Researchers at the Department of Biochemistry, Indian Institute of Science (IISc), have uncovered a long-sought mechanism employed by primitive land plants such as bryophytes (including mosses and liverwort) to regulate plant growth – a mechanism that is conserved in more recently evolved flowering plants.
Published in Nature Chemical Biology, the study focuses on the non-canonical regulation of the DELLA protein – a master growth regulator that suppresses cell division in embryophytes (land plants).
“DELLA is like a speed-breaker, but if you have that speed-breaker in place all the time, you may not be able to move,” explains Debabrata Laha, Assistant Professor in the Department of Biochemistry and corresponding author of the study. It is, therefore, crucial to break down the DELLA protein in order to promote growth. In flowering plants, DELLA is degraded when the plant hormone gibberellic acid (GA) binds to its receptor GID1, resulting in the formation of the GA-GID1-DELLA complex. Subsequently, the DELLA repressor is tagged with a chain of ubiquitin molecules and degraded by the 26S proteasome machinery.
Intriguingly, bryophytes, which were the first plants to colonise land around half a billion years ago, lack the GID1 receptor even though they produce the GA phytohormone. This raises the question of how growth and development were regulated in these early land plants.
Using the liverwort Marchantia polymorpha as a model system, the researchers found that these primitive plants recruit a dedicated enzyme called MpVIH, which produces the cellular messenger inositol pyrophosphate (InsP8) to dismantle DELLA without the involvement of gibberellic acid.
The role of VIH was confirmed when the researchers knocked out its corresponding gene using the CRISPR-Cas9 system. Plants without functional VIH enzymes exhibited serious developmental and morphological defects like compact thalli, compromised radial growth, and absence of gemma cup. These defects were rescued when the plant genome was modified to produce just one end (the N-terminal) of the VIH enzyme. Using advanced chromatography, the team found that the N-terminal contains a kinase (enzyme) domain, which catalyses the production of InsP₈.
The researchers identified DELLA as one of the cellular targets of the VIH kinase. Moreover, they observed that MpVIH-deficient plants mimic the phenotype of M. polymorpha plants overexpressing DELLA.
“We were excited to learn at this stage whether DELLA stability or activity is augmented in the MpVIH-defective plants,” says Priyanshi Rana, first author and PhD student in the Laha research group. In line with their hypothesis, the researchers found that suppression of DELLA largely restores the defective growth and development phenotypes of the MpVIH mutant plants. These findings suggest that the kinase VIH negatively regulates DELLA to promote plant growth and development.
By combining genetics, biochemistry and biophysical approaches, the researchers illuminated the mechanism underlying inositol pyrophosphate-controlled DELLA regulation in this liverwort species. Specifically, InsP8 produced by MpVIH binds to MpDELLA, promoting polyubiquitination and subsequent proteasomal degradation of the repressor protein.
Research on the DELLA protein dates back to the Green Revolution, when scientists unknowingly harnessed its potential to produce high-yielding semi-dwarf varieties. Although the mechanistic details of how it functioned were unknown at the time, modern technologies now allow scientists to aspire to manipulate this protein function, using gene-editing technology to effectively increase yield.
“As our population is growing and agricultural land shrinks, increasing the productivity of crops becomes essential,” says Laha. Given that the InsP8-regulated DELLA degradation is possibly conserved across the embryophytes, this finding might open up avenues for next generation crop species with improved yield. Studying early land plants also gives us a sneak peek into their evolution over the past 500 million years. For example, InsP₈-binding sites still exist in modern flowering plants, even though these plants destabilise DELLA through a gibberellic acid-dependent pathway. Such insights can help us understand how cell signalling pathways have evolved over time.
