One of the most intriguing stories from the evolutionary saga is the adaptation of plants to grow on terrestrial terrains that were initially inhospitable. Specific molecular pathways and messages allowed early plants to flourish away from the water. Researchers have recently highlighted one such molecular process involving a particular protein, DELLA, which has been involved in the growth regulation of early vegetations like mosses and liverworts, dating back to the ages before the Earth was blanketed by flowering plants.
The DELLA protein acts as a molecular suppressor, restraining or inhibiting plant cell multiplication. Notably, this functionality is prevalent across land plants, including critical crops such as rice and wheat. The conventional mechanism involves the hormone gibberellic acid (GA) in controlling DELLA. However, researchers indicate that the early terrestrial plants evolved an alternative pathway not dependant on GA.
A study, published in Nature Chemical Biology, enlightened the scientific community about this separate pathway in an early liverwort species, Marchantia polymorpha. This liverwort is a simplistic, green vegetal structure that expands over humid soil. The study’s insights could fundamentally influence the ways of enhancing agricultural yields by applying a more accurate control over plant growth, leveraged from keys buried in evolutionary history.
In the case of flowering plants, the DELLA protein is disassembled when the gibberellic acid binds itself to a receptor identified as GID1. This creates a triplex – GA, GID1, and DELLA – ultimately leading to DEllA’s elimination by being targeted by the cells’ trash disposing machinery, referred to as the ’26S proteasome’. On being rid of DELLA, the plant proceeds to grow. However, early terrestrial colonisers, such as mosses and liverworts, lacked the GID1 receptor and thus, necessitated an alternative DELLA-control mechanism.
The study focused on an enzyme designated as MpVIH in Marchantia polymorpha. The MpVIH enzyme synthesizes a molecular component named InsP8, falling under inositol pyrophosphates, a category of highly charged compounds that operate as chemical signals within the cell. Eliminating the gene responsible for MpVIH through gene-editing in the experimental plants, resulted in severe stunted growth, abnormal shapes and absence of gemma cups vital for their reproduction. The Scientists noticed the same defects in plants with excessive DELLA, indicating a connection between MpVIH and DELLA.
The team could mend the defective plants by incorporating only a portion of the MpVIH enzyme that played a role in generating InsP8. This substantiated that the InsP8 created by the enzyme was a significant factor. Subsequently, the researchers deliberated on the likelihood of an direct interference from InsP8 on DELLA, eventually verifying that the InsP8 latched onto the DELLA protein, instigating its breakdown.
Specifically, the InsP8’s binding with DELLA brought about an accumulation of ubiquitin molecules on DELLA; ubiquitin molecules are small proteins essential for cellular regulation. This acted as a marker for the ’26S proteasome’, signalling it to destroy DELLA. Essentially, the InsP8 in Marchantia performed the same role as GA does in flowering plants, albeit through an entirely distinct mechanism.
While being cleverly evaded by the plant sciences till now, they finally have conclusive evidence about how growth regulation could be managed in primitive plants without necessitating the GA-GIDA1-DELLA system.
Interestingly in contemporary flora, although a separate system to regulate DELLA has emerged, there is evidence to suggest that InsP8-binding sites persist. This could propose that the primitive system may be operating subtly, with potential to be reactivated. This forms the critical connection to contemporary agriculture and could further understanding and applications for human food production.
During the infamous ‘Green Revolution’, crop varieties with DELLA mutations were serendipitously selected since they produced ‘semi-dwarf’ plants, which performed better and stable under high fertilizer environments. The molecular ways in which this worked were not known back then; however, today we are beginning to see that understanding DELLA regulation via both old and new pathways could provide the exact tools needed to genetically engineer crops for higher yield, and improved resistance to adverse conditions.
The recent findings from the research provide the opportunity to adjust DELLA by not only manipulating gibberellin signals but also factoring in the ability to understand the inositol pyrophosphate pathway. This renewed dual approach allows finer tuning of plant growth under different circumstances, without having to completely eradicate DELLA, which could negatively impact stress endurance.
The growing global population and reducing arable lands have given rise to the urgent need to increase crop productivity. This study provides one potential pathway to achieve this goal.
This exploration into the cellular past also provides valuable perspective on the molecular evolution in land plants. From the ancient growth-suppressing role of DELLA to its modern-day management, each stage depicts the adaptive journey of plants to their ever-changing environment.
Despite substantial progress, various questions persist, like how MpVIH activity is regulated, the interaction of InsP8 pathway with other growth factors in the plant and how crop plants can be customized to maximize this primeval mechanism. Shedding light on these aspects could be the stepping stone to more efficient and resilient crops that can withstand the adversities of changing climate, water scarcity, and land shortage.
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