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The “Inside” Story?

An article in Science magazine (see Ref. 1 below) reports evidence supporting the hypothesis that leaf-dwelling, nitrogen-fixing bacteria may provide host plants with significant amounts of nitrogen.

In the past, we briefly explored the microbial phyllosphere, that is, the microbes – including nitrogen-fixing cyanobacteria – that dwell on the surfaces of plant leaves.

However, the bacteria in today’s story don’t dwell ON the leaves, but INSIDE the leaves.

Yes, there are bacteria (and fungi) that actually live in the spaces between the plant cells, inside the leaves. These microbes are collectively known as endophytes (as opposed to the surface-dwelling “epiphytes”).

And there is no question that most plants – even cultivated crop plants – likely serve as hosts for some endophytes.

The BIG question is: Do these endophytes affect the physiology (function) of the host plants? And, if so, how?

Presumably, the endophytes benefit, at least, from the solar-powered donut factory (i.e., photosynthesis) provided by the leaves. If the endophytes use some of the carbs (sugars) made by the plants, they are somewhat of a drain on the plant’s resources.

Then, another good question is: Why do the plants put up with these microbial “moochers”?

The likely answer is that the plants are somehow benefiting from these microbial endophytes. And, indeed, there are lots of studies showing that plants with certain endophytes may grow better (and may even survive harsh conditions better) compared to plants without these endophytes.

If this is true, then what are these microbial endophytes doing that benefits the host plant?

In other words.…

How Do Endophytes Work?

Despite all the studies showing that endophytes can improve plant growth (and may even be responsible for plant survival in extreme physical environments), precise reasons to explain HOW such endophytes do this have remained elusive.

At least a partial answer to this fundamental question may be provided by results reported in this Science magazine article (Ref. 1 below).

According to this article, at a recent scientific meeting, two separate researchers – Prof. Sharon Doty and Prof. A. Carolin Frank – presented evidence supporting the notion that that some leaf-dwelling bacterial endophytes can convert atmospheric nitrogen gas (N2) into a more biologically usable form (NH4+), which is then used by the host plants.

These results provide evidence that one reason that some bacterial endophytes benefit their hosts is by providing the plant with a source of nitrogen. Plants are often deficient in this essential mineral nutrient, especially in nutrient-poor soils.

Simply put, some of the bacteria living inside plant leaves may actually be fertilizing the plant with nitrogen.

However, this is a controversial hypothesis, and it has met with skepticism from some in the scientific community.

“That’s a radical notion, because nitrogen fixation is generally thought to happen primarily in bacteria-rich nodules on the roots of legumes and a few other plants, and not in the treetops. “We are completely fighting dogma,” says Doty, a plant microbiologist at the University of Washington, Seattle.” (from Ref. 1 below)

One reason for this skepticism is that the bacterial enzyme (nitrogenase) that converts N2 into NH4+ is significantly inhibited by oxygen gas (O2), which, of course, is quite abundant in photosynthesizing leaves.

Despite such objections some scientists “…are now cautiously embracing the idea. “There’s a change in attitude, not from skepticism to believing but from skepticism to cautious questioning,” says Gerald Tuskan, a plant geneticist at Oak Ridge National Laboratory in Tennessee. Tuskan and his colleagues have isolated about 3000 microbes from poplar, many of which are equipped with nitrogenase. Some sequester themselves in biofilms with oxygen-limited compartments, where nitrogenase could function even in the leaf’s oxygen-rich environment.” (from Ref. 1 below)

Why is this important?

Frank and Doty suspect that nitrogen-fixing leaf bacteria may be widespread, and, if transferred to crops, could help boost yields on marginal soil.” (from Ref. 1 below)

“Other plant biologists, although far from convinced, are paying attention. “If there’s an unrecognized set of nitrogen fixers in a wide number of [tree] species, that’s a big deal,” says Douglas Cook, a plant and microbial biologist at UC Davis.” (from Ref. 1 below)

References

1. Pennisi, E. (2015) “Leaf bacteria fertilize leaves, researchers claim.” Science, Vol. 348, pp. 844-845. (Abstract).

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