American botanists have discovered that the spatial orientation of plants is determined not only by their tendency to grow upwards, but is actually the result of a balance between two signaling systems. During their research, they described the SLQ1 protein, which actively stimulates stem growth downward. This finding challenges the traditional view of gravitropism, revealing that plants balance between "upward" and "downward" signals.

How Plants Determine Growth Direction

The ability of plants to sense gravity and grow against its direction (negative gravitropism) is linked to proteins from the LAZY family. Specialized statocyte cells contain statoliths—heavy starch grains associated with LAZY proteins. Under the influence of gravity, statoliths settle at the bottom of the cell, where LAZY proteins interact with the membrane and alter the flow of the hormone auxin, causing the stem to bend and grow upwards.

If LAZY genes are removed, mutant plants (so-called lazy mutants) lose their ability to grow vertically and instead sprawl along the ground. Previously, it was believed this happened because the plant lost its ability to orient itself to gravity, simply "falling" and growing horizontally.

The Discovery of the SLQ1 Protein

In a new study published in the journal PNAS, scientists conducted a genetic screening of Arabidopsis mutants lacking all major LAZY genes. Their goal was to find an additional mutation that could restore the disrupted mechanism of gravitropic growth. As a result, they discovered a mutant that, despite lacking LAZY proteins, began to grow vertically again. Genetic analysis revealed that this "recovery" was due to a mutation in a gene named SLQ1 (Suppressor of Lazy Quadruple 1). Researchers studied the structure of the SLQ1 protein, its cellular localization, and its interaction with the hormone auxin.

The study showed that "lazy" plants sprawl along the ground not because of weakness or loss of orientation, but due to the active function of the SLQ1 protein. In the absence of LAZY, it is SLQ1 that controls the flow of auxin in such a way that the plant hugs the soil (positive gravitropism).

Balancing Signaling Systems

Under normal conditions, both systems operate in the plant: a strong LAZY signal pulls the stem upward, while a weaker SLQ1 signal pulls it downward. As a result, LAZY dominates, and the plant grows vertically. The SLQ1 protein works in tandem with the SETH6 protein, forming complexes at the contact points between the endoplasmic reticulum and the cell membrane, which affects hormone transport.

The most unexpected discovery was that when both systems—"upward" (LAZY) and "downward" (SLQ1)—are disabled, the plant does not completely lose its orientation and retains a weak ability to grow upwards. The mechanism of gravity sensing independent of LAZY remains to be studied.

A New Understanding of Plant Growth

Plant growth is not the execution of a single command, but a dynamic balance between competing molecular pathways. By adjusting the balance between LAZY and SLQ1 proteins, a plant can precisely control the angle of branch growth, which is crucial for forming an optimal crown architecture. Further study of mechanisms independent of LAZY may lead to a revision of existing theories of gravitropism.