Why do plant have spines? Spines are common in many terrestrial plants, habitats, and biomes. Why would plants produce spines and exactly what are their functions? These questions have for a long time fascinated both botanist and non-botanist alike. Indeed, even the Bible has a say on the origin of plants spines! Genesis 3: 17 – 18 says “cursed is the ground because of you…it will produce thorns and thistle..”! Thus, it appears spines emerged on plants as a punishment to humankind after the fall of Adam in the garden of Eden.
Scientists, however, believe spines evolved to protect plants against large herbivores (e.g. giraffe, elephants, rhino). There are, at least, three lines of evidence in support of this hypothesis. Researchers have shown that, in Africa, many spiny species emerged during the rapid divergence of the lineages of large herbivores1. Secondly, spiny plants tend to be more abundant in environments with high large herbivores pressure1. Several experiments have shown that large herbivores struggle to munch on plants with spines than those without spines2,3. Spiny plants are also common in arid and dry environments. Here, spines are thought to have ecophysiological functions, but this has rarely been experimentally tested.
An interesting observation made by ecologists is that different spiny plants produce spines during different growth stages. The question then is “what controls spine emergence in plants”? There are several ways to answer this question, but one easy approach is to consider where spines come from. It seems like a rather straight forward question. However, researchers have only recently attempted to answer it and by so doing are beginning to unravel the morphological and genetic basis of plant spines.
Morphologically, plant spines are derived from modified leaves, stipule, branches, cortex, or epidermis. People commonly use the words; “spine”, “thorn” and “prickle” interchangeably to refer to any stiff sharp-pointed structure on plants. However, these terms refer to different types of structures. As illustrated in Fig.1; spines are modified leaf or stipule, thorns are deformed branches, and prickles are derived from cells of the cortex or epidermis. Why does it matter to make distinctions between spine, thorn, and prickle? As can be seen from Fig.1, it turns out that, at a young age, emergence of “spines” on a plant strongly depends on which plant organ is modified. This means that species with spines and prickles are able to produce “spines” earlier than those with thorns4.
A number of recent studies are beginning to unravel the genetic basis of spines and seem to suggest that the development of spines, thorns and prickles are driven by distinct genetic regulators. A study5 on the genus Vachellia (where spines are derived from modified stipules) has shown that temporal decline in the microRNAs miR156/miR157 and a corresponding increase in their target – SPL transcription factors – is intricately linked to, among other things, the swelling and elongation of stipules into spines. Thus, it appears, the MiR156-SPL-pathway may control the development of stipular spines. Researchers from Yale6 have also recently demonstrated that two Citrus (which produce thorns) genes; TI1 (THORN IDENTITY 1) and TI2, encoding TCP transcription factors, are responsible for shutting down stem cell activity in branches leading to the formation of thorns. In another study on Rosa chinensis7, researchers have observed that RcTTG2 transcript (a WRKY transcription factor) accumulate at higher levels in stems presenting prickles. This study suggests that RcTTG2 is a positive regulator of prickle presence in rose.
Spines, thorns, and prickles are important plant traits that help to protect plants against herbivores. However, under cultivation, they can become a nuisance. For instance, attending to spiny plants in home garden or orchard requires extra care. The growth and elaboration of spines also siphon resources from other plant parts (e.g. growth of new leaves) and reduces plant productivity (e.g. fruit production). Understanding their genetic regulators is an important first step to producing spine-free varieties of plants of high economic and ornamental values (e.g. rose and citrus).
1. Charles-Dominique, T. et al. Spiny plants, mammal browsers, and the origin of African savannas. Proc. Natl. Acad. Sci. 113, E5572–E5579 (2016); 2. Cooper, S. M. & Owen-smith, N. Effects of plant spinescence on large mammalian herbivores. Oecologia 68, 446–455 (2001); 3. Charles-Dominique, T., Barczi, J.-F., Le Roux, E. & Chamaill e-Jammes, S. The architectural design of trees protects them against large herbivores. Funct. Ecol. 31, 1710–1717 (2017); 4. Armani, M., Charles-Dominique, T., Barton, K. E. & Tomlinson, K. W. Developmental constraints and resource environment shape early emergence and investment in spines in saplings. Ann. Bot. 124, 1133–1142 (2019); 5. Leichty, A. R. & Poethig, R. S. Development and evolution of age-dependent defenses in ant-acacias. Proc. Natl. Acad. Sci. 116, 15596–15601 (2019); 6. Zhang, F. et al. Reprogramming of Stem Cell Activity to Convert Thorns into Branches. Curr. Biol. 30, 1–11 (2020); 7. Saint-Oyant, L. H. et al. A high-quality genome sequence of Rosa chinensis to elucidate ornamental traits. Nat. Plants 4, 473–484 (2018)
Mohammed Armani 