Extreme parasitism: Balanophora convinces a host to grow into its tissue

The Balanophoraceae family, when stumbled upon in the forest, might be dismissed as mere fungi, thanks to their peculiar mushroom-like structures. However, these aren’t your average mushrooms. They are actually inflorescences, small gatherings of flowers.

Extreme parasitism 

Rather than simply taking nutrients from host plants using a haustorium, as seen in many parasitic species, Balanophora takes parasitism to another level. It encourages the host plant’s vascular system to grow into its tuber. 

This results in an unprecedented underground organ formed from both the host and parasite tissue. It’s within this chimeric tuber that Balanophora steals nutrients from its host.

Dr. Xiaoli Chen of BGI Research and her team set out to explore the evolutionary origins of these subtropical parasites. 

Shrinking genomes

In collaboration with Dr. Sean Graham, a botanist from the University of British Columbia, Dr. Chen compared the genomes of Balanophora and Sapria, another extreme parasitic plant. While they belong to different families and have varying appearances, their genomic narratives turned out to be surprisingly similar.

According to the study, both Balanophora and Sapria displayed record shrinkages for flowering plants, losing 28 and 38 percent of their genomes, respectively, as they evolved to full-blown parasitism. 

“The extent of similar, but independent gene losses observed in Balanophora and Sapria is striking,” said Dr. Chen. “It points to a very strong convergence in the genetic evolution of holoparasitic lineages, despite their outwardly distinct life histories and appearances, and despite their having evolved from different groups of photosynthetic plants.”

Unexpected discovery 

Both plants displayed an almost total loss of genes associated with photosynthesis, which is not surprising given that they give up their photosynthetic ability. 

However, what was unexpected was the discovery of the loss of genes vital for other biological functions such as root development, nitrogen absorption, and flower development regulation. 

In essence, these parasites seemed to have discarded a significant amount of gene families typically found in green plants, only retaining crucial genes or their copies.

Strategic gene loss

An astonishing revelation was the parallel loss of genes related to the synthesis of a significant plant hormone – abscisic acid (ABA) – in both plants. 

Even in the absence of these genes, ABA hormone accumulation was found in Balanophora’s flowering stems, with ABA signaling response genes still intact in both parasites.

“The majority of the lost genes in Balanophora are probably related to functions essential in green plants, which have become functionally unnecessary in the parasites,” said Dr. Graham. 

“That said, there are probably instances where the gene loss was actually beneficial, rather than reflecting a simply loss of function. The loss of their entire ABA biosynthesis pathway may be a good example. It may help them to maintain physiological synchronization with the host plants. This needs to be tested in the future.”

Study implications 

Highlighting the broader context, Dr. Huan Liu of BGI Research mentioned the 10KP project, which aims to sequence 10,000 plant genomes. 

“The study of parasitic plants deepens our understanding of dramatic genomic alterations and the complex interactions between parasitic plants and their hosts,” said Dr. Liu. 

“The genomic data provides valuable insights into the evolution and genetic mechanisms behind the dependency of parasitic plants on their hosts, and how they manipulate host plants to survive.”

The research is published in the journal Nature Plants.

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