plant physiology – University of Bristol Botanic Garden

The symbiotic relationship between legumes and soil bacteria has been known for well over a century. The intimate details of this relationship, however, are only recently being revealed. It is a very active area of research as understanding this symbiotic relationship could lead to strategies that help reduce the environmental impacts of food production.

Rhizobia nodules on the roots of cowpea
(Vigna unguiculata). By Stdout
[GFDL (http://www.gnu.org/copyleft/fdl.html),
via Wikimedia Commons.

Special soil bacteria – known as rhizobia – reside within the nodules of legumes, such as peas, lentils, beans, alfalfa and clover, which are found along the roots of these plants. The bacteria take nitrogen from the air and convert it into ammonia, which the plant is able to use – a process known as nitrogen “fixing”.

This allows legumes to grow well in nitrogen-poor soils. This nitrogen is taken up in the plant material, which can then be worked back into the soil as a natural fertiliser for subsequent crops.

While this all might sound very straight forward – there are details about this relationship that remain unclear. How do the bacteria get into the nodules? Are there signals that the plant uses to stimulate the bacteria to produce nitrogen?

An answer to a century-old debate

In 2011, researchers from the John Innes Centre in Norwich answered the mystery of how nitrogen-fixing bacteria crossed the cell walls into the nodules of legumes.

It had been a century-old debate as to whether bacteria produced the enzymes to break down the cell walls or whether the plant did. The researchers showed that it was the plant which supplied the enzymes to break down its cell walls in order to give the bacteria access.

How legumes communicate with their symbiotic bacteria

In 2010, Stanfordresearchers discovered the gene in plants that triggered the chemical signal required for the bacteria to fix nitrogen. They found that the rhizobia bacteria would just sit around in the legume nodules if the plant failed to produce the protein that’s required to spur the bacteria into nitrogen fixing mode. This was only part of the communication story.

It is energetically costly for the plant to produce and maintain the root nodules in which the bacteria live; usually the benefit of having a supply of nitrogen outweighs this cost. If there is sufficient useable nitrogen in the soil, however, the plant is able to reduce the number of nodules on its roots.

Communication between the shoots of the plant and the roots of the plant help regulate the number of nodules. The leaves transmit a signal to the roots to either develop more or get rid of rood nodules, depending on circumstances. The roots communicate back up to the leaves using molecules known as peptides.

Research published recently has now discovered that the plant shoots use plant hormones, known as cytokinins, which travel down the phloem into the roots to help regulate nodule development.

The environmental benefits of understanding legumes

Understanding the symbiotic relationship between legumes and soil bacteria is not simply a matter of scientific curiosity. The ability for legumes to produce natural nitrogen fertilisers is a trait that US researchers would like to potentially transfer to non-legume crops as a way of reducing the environmental impact of agriculture.

Manufacturing nitrogen fertilisers for non-legumes is extremely resource intensive. It has been estimated that to produce 68 kg (150 lbs) of nitrogen fertiliser – enough for one acre of corn – would be the equivalent of driving a car 1,046 km (650 miles).

Beyond that, nitrogen fertilisers release the powerful greenhouse gas, nitrous oxide, after they’ve been applied. Excess fertilisers also runoff agricultural land into rivers and lakes and eventually out into the ocean. This influx of nitrogen can provoke algal blooms and create oxygen deplete dead zones.

Therefore, there is great incentive to fully understand this relationship legumes have with soil bacteria. The environmental impact of agriculture could be significantly reduced by utilising legumes with their natural nitrogen fertiliser more by using them in more marginal land and using traditional breeding programs to select for drought resistance or temperature tolerance. In some countries, genetic engineering might even be used to introduce nitrogen-fixing abilities into non-legume species. Genetic modification, however, can be an inflammatory issue with considerable debate as to its pros and cons, particularly with respect to its use in food products.

Two days ago I purchased an alarmingly large number of strawberries. I couldn’t help myself. Grown in Cheddar, these sweet little ripe morsels are a welcome break from the onslaught of last year’s apples and a plethora of citrus. When you try to eat seasonally and with reduced transportation miles, you appreciate the appearance of new season fruit that much more.

