Effect of Induced Mutations in the Barley Genes xan-n, xan-i, xan-j and xan-b

Popular Abstract

Decoding Colors: How mutations in barley genes affect pigmentation

Plants are an incredibly diverse group of organisms that share one crucial component: the presence of a green pigment called chlorophyll. Chlorophyll plays a pivotal role in capturing light energy and facilitating the process of photosynthesis, which converts this light energy into chemical energy. The green appearance of plants is a consequence of this particular process: all colors of light except green light are used by the plant to synthesize sugars. Therefore, it is especially notable when the process is disturbed, and the green color vanishes. Barley stands out as a compelling subject of study due to its immense importance in sustaining the global human population. Understanding the cellular function with a direct connection to developing better crops, amplifies this research from fundamental lab science to a real-life purpose. The absence of green pigmentation in certain barley mutants is what I explored in this project. These barley mutants were created through genetic modifications in the mid-20th century, resulting in the emergence of four distinct plants exhibiting unique characteristics. These mutants were named xantha-n, xantha-i, xantha-b, and xantha-j, based on their physical appearance, and the specific genetic mutations responsible for their distinct traits were identified.

I approached understanding the impact of the mutations on the plants from three key angles: theoretical comparison to well-investigated plants like Arabidopsis, a whole-plant perspective of how the mutants grow, and a cell-focused perspective of what the plant actually contains. The latter contained a review of chlorophyll content, the precursors of chlorophyll in its biosynthetic pathway, and a general regard of overall protein presence. In combination, these results can hint at disruptions in the intricate developmental process of the plant cell. Central to this process is the formation of the chloroplast, the cellular compartment where photosynthesis occurs, and the assembly of essential components for photosynthesis. These aspects are finely intertwined, forming a complex regulatory network to ensure the healthy development of cells and the effective functioning of photosynthesis. In other words, nothing is left to chance – and every given small part of the cell matters with great impact.

It was not surprising to find that the four mutants exhibited substantial variations in their affected cellular functions, given the interconnected nature of processes within the cell. However, a common thread among them was the disturbance in cellular compartment development rather than a direct impact on the chlorophyll biosynthesis pathway. The proteins associated with the mutated genes highlighted the extent of interdependence between plant cell developmental processes.
In xantha-n, the mutations led to functional disruption of the protein XanN, which is an enzyme known as superoxide dismutase. XanN appears to be a vital component of a complex tool responsible for chloroplast construction and the removal of harmful reactive oxygen species (ROS). The absence of XanN resulted in arrested chloroplast development in the presence of light, leading to a white color, the complete loss of chlorophyll and its precursor, as well as a marker for photosynthesis known as Rubisco.
Conversely, xantha-b mutants exhibited a pale green appearance and retained some chlorophyll. The severity of the mutation correlated with the plant's color and chlorophyll content. XanB, the protein the product of gene xan-b, was suggested to be involved in supporting the mitochondria in properly constructing its genetic components. XanB belongs to a family of proteins known as Pentatricopeptide Repeat Proteins, which can bind to nucleic acids like DNA and RNA. Unlike in xantha-n, chloroplast development was not found to be inhibited in xantha-b mutants.
In xantha-i, the mutations appeared to interfere with the breakdown of proteins involved in photosynthesis, resulting in plants with a yellowish hue. Light energy has a strong potential of damaging proteins which consequently need to be replaced to ensure continuous functionality of the photosynthetic process. XanI is indicated to play a role in degrading light-damaged proteins in the chloroplast. Although other proteins are highly similar to XanI, none can replace its function as a protease in its distinct cellular context, leading to the yellow appearance.
Xantha-j, on the other hand, presented a clear case where the mutation affected the final step in the chlorophyll biosynthetic pathway. Without XanJ, there cannot be chlorophyll. In spite of this clear functional definition, the plant's appearance continues to be difficult to understand with the methods applied. The higher the light, the more a specific mutation in xantha-j showed burned areas in the leaf while another did not.
In summary, this study illustrated that disruptions in various cellular processes could alter the plant's appearance dramatically. To gain a more comprehensive understanding of these mutants, a microscopic examination of cellular compartment development could yield valuable insights into how the photosynthetic apparatus can be affected in diverse ways and how these effects manifest in the plant's pigmentation.


Master's Degree Project in Plant Science, 45 credits, 2023
Department of Biology, Lund University

Advisor: Mats Hansson
Molecular Cell Biology Unit, Department of Biology

• author: San Torcuato, Anna
• supervisor: Mats Hansson
• organization: Degree Projects in Biology
• course: BION02 20222
• year: 2023
• type: H2 - Master's Degree (Two Years)
• subject: Biology and Life Sciences
• language: English
• id: 9140776
• date added to LUP: 2023-11-02 11:58:45
• date last changed: 2023-11-02 11:58:45