary adaptation.
These scientists showed how the effects of yeast rhizophagy on tomato and sugar cane plants improved many aspects of growth:
“The addition of live or dead yeast to fertilized soil substantially increased the nitrogen (N) and phosphorus (P) content of roots and shoots of tomato (Solanum lycopersicum) and young sugarcane plants. Yeast addition to soil also increased the root-to-shoot ratio in both species and induced species-specific morphological changes that included increased tillering in sugarcane and greater shoot biomass in tomato plants.”
Plants that had absorbed yeast were bulkier, had better tissue nutrient levels, and produced 56% and 47% greater root mass! The growth promoting effects were found to occur with both dead and living yeast cells. The yeast are able to secrete plant hormones such as auxin, and when living or dead they deliver a load of organic nutrients to the plant roots.
Other studies done with rice plants showed that yeast are able to live in the plants for weeks after colonizing roots. A strain of yeast called CtHy was given to experimental rice plants and tests were done to see how they affected growth:
These scientists reported that:
“CtHY showed a strong ability to colonize roots by increasing in number after inoculation. The dynamics of colonization were similar to that of the endophytic PGP yeast Williopsis saturnus, which increased in number from about 5 × 102 cfu g−1 root fresh weight to 5 × 103 cfu g−1 root fresh weight within 3 weeks after inoculation onto maize roots (Nassar et al., 2005). That the colonization of CtHY resulted in 15–35% increases in root dry weights of rice seedlings clearly indicates its potential to promote plant growth and validates its inclusion and prospects for field use…”
The type of yeast being tested here on rice could produce auxin the root growth hormone, it can dissolve and mobilize phosphorus, and possesses a useful enzyme called ACC-deaminase that reduces ethylene buildup in plant tissues. (Ethylene buildup slows growth and eventually kills plant cells.)
Given the fact that this type of yeast can colonize the roots for weeks makes it a prime target for rhizophagy. Seeds coated in this yeast resulted in improved growth of the young seedlings, with 18% longer roots and 35% longer shoots.
When applying yeast strains to experimental crops of sugar beets, similar growth promoting effects were seen:
The researchers doing this work found that yeast inoculated sugar beets had more photosynthetic pigments than non-inoculated ones, and this produced a sweeter and more protein rich beet crop.
Rhizophagy of yeast cells is something all growers should remember to exploit; yeasts have a unique eukaryotic metabolism and roots of plants will “brew” their own growth enhancing yeasts right inside their tissues if they are present in the soil.
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