It’s fair to say that the 2024 season is proving to be memorable in all the wrong sorts of ways. Although the weather is always beyond our control, creating crops and soils that are more resilient to challenging conditions is within our grasp.
Understanding of not just the composition of soil biology, but the function of the soil microbiome is a growing area of research and one of the most exciting when it comes to helping plants to help themselves. The symbiosis between plants and endophytes is perhaps one of the most important things to foster as we strive to reduce dependence on inputs and to use them more efficiently.
With that in mind, we wanted our technical group to be among the first to know that the consortium of endophytes in Unium’s leading biological seed treatment TIROS is coming back this season by popular demand as TIROS Max.
While the endophytes may be the same, the formulation has been further improved and marks a leap forward in biological seed treatment technology. The advanced formulation results in 13% more Colony Forming Units per seed and greater flexibility in treatment, with batch sizes of just one or five tonnes now possible in addition to the previous 10-tonnes of seed.
The advanced formulation of TIROS Max is in powder form, removing a liquid phase when preparing the seed treatment. It contains a prebiotic, or extender, which provides nutritional support for the endophytic bacteria as they form a relationship with the germinating plant and a proprietary biostimulant material to provide an additional boost during the all-important germination and establishment phases.
The tried and tested technology in TIROS Max helps increase the success of the biological seed treatment where historically many fail to associate with the plant or provide variable results.
We recently sat down with Professor James F White or Rutgers University in New Jersey to ask him more about the symbiotic relationship between endophytes and plants and the role of biostimulants microbes, can they really help?
Endophytes with Prof James White
Q&A – click the questions to read the answers
Prof White: Plants use endophytes to adapt to the environmental conditions they find themselves in, but they are also critical not only for the plant’s growth and development, but for their resilience against environmental stress and disease.
Endophytes work to suppress disease in plants by colonising fungi such as Fusarium (which causes damping-off) and altering how it grows, thereby making it less virulent. Plants can switch endophytes depending on their needs at any time.
One of the roles of endophytes is in providing nutritional products to plants and the discovery of how plants utilise these endophytes is relatively new. The interaction was first observed by a group of Australian microbiologists in 2010 and dubbed rhizophagy, but at Rutgers university [in New Jersey] we delved further into the detail and first described the rhizophagy cycle in which plants seem to ‘farm’ microbes for their benefit.
Prof White: The process gets underway by plants secreting exudates at their root tips which serve as signalling molecules to attract beneficial communities of microbes.
Once attracted to the root tip meristem, microbial communities are internalised by the plant – the exact mechanism hasn’t been described yet. Once inside the root tissues, the plant produces superoxide which oxidises the bacteria, stripping off the bacterial cell walls to leave protoplasts. It is from these that the plant extracts nutrients.
What happens next emphasises how plants are dependent on microbes not just to help feed them, but also for their root hair development – these massively increase the surface area for absorption of nutrients and water. They do this by secreting substances such as ethylene, nitric oxide and other hormones that trigger root hair elongation.
Some of the bacteria that survive the process are then ejected back into the soil from the tips of elongating root hairs, where root exudates encourage their cell walls to reform. And the cycle begins again as the microbes migrate away from the root to acquire nutrients and then return to the root tip, attracted by plant exudates. Rhizophagy is a system common to all plants with root hairs and without it, root hairs can’t develop.
Source: Chang X et al. 2023. Endophyte symbiosis: evolutionary development and impacts of plant agriculture. Grass Research 3:18
Prof White: Because the symbiotic relationship between plants and endophytes is especially important in their early development stages, a seed producing plant will place some of its endophytes onto and into its seeds so that endophytes are in situ when the seed germinates – aiding root hair development and elongation as well as supply of nutrients to the seedling. Later, some plants will form mycorrhizal associations which will take over the process of acquiring nutrients.
It’s a process that’s understood by many regenerative farmers that home-save seed because they already contain the endophytes already adapted to the seeds’ growing environment.
Modern farming practices have left many soils with depleted microbial communities and the extensive use of fertilisers means plants are less dependent on microbes for nutrient supply.
Prof White: Biostimulant microbes have a useful role. Mostly they’re available as soil microbes but sometimes they come from plants. When you apply them to the seed, they’ll replace some of the microbes that were lost earlier on so that when the seed germinates, the plant will internalise these microbes and kick-start the rhizophagy cycle.
