Dear Zhang Apple,

The increase of bacterial and fungal numbers near a plant root surface is known as rhizosphere effect. The term was introduced by Lorenz Hiltner in 1904 and subsequently rhizosphere defined as root's zone of influence was subdivided into rhizoplane (root surface) and rhizosphere proper (adjacent soil). In practice it's not easy to delimit them because different plants can have very different rhizosphere extension. The incease of microorganisms (both in species richness and numbers) is due primarily to root exudation which is more prominent in closest neighbourhood to root itself. For more details and references you can see:

Curl, E.A. and B. Truelove: The Rhizosphere. (Advanced Series in Agricultural Sciences, Vol. 15) Springer-Verlag, Berlin-Heidelberg-New York-Tokyo, 1986. http://link.springer.com/book/10.1007%2F978-3-642-70722-3

Zoe G. Cardon and Julie L. Whitbeck. The Rhizosphere An Ecological Perspective. Elsevier. 2007. ISBN: 978-0-12-088775-0 http://www.sciencedirect.com/science/book/9780120887750

R Pinton, Z Varanini, P Nannipieri (eds.) The rhizosphere: biochemistry and organic substances at the soil–plant interface. 2nd edn. Boca Raton, FL: CRC Press. 2007. https://www.crcpress.com/The-Rhizosphere-Biochemistry-and-Organic-Substances-at-the-Soil-Plant/Pinton-Varanini-Nannipieri/9780849338557

Best wishes,

Elena

Dear Elena,

Thank you for your kind reply. Maybe the root exudation makes contribution to the increase. But our result on bacteria showed that the diversity was lowest near the root (Bulk soil> Loosely attached to root soil>Tightly attached to root soil). Besides, Edwards (2014, PNAS) study about root-associated bacteria of rice also found that the speciese richness was lowest in the rhizoplane compartment (Bulk soil~Rhizosphere>Rhizoplane). The study also observed the microbiome acqusition process and suggeseted that the rhizoplane plays a selective gating role. As far as I understand, the micro-environment near the root may act as a filter which made the decrease of the diversity. But my results in fungi (Bulk soil>Tightly attached to root soil>Loosely attached to root soil) betrayed the idea. So I feel a little confused about the result. I wonder how can I explain this phenomenon?

Edwards J, Johnson C, Santos-Medelln C et al. Structure, variation, and assembly of the root-associated microbiomes of rice.2014. http://sci-hub.io/10.1073/pnas.1414592112

Zhang Apple,

Elena makes some really useful points above. The interaction is a two way process and the plant wants to encourage the right microbiology around it's outer surfaces.

You will I guess already be aware of fungal mycorrhizal associations with plant roots, these associations are truly beneficial to both parties. Many edible fungi are Ceps are associated with trees.

The rhizosphere is a very interesting aspect of modern microbiology. The whole subject of microbial interactions with other living surfaces is part of the paradigm shift is looking for help from the beneficial microbes rather than trying to fight the bad guys.

This is receiving lots of research effort from many companies who see it's potential in improving nutrient uptake, reduced pesticide usage etc

I found the two attached papers very interesting with this regard.

I think you are working in a very inciting area of research, good luck with your studies.

Andrew Goddard

Andrew Goddard,

Thank you for your consideration! I agree with your points that the micro-environment of the rhizosphere will filter the microorganisms which discomfort in the environment. This may can explain the decrease of fungal alpha diversity compared with bulk soil. But this can not explain the increase of fungal alpha diversity in Tightly attached to root soil compared with it in Loosely attached to root soil. (The alpha diversity in my study is Bulk soil>Tightly attached to root soil>Loosely attached to root soil). So I still can not explain why the alpha diversity increased in the tighly attached to root soil than loosely attached to root soil?

Dear Zhang,

Agree with Dr Goddard, mycorrhization can be a very significant influence. If your plants were mycorrhizal, effect of fungal symbiont can interfere a rhizosphere one and the resulting effects, up to my own observation, can be rather different according to plant and fungal species. I worked with ectomycorrhizal tree species myself and I didn't divided rhizosphere (non-mycorrhizal root envronment) into rhizoplane and strictly rhizosphere zones to compare it with mycorrhizosphere (mycorrhizal root zone). My data have shown that effects were different, but the main pattern was increase both bacteria and fungi in rhizosphere and fungal suppression along with bacteria stimulation in mycorrhizosphere. If you are interesting in it, you can see the papers on my profile page https://www.researchgate.net/profile/Elena_Voronina6.

Another moment which seems significant to me is to reveal what microorganism groups comprised the diversity, because in my case the most prominent influence was no total elimination or appearance of some new species but reestablishing dominant complexes and drastic changes in numerical characteristics both in rhizosphere and mycorrhizosphere comparing to bulk soil. Up to my personal experience in such a research, there is no single "true" pattern of influence and we have to analyze any case separately. Besides, the rhizoplane zone undergo effects of microorganism strong competttion and probably antagonistic effect of intraradical fungal biota against extraradical one.

Best wishes,

Elena

Dear Elena,

Thank you for your patient guidance. I think I really konw how to explain it and mycorrhization in rhizoplane may make contribution to the increase. I will read your paper to find the different increased pattern in rhizosphere.

Thank you for your time again!

I am agree with Elena Yu. Voronina. 

Very interesting discussion of great level. Another opinion is to consider, in addition to all the other possibilities discussed, the amount of microbial biomass each "compartment" or fractionated sample have. It is one very simple: if the total microbial biomass per gram is in one case very low, this due to the experimental procedures or to the actual condition in the plant and in the soil, the alpha diversity would be a lot lower just because of that reason. Also, the diversity of the neighbouring niche (soil) that use to be extremely high, may transfer bacterial types that are influencing to closer sampling fractionations.  

Howard makes a good point here. The total biomass as well as the diversity are two connected, but also interesting aspects to these associations. There is also the question of viability? 

Andrew

All above answers are highly appreciable and important. As per Elena Yu. Voronina it can be The rhizospheric effect that can cause positive or negative effects.