2018_05 Cool Rice
"Cool down" the plant version
Stress is part of life and helps us to grow with a challenge. However, when organisms are sensing stress all the time, they become sick. Many civilisation disorders are linked with impaired processing of stress signals. When a stress signal had been deployed, it has achieved its function and therefore has to be switched off again. This holds true for plants as well: Here, it is jasmonic acid that steers perception of and adaptation to stress. In previous work we could show that salt stress in grapevine cells leads to successful adaptation, when the level of jasmonic acid increases rapidly, but also decreases afterwards. When the levels remained elevated this led to cellular suicide (so called programmed cell death)
Thus, it is not only the molecular nature of a signal (in both cases jasmonic acid), but also the temporal pattern of activation and dissipation of a stress signal that matters (we use the term signature to refer to this phenomenon).
In fact, plants have developed different approaches to switch off stress signals. For instance, the processing of the jasmonic acid signal is blocked by so called jaz proteins. Interestingly, the genes encoding these "off-switcher" are among the first that are activated by jasmonic acid. This will constitute a negative feedback which will ensure that the stress signal jasmonic acid will disappear rapidly, once it has been deployed.
Can we get "cooler" rice?
These considerations and results inspired the following idea: if we tailor the jasmonate signature, we should render rice (a plant highly susceptible to salt stress) "cooler". This idea was put into reality by combining the parts of two genes: we used the protein-coding sequence of the gene JAZ8 (among the numerous jaz genes of rice, this member is the central quencher for salt stress) and placed this sequence behind a promoter that is stronlgy activated by salt stress. We used here the promoters of zos11 and zos12, gene switches with high salt responsiveness. When this chimeric construct would be introduced into a cell, this should not change anything under normal conditions, because the promoters would remain silent. The cell should therefore develop normally. However, in the very moment, when it would be challenged by salt stress (producing a lot of jasmonic acid), the promoter should become active and drive the formation of the JAZ8 protein, which would immediately switch off the stress reaction, such that a perfect jasmonate signature would be generated (rapid activation, rapid switch-off). We tested this first with tobacco BY-2 cells that are easier to handle and could show that our strategy allowed to steer the level of jaz8 expression by salt and jasmonic acid. The cells harbouring our chimeric construct were able to cope with salt stress and even continued to proliferate. Encouraged by this result, we changed to rice. Genetic engineering of rice is a cumbersome activity and it took almost three years, until we succeeded, with the help of colleagues in Montpellier, to get enough seeds from these engineered plants to do our tests. Eventually, we succeeded, and it worked. The engineered rice produces more jaz8 under salt stress and does not shed the leaves, but tries to sequester the salt in the vacuole, such that the plants continue to grow.
What is the use of this research?
We do not pursue the plan to address the problem of salt stress by transgenic rice plants (in Southeast Asia a huge problem - Bangladesh is almost every year flooded by the sea). Our work showed that tailoring of jasmonate signatures can improve stress resilience of rice. Together with colleagues from the International Rice Research Institute (IRRI) on the Philippines we search for variants of the jaz8 promoter in wild species or old land races of rice. This "cool" version of the gene switch can then be crossed into high-yielding varieties. Since the desired gene sequence is known, one can screen the progeny of the cross and find the individuals that have got the "cool" version of the jaz8 switch, while harbouring the genes for high yield from the other parent (so called smart breeding). These plants are not transgenic, since only natural sexuality was used to generate them. However, genetic engineering was used to design this approach.
139. Peethambaran PK, Glenz R, Höninger S, Islam S, Hummel S, Harter K, Kolukisaoglu Ü, Meynard D, Guiderdoni E, Nick P, Riemann M (2018) Salt-inducible expression of OsJAZ8 improves resilience against salt-stress. BMC Plant Biol 18, 311 - pdf