Non-climacteric fruit, such as strawberries, do not continue to ripen once picked

Strawberries have to be picked at their peak of ripeness as
they don’t ripen any further once they’re separated from the
plant – known as non-climacteric fruit.
Photo credit: Nicola Temple

The moment I placed the box on my kitchen counter, however, I felt as though a timer began counting down on a bomb. But rather than finishing off with an explosion, it would be more of a moldy, decayed mess of fruit wasting away. In response, I did as my mother before me did, and I issued relentless alarm calls to my family, “Eat strawberries…strawberries would go well with that…why are you eating that pear? EAT strawberries!” Luckily the troops rallied and I’m happy to report that there was no waste.

This strawberry time bomb is more technically that stage between when a fruit has reached its peak ripeness and when it first starts to deteriorate. Strawberries, unlike some other fruits, do not continue to ripen when picked and so they have to be picked when they are perfectly ripe otherwise they will taste somewhat inferior. The rotting timer starts the minute the strawberry is picked and is running down from field (or poly tunnel) to consumer. So why is it that strawberries don’t ripen further after they’re picked, but fruits like tomatoes do?

Ethylene and rapid respiration: qualities of the climacteric fruit

The answer lies in some basic plant physiology. Some fruits produce a lot of ethylene and undergo rapid respiration during ripening, which means the fruits continue to ripen even once they are separated from the plant. These are known as climacteric fruits. As one would expect, non-climacteric fruits produce very little ethylene, do not undergo periods of rapid respiration and do not ripen any further once picked from the plant.

Ethylene plays a major role in the regulation of the ripening process and affects the rate at which the fruit ripens. Producers use this to their advantage. Bananas, for example, are picked hard and green and stored mature but unripe. When a retailer places an order, the bananas are placed in a room and ethylene is pumped in to ripen the fruit up for sale.

Ethylene is even used by industry as a de-greening agent for non-climacteric fruits, such as citrus. It is used to break down the green chlorophyll pigment in the peel of many citrus fruits, like orange and lemon, which essentially makes a somewhat unripe fruit appear ripe to the consumer.

The genetic regulation behind the climacteric characteristics of plants is very complex and not yet completely understood. For example, different melon varieties can be climacteric or non-climacteric. If a climacteric melon is crossed with a non-climacteric melon, the fruit is generally climacteric, suggesting it might be a genetically dominant character trait. Yet, other experiments that have crossed two non-climacteric melons have generated climacteric melons. This implies that the trait is more complex than a dominantly inherited trait.

Examples of climacteric versus non-climacteric fruits

There may be a few items on these lists that make you take a second look as we don’t commonly think of them as fruits, but rather as vegetables. However, aubergines, courgettes and cucumbers are indeed fruits.

Climacteric Fruits	Non-climacteric fruits
Apple	Aubergine
Apricot	Bell peppers
Avocado	Cherries
Banana	Citrus fruits
Cantaloupe	Courgettes
Fig	Cucumber
Kiwi	Grapes
Mango	Lychee
Passion fruit	Most berries
Peach	Pomegranate
Pear	Strawberries
Plum	Pineapples
Tomato	Watermelon

How to store climacteric fruits and non-climacteric fruits

Tomatoes are a climacteric fruit - they continue to ripen after picking.

Different varieties of tomatoes, a climacteric fruit, on
display at a French market. Photo credit: Shelby Temple.

Knowing the difference between your climacteric and non-climacteric fruits can help you store them appropriately.

Climacteric fruits are best stored at room temperature. They are picked before they are ripe and refrigeration can slow the ripening process. Since these fruits will continue to ripen after picking, they generally have a shorter shelf-life, but refrigerating them once they have fully ripened could extend the shelf life somewhat.

Non-climacteric fruits, on the other hand, are picked when fully ripe and are best stored in the refrigerator to slow their deterioration. They generally have a longer shelf-life as they don’t continue to ripen (though I don’t consider this to be true of berries).

Don’t store climacteric fruits with non-climacteric fruits as the ethylene produced by fruits such as bananas can speed up the rotting process of an already ripe fruit. However, this natural ethylene production can also work to your advantage. Avocados, for example, are often sold hard as rocks and if you wish to speed up the ripening process, you can store them with bananas in a paper bag on the counter.

The climacteric character of fruit is an active area of research due to the direct applications for the way we pick, transport and store our food. As much as I am an advocate for scientific solutions, I hope overindulging on the sweet delicious fruit of local strawberries during this precious time of year is never resolved – it is simply a matter of tradition.

Category: plant physiology

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