By Katharina Schoebi, Checkbiotech

When plants experience a drought, they produce special compounds, such as the amino acid proline. Recently, Japanese researchers found that petunias can better survive a drought when they contain the gene for a key enzyme in proline biosynthesis .

Water deficiency is a serious problem for agriculture, and drought tolerance has become a major issue of late. This is in part due to the increased attention that has been given to water conservation. The problem is still growing and enforcing a strong demand for generating new crop varieties with improved drought tolerance. Irrigated plants would benefit from a more efficient water use, and simultaneously also the economics of production would be improved.

Nowadays, genetic engineering offers the possibility to identify genes that render plants tolerant to drought. Various experiments revealed that drought activates a variety of metabolic and defense systems in plants, for example: the accumulation of sugars, the sugar substitute mannitol and the amino acid proline.

Of the three defense systems, the pathway for proline production is well-known, since a considerable amount of research has been done with it in the past. An enzyme called D1-pyrroline-5-carboxylate synthetase (P5CS) plays a key role in the production of proline. Past research revealed that transgenic tobacco (Nicotiana tabacum) and rice (Oryza sativa) plants overproducing P5CS synthesized more proline than untransformed plants, which helped them better tolerate too much, or a lack of water.

Noticing the ability of P5CS to help plants survive in drought conditions, Dr. Yoshiba from the Central Research Laboratory in Saitama, Japan, and his colleagues wondered whether P5CS from Arabidopsis thaliana (AtP5CS) and from Oryza sativa (OsP5CS) would display similar functions in a commercially important plant like the petunia (Petunia hybrida).

In order to help petunias produce more proline, the research group first introduced the AtP5CS and OsP5CS genes into petunias. Afterwards, they compared the growth and proline content of these plants with that of untransformed petunias under drought stress (without water at 25° Celsius).

The untransformed petunia plants showed only little or no growth in the absence of water, and they did not survive after rewatering. The proline concentration in these plants amounted to 60 percent of total amino acid content, whereas untransformed plants growing under normal condition did not produce proline.

The genetically engineered Petuniae hybridae, however, showed normal growth. Dr. Yoshiba’s team discovered that the proline content in these petunias was up to 3.5 times the content of untransformed plants growing under normal conditions.

The researchers could further observe that the higher proline content in the transformed petunias rendered them more tolerant to drought and that after rewatering, up to 53 percent of the plants were revived. Dr. Yoshiba and his colleagues therefore concluded that there must be a correlation between the survival rate and the proline accumulation in genetically modified petunias.

To investigate the influence of proline on plant growth, the researchers added some additional proline to young untransformed petunias. After the addition of proline, the plants showed clear stress symptoms: they accumulated about 100 times more proline, their root growth was retarded and their leaves became yellowish. The severity of these stress symptoms was directly correlated to the addition of proline.

Compared to previous experiments with Arabidopsis in Dr. Yoshiba’s lab, however, the proline content in the young untransformed Petunia hybrida was up to 18 times higher, which reduced root growth and leaf production. Thus, the researchers suggest that young untransformed petunias are hypersensitive to sudden increase in proline.

In contrast, adult transgenic petunias had a proline content as high as one percent of the total amino acids and did not show any stress symptoms. In adult transgenic tobacco plants, however, almost half of the total amino acid content was proline and the plants showed an enhanced drought tolerance.

From these results, the researchers suggest that the proportion of proline to the total amino acid content – rather than the concentration – is what influences the growth of petunias under drought conditions. In other words, when a plant constantly produces higher levels of proline it is more capable of surviving, when proline levels increase due to drought conditions.

Looking at the prospect of increasing proline amounts in other plants due to the success of this project, Dr. Yoshiba was cautious and explained, the ability for other plants to tolerate proline might depend on the plant species or the growth stages.

Dr. Yoshiba’s work provides a good corner stone for producing drought tolerant petunias through the use of AtP5CS and OsP5CS. By so doing, the increasing demand for water conservation can be met, while also providing florists with a better product. Most notably however is that Dr. Yoshibi’s work has laid the ground work for producing many other varieties of water-saving flowers and crops.

Katharina Schoebi is a biologist and Chief Science Writer for Checkbiotech. Contact her at katharina.schoebi@bluewin.ch.

Yoshu Yoshiba et al. Effects of free proline accumulation in petunias under drought stress. Journal of Experimental Botany. (2005) 56, pp. 1975-1981

Link to the abstract: http://jxb.oxfordjournals.org/cgi/content/abstract/56/417/1975

Contact:
Yoshu Yoshiba
Central Research Laboratory
c/o Advanced Research Laboratory
Hitachi Ltd.
2520 Akanuma
Hatoyama-cho
Hiki-gun
Saitama 350-0395
Japan
Fax: +81 49 296 6006
Mail: yoshiba@harl.hitachi.co.jp