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mikey

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Root Plasticity
« on: September 28, 2008, 03:54:38 AM »
Root Plasticity: A Crop’s Lifeline vs. Fluctuating Soil Moisture Stress
Roots, the hidden half of a plant, are less studied than its above-ground counterparts. This was on the mind of Dr Roel R. Suralta, PhilRice agronomist, when he was starting his research on roots.

The seeming neglect of root research challenged him to make roots the subject of his dissertation at the Nagoya University, Japan. Titled “Significance of Root System Development Responses to Transient Soil Moisture Stress Conditions for Adaptation of Rice Plants”, his thesis centered on root plasticity or the ability of a plant’s root system to change developmentally and functionally in response to changing soil conditions, thus maintaining overall plant growth.

“Specifically, my study focused on the plastic root response not just to simple constant soil moisture stresses such as drought (deficit in water) and waterlogging (excess in water) but also to a combination of these stresses and how the response contributes to the maintenance of shoot growth in rice,” Dr. Suralta explained.

FLUCTUATING “FORCES” IN THE SOIL
Roots are in direct contact with the soil and are therefore the first to encounter stress factors in the soil. They are potentially the first sites of damage and offer the first lines of defense of plants exposed to soil stress. In terms of soil moisture, drought and waterlogging are two of the most critical stresses.

“Well-documented plastic root responses include lateral root production, especially L-type roots, and aerenchyma development under drought and water
logged conditions, respectively,” Dr. Suralta said.

L-type lateral roots are long and branching fine roots emerging from either the seminal and nodal roots of rice. They increase the efficiency for water and nutrient uptake under drought. On the other hand, aerenchyma formation refers to tissue growth within the roots with air passages. Under waterlogged conditions, the air passages due to aerenchyma formation, allowing the roots to develop and function by facilitating internal oxygen diffusion.

Under actual field conditions, the soil normally experiences fluctuating soil water regimes due to the intermittent nature of rainfall and irrigation systems. This scenario is especially true in rainfed lowland rice systems that use water saving technologies such as alternate wetting and drying.

“Under fluctuating soil moisture conditions such as these, the quickness of the root to respond is very important for the rice plant to adapt and maintain its root growth and functions,” he averred. Therefore, “it is more meaningful to study root responses to soil moisture fluctuations than to focus on individual moisture stresses.”

Because modes of root adaptation differ under soil moisture stress conditions, two important questions needed to be addressed, he said.

First, can deep and extensive roots, formed in response to drought, immediately enhance its aerenchyma to facilitate oxygen diffusion to the roots upon sudden waterlogging? Second, can shallow roots with enhanced aerenchyma, developed under waterlogged condition, rapidly expand under progressive soil moisture deficit to meet the demand for water?

“The answers are essential because the inability of roots to acclimate to sudden changes in soil-water regimes affects its growth and functions, and thus, crop productivity,” he emphasized.

GENOTYPIC DIFFERENCES IN ROOT RESPONSES
To compare genotypic differences in response to fluctuating soil moistures, “I initially compared the growth of aerobic and irrigated lowland genotypes under short duration soil moisture fluctuations in hydroponics,” Dr. suralta said.

To simulate the soil moisture fluctuations, two transient moisture stress treatments were used: transient stagnant (oxygen-deficient) to drought and drought to oxygen-deficient conditions. Plants were exposed under each stress for a week.

“Aerobic genotypes have the potential to provide basic information on root response under soil moisture fluctuations because they tolerate occasional flooding and have improved lodging resistance, high harvest index, high input responsiveness, tolerate occasional flooding, and a combined characteristics of a drought-resistant upland and high yielding irrigated lowland varieties,” he explained further.

Initial findings showed that aerobic genotypes were better than irrigated lowland genotypes under soil moisture fluctuations “because their lateral root production were less inhibited under drought preceded by waterlogged condition. They also have the ability to enhance aerenchyma under waterlogged conditions even when preceded by drought.”

