Polygalacturonase: Does it affect plant shape?

Monika Raman, PSG College of Technology, Coimbatore

Plant architecture regulation is a priority in recent breeding efforts. Previous researches have improved the understanding of the genetic regulation of plant architecture, but understanding their morphology at the cellular level is as critical. 

Cell division and cell expansion work together to shape organs, and the cell wall plays a vital role in both. The cell wall must relax due to its stiff nature for a cell to elongate. Plants can reduce the rigidity of their cell walls by degrading the polysaccharides that make them up. Pectin, a polymer having a backbone made up of galacturonic acid (GalA) units, is one of the polysaccharides found in plant cell walls. Pectin degrading enzymes like polygalacturonases play a crucial role during cell development. It catalyzes the hydrolytic breakage of the bonds between GalA units. Although polygalacturonases are known to aid cell proliferation, their role in organ morphogenesis remains unknown.

Leaf morphology:

The development of adaxial–abaxial polarity is critical for leaf shape. The densely packed palisade cells on the adaxial side have a characteristic cylindrical form, whereas the spongy mesophyll cells on the abaxial side are less regularly organized and loosely packed, and they can be easily distinguished in wild-type plants.

Aside from adaxial–abaxial patterning, leaf form is determined by epidermal cell layer proliferation. Epidermal cells stop dividing and start expanding when the leaf blade matures and takes on its mature shape. This movement happens in a basipetal manner, commencing at the leaves tip and working its way down to the base.

Since the functions of Polygalacturonases in morphology is yet to be clearly understood, to deal with this issue, Yang Yang and colleagues from Hainan Institute of Zhejiang University, Sanya, China, screened a set of polygalacturonase mutants. They discovered a Polygalacturonase45 (PG45) mutant with curved rosette leaves. They chose to look into this mutant further and discovered that it had other developmental flaws, such as fewer floral organs and a bushy appearance. The researchers started to look at what causes the curving rosette-leaf phenotype because they were intrigued by the subject of how a polygalacturonase gene might alter plant shape.

How does polygalacturonase affect plant morphology?

The authors cut cross-sections of rosette leaves to see if the twisted leaves were due to disrupted leaf polarity. Yang Yang, the lead author of this study said, “In the mutant, we could hardly distinguish the palisade mesophyll layer from the spongy mesophyll layer”. The absence of adaxial–abaxial patterning in the mutant implies that PG45 is important in leaf polarity formation or maintenance. 

The authors stated that they selected slices that were 25% or 75% from the leaf tip and measured the proportion of cells browsing the mitotic index in each to observe how the epidermis of the PG45 mutant grows. They did this using a fluorescent marker to visualize the mitotic spindle and phragmoplast microtubules in dividing cells. In wild-type and mutant plants, the mitotic index was similar amongst leaf areas. According to this computation, PG45 does not affect the proportion of cells that undergo mitosis in a particular leaf region.

Following that, they determined the mitotic index ratio between the 25% and 75% regions. They found that the mitotic index ratio of PG45 leaves was lower in the adaxial epidermal layer than in the abaxial epidermal layer, while there was no difference between the adaxial and abaxial epidermis in wild-type leaves. These outcomes show that PG45 deficiency causes a faster transition from cell division to cell expansion in the adaxial epidermis, resulting in an uneven cell count on both epidermal sides.

Cellular functions of PG45:

Anderson, the co-author of this study said, “To gain insight into the cellular function of PG45, we also analyzed pectin metabolism in wild-type, PG45 mutant, and PG45-overexpressing plants”. Oligogalacturonides (OGs) or GalA monomers are released when pectin degrades. The author’s utilized high-performance size exclusion chromatography coupled with mass spectrometry to evaluate the OG content. According to the author, the mutant accumulated more OGs made up of six GalA units (GalA6) than wild-type plants. GalA6 levels were lower in overexpressed PG45 plants than in wild-type plants, but GalA2, GalA3, and GalA4 levels were higher. These results imply that PG45 may degrade GalA6 specifically, resulting in the buildup of shorter OGs.

How PG45 impacts leaf polarity and the transition from cell division to cell growth is still unknown. “The Oligogalacturonides material, we believe, will be significant,” Anderson continued. Auxin and cytokinin signalling is known to be affected by OGs of different duration. These hormonal routes are essential for leaf polarity and cell division length during leaf initiation, respectively. As a result, PG45 may affect leaf polarity by generating OGs that feed into the auxin and cytokinin pathways. 

Yang Yang and colleagues concluded that polygalacturonase regulates the balance between cell division and growth. Still, further studies are required to determine the biochemical mode of action of PG45 by analyzing its structure and characterizing its biochemical activity on specific substrates. They may provide more insight into the role of cell wall modifications during plant development. 

Also read: ddPCR: Analyzing three DNA repair pathways together!

References:

• Yang, Y., Anderson, C. T., & Cao, J. (2021). Polygalacturonase45 cleaves pectin and links cell proliferation and morphogenesis to leaf curvature in Arabidopsis thaliana. The Plant Journal, 106(6), 1493–1508. https://doi.org/10.1111/tpj.15308
• Verhage, L. (2021). Get in shape – how a polygalacturonase affects plant morphology. The Plant Journal, 106(6), 1491–1492. https://doi.org/10.1111/tpj.15366

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Author info:

Monika Raman is an undergraduate student pursuing her final year B. Tech in Biotechnology. She is an enthusiastic Biotech student aspiring for an opportunity to develop skills and grow professionally in the research field. Extremely motivated and possess strong interpersonal skills. Read some of her published articles at BioXone:

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