Amazing stuff!
"When inner and outer cell layers of sepals – the leaf-like plant organs that cover and protect the flower bud before it opens – all grow upward, the organs stay smooth and uniformly stiff, which is optimal for the plant, according to a new study.
The study ... compared a wild type Arabidopsis plant with a mutant one that grew with wrinkled sepals. For the first time, the researchers were able to image both the inner and outer layers of the sepal to understand factors that controlled smoothness and wrinkles. ..."
From the highlights and abstract:
"Highlights
• Smooth sepal formation requires aligned growth directions and comparable stiffness
• Ectopic expression of AS2 causes buckling of the outer epidermis
• Buckles form due to misaligned growth directions and unequal epidermal stiffness
• Buckling promotes the PIN1 convergence to initiate outgrowths
Summary
Nature exhibits organs of various shapes and forms, ranging from smooth to undulated morphologies.
How cells coordinate their growth to produce smoothly shaped leaves and leaf-like organs such as sepals remains unclear.
We identified a mutant, as2-7D, that exhibits ectopic expression of ASYMMETRIC LEAVES 2 (AS2) on the outer epidermis.
Our analysis reveals that ectopic AS2 expression causes the outer epidermis of as2-7D sepals to buckle during early stages of sepal development.
Buckling is caused by conflicting cell growth directions and unequal tissue stiffness across the epidermal layers.
Overexpression of cyclin-dependent kinase (CDK) inhibitor Kip-related protein 1 (KRP1) in as2-7D aligns the growth directions of the outer epidermal cells along the longitudinal axis, increases the overall stiffness of the outer epidermis, and restores sepal smoothness.
Further experiments suggest that buckling promotes the convergence of auxin efflux transporter protein PIN-FORMED 1 (PIN1) to initiate pointed outgrowths. Thus, we show that growth along the longitudinal axis during early developmental stages and comparable stiffness across both epidermal layers of Arabidopsis thaliana sepals are essential for smoothness, as seen in the wild type.
Our findings suggest that in addition to molecular cues influencing tissue mechanics, tissue mechanics can also modulate molecular signals, giving rise to well-defined shapes."
Growth directions and stiffness across cell layers determine whether tissues stay smooth or buckle (open access)
Graphical abstract
Figure 1. The ectopic expression of AS2 on the outer epidermal layer disrupts sepal smoothness in as2-7D mutant
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