THERE IS ROOT PLASTICITY UNDER TRANSIENT MOISTURE STRESSES
While aerobic genotypes demonstrated better response to fluctuating soil moisture than irrigated lowland genotypes, it is difficult to conclude that this is solely due to root plasticity without eliminating bias arising from inherent trait differences like plant size and growth duration under non-stressed condition.

Using near isogenic rice lines, specifically 54 chromosome segment substitution lines (CSSL) from a cross between Nipponbare (japonica variety) and Kasalath (indica variety), Dr. Suralta and his co-workers were able to test their hypothesis.

“These lines carry almost the same genetic makeup with the recurrent parent Nipponbare except for few chromosome segments that are substituted with the one from the donor parent Kasalath,” Dr. Suralta added.

From these 54 CSSLs, he selected CSSL47, a line that showed no significant differences in shoot and root growth with Nipponbare under nonstressed condition but showed more plastic root response than Nipponbare under transient moisture stresses. Thus, CSSL47 was used to precisely compare its growth response against Nipponbare.

ROOT PLASTICITY ON SHOOT GROWTH
The study also aimed to prove the relationship of plastic root growth under soil moisture fluctuations to the maintenance of shoot growth.

Both genotypes were then subjected to two transient moisture stress treatments and continuously waterlogged condition as control for total growth duration of 36 to 38 days. Specially designed root boxes were used to observe growth and physiology.

Under transient drought to waterlogged conditions, the seedlings were grown under 10% constant drought for 21 days prior to waterlogged condition for another 15 days.

Under transient waterlogged to drought conditions, the seedlings were grown for 14 days in waterlogged condition prior to progressive soil drying at different soil moisture contents for another 21 days.

Results showed that the reduced root growth of Nipponbare was due to the loss of its ability to enhance aerenchyma under oxygen deficiency when preceded by drought, thus, reduced root axis elongations.

On the other hand, “the reduced root system development of Nipponbare was due to its reduced ability for lateral root production under progressive soil drying at low soil moisture when preceded by oxygen deficiency.”

The CSSL47, which showed more plastic root growth under both transient conditions, had the ability to enhance aerenchyma formation and lateral root production.

“We observed that CSSL47 have more plastic root response than Nipponbare under both soil moisture fluctuations but not under constant soil moisture stresses,” Dr. Suralta said.

“CSSL47 is a very unique line because it may contain putative regions in a chromosome controlling traits in its substituted segment donated by the Kasalath parent, which can only be expressed under soil moisture fluctuations. This only implied that the QTLs associated with aerenchyma formation and lateral root production under soil moisture fluctuations may differ from those under constant soil moisture stress,” he added.

Dr. Suralta, and other researchers are now analyzing the growth performance of these two genotypes under field condition with intermittent irrigation system to impose continuous alternating occurrences of drought and waterlogged conditions.

“Of course we are after the benefits of root plasticity not just to the total shoot growth but also yield. CSSL47 exhibits potential for identifying QTLs that control lateral root production and aerenchyma development under soil moisture fluctuations,” he said.

ADJUSTING TO FLUCTUATING SOIL MOISTURE STRESSES
To plants, the roots are the key.

“You just have to identify the desirable root traits under soil moisture fluctuations. The key lies in lateral root production and aerenchyma development under progressive drought preceded by waterlogged conditions and sudden waterlogging preceded by drought, respectively,” Dr. Suralta affirmed.

“The expression of these traits should not be lost but must be maintained under fluctuating soil moistures for better adaptation, otherwise growth will be affected,” he added.

With this, the study’s initial assumption stood out as true, that “the plastic responses of root traits are useful in maintaining crop growth under soil moisture fluctuations.” Thus, this would be an important consideration for improvement of rice cultivars suited for growing environments with fluctuating soil moistures.

The ability of the roots to adapt to these changing soil moisture stresses is indeed the solution.



 

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