In a groundbreaking study published in Nature Plants, researchers from the Center for Excellence in Molecular Plant Sciences at the Chinese Academy of Sciences and Hubei University have provided unprecedented insights into the rise of seed plants. Examining the evolution of specialized cell walls, the study challenges long-standing assumptions, revealing a key role played by suberin lamellae in the ascent of seed plants over 300 million years ago.
Seed plants, constituting two-thirds of all plant species and dominating the Earth’s flora, emerged as the predominant life forms during the Carboniferous period. This era, characterized by towering tree ferns, saw the formation of coal resources that persist today. However, a climatic shift at the end of the Carboniferous period created a cold and arid environment, leading to the decline of ferns and the subsequent rise of seed plants.
Central to the researchers’ investigation was the exploration of roots, particularly the endodermis—the core of the root responsible for water and nutrient transport. The endodermis features a Casparian strip and suberin lamellae in its cell wall, acting as barriers to prevent the free diffusion of substances.
Contrary to previous assumptions, the researchers discovered that the Casparian strip is present in all vascular plants, including ferns, lycophytes, gymnosperms, and angiosperms. However, suberin lamellae are exclusive to gymnosperms and angiosperms, collectively known as seed plants.
The study delved into the molecular evolution of genes related to suberin lamellae formation, uncovering a significant expansion in the common ancestor of seed plants. This expansion, resulting from gene duplication, indicated functional innovations that allowed for the emergence of genes responsible for synthesizing suberin lamellae.
To validate their findings, the researchers examined homologous genes in various plant groups, confirming that the expanded genes in gymnosperms and angiosperms initiated suberin lamellae formation, enhancing drought adaptability. This led to the hypothesis that the rise of seed plants was facilitated by the emergence of suberin lamellae, providing waterproof properties that improved drought resistance.
Experimental evidence supported this hypothesis, demonstrating that suberin-deficient Arabidopsis plants were more sensitive to drought. Advanced analytical techniques, including Raman spectroscopy and nuclear magnetic resonance, highlighted the crucial role of suberin lamellae in enhancing the efficiency of vascular water transport.
The researchers proposed a model for the rise of seed plants, suggesting that in the moist conditions of the Carboniferous period, fern plants thrived due to their higher water and nutrient absorption efficiency. However, the onset of a dry climate at the end of the Carboniferous period favored seed plants with evolved suberin lamellae, providing a more efficient water transport system and enhanced drought tolerance.
This study not only unveils the mystery of the Casparian strip and suberin lamellae but also introduces a new perspective on the evolutionary advantage that drove the rise of seed plants. The identification of suberin lamellae’s crucial role in plant adaptation to adverse conditions, such as drought, holds significant implications for enhancing drought resistance in crops and advancing our understanding of plant tolerance mechanisms.