A broken strand of DNA is one of the most serious threats a cell can face. Across plants and animals, a protein called H2A.X has long been known as one of the cell’s first responders to such emergencies, helping detect and repair damaged DNA. Turns out, this molecular firefighter had another job altogether in rice plants.
A new study at NCBS found that H2A.X does far more than respond to crises. In rice, this protein quietly influences how roots grow, when plants flower, how green their leaves become, and even the colour of their stems and seeds. The findings uncover an unexpected role for H2A.X as a regulator of plant development, acting through subtle changes in the way DNA is packaged inside the cell.
Inside the nucleus, DNA is wrapped around proteins called histones, forming a compact structure known as chromatin. Histones are not just passive spools that hold DNA together. By carrying chemical tags and variants, they influence which genes are switched on and which remain silent. H2A.X is one such histone variant. Although scientists have extensively studied its role in DNA repair, recent reports of its presence in gene bodies in the model plant Arabidopsis have suggested additional functions.
“Because H2A.X is found throughout the genome even in the absence of DNA damage, we suspected that it might be doing something more than repairing DNA,” says Aravind Madhu, the lead author of the study. To investigate this, Aravind and team engineered transgenic rice lines in which H2A.X was either knocked out or overexpressed. Plants without H2A.X developed longer roots and greener leaves, flowered later, and produced more tillers. However, they also suffered from reduced fertility, producing fewer and smaller seeds. Plants in which H2A.X were overexpressed, displayed a different set of stress associated abnormalities. They had shorter primary roots, increased levels of anthocyanins (the pigments responsible for red and purple colours) and defects in reproductive structures.
To understand what was changing inside these plants, the team performed RNA sequencing. Hundreds of genes involved in root growth, photosynthesis, and pigmentation behaved differently when H2A.X levels were altered. These changes had little to do with DNA damage responses, but were more associated with plant development. Instead, the answer lay in the altered histone marks decorating the genome. Cells use a wide range of histone modifications to control access to genes. One such modification, called H3K4me3, is usually associated with transcription start sites of the active genes. Genes carrying this mark are generally easier for the cell to read and express. Using genome-wide experiments, Aravind and team discovered that plants lacking H2A.X accumulated far more H3K4me3 marks than normal.
“H2A.X appears to act as a brake,” says Aravind. “It prevents active chromatin marks from spreading too widely across the genome,” he added. Without H2A.X, these activating marks spread across thousands of gene bodies, particularly those involved in development. This molecular imbalance helps explain the dramatic changes in plant growth and reproduction seen in the mutant plants.
The team discovered that H2A.X also inhibits another histone variant called H2A.W, which is found only in plants. Unlike H2A.X, H2A.W is associated with tightly packed regions of the genome and with histone marks that silence genes and transposable elements (stretches of DNA capable of moving around in the genome).
The research team combined ChIP seq experiments for H2A X, H2A.W and their associated histone marks, and found evidence that the two histone variants compete for space on the DNA. In plants lacking H2A.X, H2A.W spread into new regions, bringing along the repressive histone mark - (H3K9me2). In other words, H2A.X helps maintain a delicate balance between activating and silencing signals within the genome. This balancing act may be particularly important in rice. Compared to model plants like Arabidopsis, the rice genome contains many more transposable elements interspersed among genes. Managing which regions remain active and which stay silent is therefore a far more complicated task.
Plants that overexpressed H2A.X resembled plants experiencing stress. They accumulated anthocyanins, molecules often produced under harsh conditions, and displayed gene-expression patterns similar to those seen after DNA damage. This suggests that the protein may help plants coordinate the transition between normal growth and stress responses. The researchers also explored the role of FACT, a protein complex already known to incorporate H2A.X onto DNA. Plants in which FACT was knocked down displayed many of the same defects as H2A.X mutants, indicating that at least some of FACT’s developmental functions depend on this histone variant.
“It is extremely valuable to learn that a master regulator of this kind is contributing to yield traits. The genes under H2A.X histone variant’s control are useful candidates to improve crops,” says Prof. PV Shivaprasad, the Principal Investigator of the study.
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