Biologicals can also act as a supplement in soils where microbial communities are depleted. “In the longer term, once roots start growing, other microbes from the soil or microbes that happen to still be in the plant seed will come in and take over. And in many cases, the microbes you put on will become less important.
However, there are some biostimulant microbes that have been isolated from plants and the intention with those is that they establish a more permanent relationship with the plant, with the proviso that the consortium selected is adapted to the crop plant and the plant needs that community in its environment.
And this is precisely what plant-derived TIROS Max aims to achieve.
Prof White: It’s not just about root hairs… not all endophytes get stuck in the rhizophagy loop, instead the plant moves some of the bacteria from the root and spreads them throughout its tissues. Many plants will put these bacteria into their leaf hairs (trichomes) where they feed them plant sugars in exchange for the nitrogen these endophytes will fix from atmospheric nitrogen.
When looking at trichomes in super-efficient invasive weeds at Rutgers university, we’ve been able to see nitrogen around the bacteria in the trichomes and believe this nitrogen fixation is happening in trichomes all over the planet.
Trichomes on seed plants stained using acidified diphenylamine stain to visualize nitrate (purple; arrows) around clusters of bacteria in hairs. (a) Two trichomes (at arrows) on petals of Canadian goldenrod (Solidago canadensis; Asteraceae) (bar = 10 µ). (b) Trichome on young leaf of common mullein (Verbascum thapsus; Scrophulariaceae) showing accumulation of nitrate around bacterial mass in head of trichome (bar = 10 µ).
Source: Chang X et al. 2023. Endophyte symbiosis: evolutionary development and impacts of plant agriculture. Grass Research 3:18
Trichome research
Last year, researched at Prof White’s lab published an open access paper in Grass Research which looks at the evolutionary development of endophyte symbiosis and impacts on agriculture (well worth a read). It describes the role of above-ground hairs in plants and the scientists conclude that these hairs on the leaf and stem also function to replicate microbes and foster plant growth (in an echo of the root’s rhizophagy cycle).
Trichomes function like tiny root nodules and in their simplest form are filamentous structures that often have microbes inside them. Microbes tend to accumulate at these hair tips and in pits along the side of the trichome which are only present in some species. Because there are multiple interactions when filamentous trichomes also have lateral pits, these are thought to be more efficient at extracting nutrients.
A third type of trichomes exists which is glandular in structure, and these have been shown to be very efficient at absorbing nitrogen from the air. These plants also secrete higher levels of antioxidants as oxygen inhibits nitrogen fixation in bacteria. A secondary effect of these increased levels of antioxidants in plant tissues is a greater tolerance to oxidative stress when plants come under environmental stress or are attacked by disease. And that’s why Prof White says a healthy plant is one that contains endophytes.
Other plants form leaf nodules – similar to root nodules in legumes – that contain endophytes that fix atmospheric nitrogen, enabling them to grow under nutrient-poor conditions. So, while all plants may have the ability to fix atmospheric nitrogen to a degree, some plants are super-efficient at doing this.
The endophytes in TIROS Max were discovered in wild poplar and willow plants that were thriving in extremely unfertile conditions on the banks of the Snoqualmie River in Washington State. In studies carried out by Prof Sharon Doty of University of Washington, endophyte strains were isolated from their vascular tissues that fix atmospheric nitrogen and sequester nutrients such as phosphate, potassium, zinc and other microelements. It’s a consortium of these plant-derived endophytes that form the basis of Unium’s advanced biological seed treatment.
Summary of how TIROS Max can support crops
Trials Data
Same endophytes as we had in TIROS, but more of them per seed in TIROS Max for more consistent performance.
It’s back and it’s better
The next generation of biological seed treatment
Join us for a webinar with
Prof James White | Prof Sharon Doty | Ben Taylor-Davies
2nd July at 6pm
Both endophyte experts will be giving us a brief update on their research before joining Ben Taylor-Davies in an interactive ‘fireside chat’. This is your chance to ask these legends of endophyte research and agroecology anything and everything you’d like to know about endophytes, such as whether microbial biostimulants really do help and how we can support the soil microbiome.
We’d like to thank the British On-Farm Innovation Network (BOFIN) for hosting the webinar for us as part of its TRUTH project. Unium will be supporting members of BOFIN’s Soil Circle with on-farm trials of TIROS Max and TARBIS in the 2024/25 season.