Understanding flower development – targeted increase of plant biomass

In times of a rapidly growing world population, advancing climate change and a shortage of arable land, securing global food ­supplies is one of the greatest global challenges facing humanity. In order to meet this challenge, a drastic increase in agricultural production is an outstanding goal, in addition to a fairer distribution of food. Increasing plant biomass, i.e. increasing the yield of crops, is of fundamental importance in this context and is consequently the focus of modern plant breeding. In the “Functional and Applied ­Genomics” department of Fraunhofer IME at the locations Aachen and Münster, we investigate the floral induction mechanisms of plants. Based on this knowledge, we can increase biomass and yield of plants using modern breeding methods.

The timing of flower induction in plants is a tightly regulated process, as it determines the reproductive success of the respective species. Both external environmental stimuli (exogenous factors), such as the day length or a mandatory exposure to cold, as well as internal (endogenous) factors, such as the age of the plant, play important roles in this regulation. Even though the basic factors of flower induction are similar in many plants, this is a species-specific process, since the architecture of the plant body, the habitat and the age entering the flowering stage are often very different. Thus, a basic understanding of the decisive factors and processes is of fundamental importance, especially for conventional, but also for alternative, novel crops, as they directly influence the yield. The rapidly growing world population and the effects of climate change, such as prolonged periods of drought or flooding, are the driving forces behind modern plant breeding in terms of increasing yields and producing more adaptive plants.

The central factor of flower induction of almost all plants studied to date is responsible for triggering the entire cascade of flower formation and development: This so-called Flowering Locus T (FT) is within the focus of our research group. We aim to understand how FT is regulated, along with its downstream genes. Ultimately, we hope to be able to increase the biomass and thus the yield of crops by specifically modulating FT.

<b>External environmental factors and internal plant processes determine the timing of flowering. </b> <br> This process is strictly regulated, as it is essential to ensure the existence of the respective species. At the molecular level, all these factors target the expression of FLOWERING LOCUS T (FT), a central mobile stimulus of flower induction described in many species. For our model species tobacco we could identify a special case of FTs: In addition to flower-activating FTs (FT4-5), tobacco also possesses flower-inhibiting FTs (FT1-3), which are transported via the phloem into the shoot apical meristem (SAM) where they compete for FD and thus determine the optimal flowering time.

Flowering Locus T (FT) – the key regulator of flower development

As early as the 1930s, botanists in pioneering studies on floral induction concluded that there must be a mobile flowering ­stimulus - a so-called florigen - which is formed in the leaves and then transported to the tip of the shoot. Here it terminates the vegetative growth of the plant by mediating the transition from a vegetative to a reproductive meristem. Many decades later, this mobile stimulus of floral induction was described on a molecular level as Flowering Locus T (FT).

In innumerable plants, his regulator is formed in the conductive tissues of the leaves and then transported via the phloem from the leaves to the apical meristem of the shoot tip(s). Here FT interacts with its partner, the so-called FD, and subsequently activates all processes necessary for floral induction and flower development. Although FT is the central module of floral induction across species, it is regulated by diverse factors in different plant species. Depending on the location of the plant, day length plays a decisive role in the availability of FT, while other plants need a period of cold (winter) to activate FT and initiate their flowering process. Our research group is working on elucidating the mode of action of FT in plants of the Solanaceae family. This includes our model plant tobacco as well as important crop species such as potato and tomato.

The targeted suppression of flowering leads to an enormous increase in biomass - not only under greenhouse conditions.

Yield increase by targeted modulation of activators and repressors of flower induction

Our research already resulted in the identification and characterization of several FTs. In addition to the FTs that activate floral induction, we also identified a special case of FTs in these plant species: These are FTs preventing the formation of flowers, i.e. repressing them. This is in a similar form only described for a few other crops like sugar beet or sunflower. Based on the molecular characterization of FTs that have a repressive effect on floral induction, a special focus of our current work is on the use of genetic engineering to achieve a remarkable increase in plant biomass.

By specifically modulating the plant, we have succeeded in providing larger quantities of this repressive factor and thereby keeping the plant in the vegetative growth stage. The inhibition of flowering prolongs the vegetative growth, which enables us to control the morphology and thus the yield and biomass of the plant. Such optimized tobacco plants reached heights of around five meters and thus an increase in biomass of up to 60 percent due to the leaf size alone, further increased enormously by the formation of up to 10 times more leaves. The elucidation of the mode of action of activating versus repressing FTs now allows us to precisely regulate plant architecture, flowering time and seed quantity and thus the yield realized by use of modern breeding methods such as CRISPR/Cas. In this way, we achieve economically relevant and, above all, field applicable breeding objectives.

 

Leaf biomass of tobacco plants cultivated at 15 °C increased temperature and normal irrigation. left: unmodified plant, right: FT-optimized plant

Even under stress, FT-optimized plants still maintain their positive agronomic traits

In view of global climate change and the steadily increasing world population, plants that are more resistant or tolerant to changing environmental conditions such as heat and drought will be indispensable for securing the food supply.

Our latest studies show that plants in which we have reduced the levels of activating FTs using modern breeding methods maintain their positive agronomic traits such as increased biomass and seed production even under various abiotic stress conditions. Normally, tobacco plants exposed to temperatures only 5 °C higher than normal conditions are significantly retarded and leaf biomass is substantially lower. FT-optimized plants generated with our technique, even when grown at 15 °C higher temperatures, produced biomass similar to that of wild type plants cultivated under normal conditions.

Thus, FTs acting as floral inducers are promising candidates for breeding programs aiming at adapting various crops to the effects of climate change. For example, it would be interesting to test this breeding strategy in vegetative plants such as potatoes, sugar beet or lettuce, where carbon and energy distribution into the vegetative organs is desirable and a prolonged vegetative growth phase by modulating the activity of FTs could be beneficial. In the case of potato, research on the specific functions of the different FTs is of particular interest. In this plant, tuber formation is also dependent on the availability of certain FTs and a targeted modulation of FT levels could further increase the yield of potato plants even under changing environmental conditions.

 

This project again shows that the combination of basic research to elucidate the function of various key factors in flower development with the goals of applied research, such as the targeted breeding-based optimization of these key factors, is a central, successful and effective approach. In terms of modern plant breeding, this represents a particular incentive and a major challenge.

 

 

Selected publication

 

Schmidt et al.
The Major Floral Promoter NtFT5 in Tobacco (Nicotiana tabacum) Is a Promising Target for Crop Improvement (2020) Frontiers in Plant Science, 10, 1666