What we did to plants decades ago, in an effort to save us from famine, has backfired, weakening most crops. Scientists say a major share of thanks goes to the Green Revolution.

Although a necessary move back then to increase yield and put a pause to famishment, plants’ resistance mechanisms have weakened and made them vulnerable to erratic climate change.

The good news is, plants haven’t forgotten their strength. Scientists worldwide are trying to understand how this ‘hidden memory’ of a plant’s strength can be amplified to fight against climatic stress such as high salt levels that wouldn’t let the plants thrive. Such efforts include understanding the epigenetic regulation of genes under stressful conditions.

Last year, another work in the same league came out from a collaborative study and was published in the journal Nature Plants.

Researchers at the National Centre for Biological Sciences (NCBS), Bengaluru, studied a rice-family (Oryza) specific histone gene in rice plants and demonstrated that disrupting it left plants less able to cope with high salt exposure.

H4.V: Identifying the silent variant

The research looked into histones, which are primarily responsible for organising DNA into packages. Histones can be imagined as spools that DNA wraps around to sustain its shape. The researchers studied a histone variant to know more about different traits the plants exhibit when subjected to varied stressful environments.

While the NCBS team is not the first to explore histones, their finding double-checked a special variant of histone (H4), which remained a mystery until the team discovered it.

In focus: H4.V, a special variant of H4.

As they discovered, H4.V helped the rice plants survive highly saline environments. The variant turns on certain genes under saline conditions and silences their function otherwise.

The researchers, as part of their method, modified rice plants to remove H4.V and observed that the gene activity was on par with plants under salt stress. Here, they did not expose these plants to saline conditions. Even then, the results showed that the plants behaved as if they were put in saline environments, a process called transcriptome resemblance.

The knockout game

In normal conditions (optimal exposure to salinity), H4.V is usually put up at specific regions of DNA called heterochromatin without much redistribution. But under salt stress, H4.V could redistribute and reach out to new binding areas where salt-stress protein-coding genes reside.

In the lab, researchers confirmed the pertinence of the H4V variant in a knockout experiment, as the H4.V variant gets deleted. First up, the plants already show a transcriptome resemblance “as if” they were stressed in a saline environment. Up next, knockout plants poorly respond to actual salt stress. Simply put, the knockout plants that lack H4.V show poor tolerance to saline environments.

A fortunate division of labour (H4.V & H4K5ac)

In plants with H4.V intact, salt stress causes the variant to occupy the gene bodies. However, it does not occupy the entire gene evenly. In the left-out position of the gene, another marker shows up, known as H4K5ac (an acetylation tag) and sits at a crucial site.

This site, incidentally, acts as the “start button” at the start site of transcription (read: a message for the plants to act as if they are under stress).

Simply put, while the H4.V moves out, the H4K5ac tag shows up at the start site, prompting a salt stress response. In technical words, the authors labelled it as “An Oryza-specific histone H4 variant predisposes H4 lysine 5 acetylation to modulate salt stress responses.”

The authors concluded that the function of H4.V is that of a gatekeeper. Under normal conditions, it sits silently in heterochromatin. That said, under salt stress, the variant relocates to protein-coding genes and promotes H4K5ac deposition and amplifies salt-responsive gene expression. Such a mechanism, as the authors note, could be the reason behind the semi-aquatic character of Oryza.

From lab bench to field?

Over 50% of arable land will be in salt-affected regions by 2050, and salinisation poses a major threat to contemporary agriculture. That too with less supply of water, growing rice crops has become a daunting agricultural task.

While the paper doesn’t prescribe any solution for farmers that can be directly applied in the field, the molecular research adds another step in cultivating resilient crops.

Targeting the H4.V, either by preserving or enhancing it, could become a tool for crop breeding or crop resilience programmes and improve the salt tolerance capacity of these crops.

But one needs to be cautious of overstating the application of the research beyond the laboratory set-up. A field-ready solution could take years, involving careful trials, regulatory approvals, and awareness.