The Variable Chirality of Plant Twining Stems

1. The twining direction of the stem in Mikania micrantha is variable.

Mikania micrantha, also known as the mile-a-minute weed (薇甘菊, Weiganju in Chinese), is a species of the Asteraceae family native to tropical America. In recent decades, it has become a serious invasive plant in southern China [6].

The stems of M. micrantha can take two forms: trailing (creeping) and twining. As described previously [5], the twining stems of M. micrantha exhibit both left-handed and right-handed chiralities (Figure 1F; Figure 2A-C). Different branches of the same plant can have different chirality, and even within the growth process of a single branch, the chirality can change (Supplementary video 1). However, it seems more common for the twining direction to change from left to right rather than from right to left (data not shown). Based on the survey of the numbers of the two stem chiralities of the M. micrantha in Guangzhou, China, the proportion of left-handed lianas are slightly more than 50%, but not significant (Figure 2D, P=0.11).

The Asteraceae family comprises a vast number of plant species, and although the proportion of twining stems is not high, there are still a lot of species with twining stems within this family. For instance, the variegated wax ivy (Senecio macroglossus DC.) exhibits a fixed right-handed twining stem. Additionally, we have recently observed that Pseudogynoxys chenopodioides also possesses VCTS (Figure 2E and F), suggesting that VCTS may be relatively common within the Asteraceae family.

2. The twining direction of the stem in Pleuropterus multiflorus is variable.

Pleuropterus multiflorus (Thunb.) Nakai （Fallopia multiflora, 何首乌，Heshouwu in Chinese）is a species of the Polygonaceae family and is an important traditional Chinese medicinal herb known for its root and stem. It is widely distributed throughout China and its genome has been sequenced and assembled recently [7]. Some popular literature in Chinese mentions that Heshouwu has twining stems with both chiralities, but there have been no formal reports on this. We have observed and recorded the proportion of the two chiralities in Heshouwu (Figure 3). Additionally, we have observed that the twining direction can change midway in a single stem (supplementary video 2). Currently, we only know that Heshouwu, as a member of the Polygonaceae family, exhibits variable direction twining stems (VCTS), and further observation is needed to determine if there are other members of VCTS in the family.

Similar to the case in M. micrantha, none of the two chiralities is dominant in the stems of P. multiflorus (Figure 3E, P=0.06).

3. The stems of the Hedyotis hedyotidea exhibit two twining directions

Hedyotis hedyotidea (DC.) Merr. (牛白藤, Niubaiteng in Chinese) is a species of the Rubiaceae family, characterized as a shrub or twining lianas, which is also used as Chinese herb medicine [8]. The majority of its twining stems are right-handed, but there are also left-handed ones (Figure 4). The Rubiaceae family comprises numerous species, mostly distributed in tropical and subtropical regions. The composition of twining stems within the family is complex. For example, Paederia foetida and Mussaenda pubescens exhibit fixed left-handed twining, while Morinda parvifolia and Morinda umbellata exhibit fixed right-handed twining.

4. Some members of the Codonopsis genus of the Campanulaceae family exhibit two twining directions.

Codonopsis pilosula and Codonopsis lanceolata, both known as Dangshen (党参), are well-known traditional medicinal herbs [9] and they also have VCTS (Figure 5). We have searched for images on the Plant Plus of China website (PPBC) and found that Codonopsis henryi and Codonopsis tubulosa, known as Chuan'e Dangshen (川鄂党参) and Guanhua Dangshen (管花党参), respectively, also exhibit VCTS. Therefore, it is possible that most or all twining members of the Codonopsis genus have VCTS.

5. Dendrotrophe varians exhibit two twining directions.

Dendrotrophe varians (Blume) Miquel (寄生藤, Jishengteng in Chinese) is a hemiparasitic plant with large woody vines belonging to the Santalaceae family [10]. It is currently the only known VCTS of large woody vines/lianas (Figure 6). Since plants in the Santalaceae family are not very common, it is currently unknown if there are other members of VCTS in the family. We have also observed another suspected large woody vine VCTS in Guangdong province, China; but the quantity of that plant is very limited, and there is no definitive conclusion yet.

6. The stem of Aconitum volubile has two twining directions

Aconitum volubile Pall. ex Koeue, also known as Twining Monkshood (蔓乌头, Manwutou in Chinese), is a member of the Aconitum genus in the Ranunculaceae family. Many species of Aconitum genus were also used as traditional Chinese herb medicine [11]. While most members of the Aconitum genus have erect stems, there are a few, such as A. volubile, that have twining stems. The twining stems of A. volubile also exhibit both left-handed and right-handed chiralities (Figure 7).

7. The lianas of Clerodendrum thomsoniae has two twining directions

Clerodendrum thomsoniae Balf. f., known as Bleeding Heart Vine (龙吐珠, Longtuzhu in Chinese), is a twining lianas in the Lamiaceae family. Although it is primarily a climbing vine, it can also exhibit slight twining. The twining of C. thomsoniae has a relatively long pitch, and the direction can be both left-handed and right-handed (Figure 8).

8. The growth pattern of leaf rachises in the Lygodium genus is similar to twining stems, and the twining direction is variable

So far, the VCTS we have found are all belong to the angiosperms, specifically the eudicots. We have not yet found this phenomenon in monocots, basal angiosperms, magnoliids, or gymnosperms. However, it is interesting to note that similar phenomena exist in ferns [2]. In the Lygodium genus (海金沙, Haijinsha in Chinese) of the Lygodiaceae family, the leaf rachises can grow indefinitely and twine around support structures, resembling stems in flowering plants. We have examined L. japonicum, L. microphyllum (Figure 9), and L. flexuosum and found that their leaf rachises exhibit both left-handed and right-handed twining directions. The right-handed leaves were significantly more than the left-handed ones (Figure 9F, P=5.8E-11). The left-handed leaves are less than 20%, rare but still easy to encounter.

We speculate that members of the Lygodium genus may all have two twining directions. Although they twine with their leaf rachises rather than stems, the similar growth pattern suggests that they might have similar regulatory mechanisms. Additionally, ferns as an evolutionary outgroup can provide insights into the evolution of twining stems in seed plants.

9. The relation between twining chirality and taxa

Previous study has shown that there was no clear relation between the twining direction and phylogeny at the family level [12], and may have some relation at the genus level [13]. Here, to examine the evolutionary relationships of known taxa with VCTS, we constructed a phylogenetic tree that includes the families of twining stem plants commonly found in China (Figure 10).

As previously described, the majority of twining stems possess right-handed chirality. Most known families have exclusively right-handed FCTS (red fonts). These families can be divided into two categories: one with numerous twining stem members, such as the Convolvulaceae and Menispermaceae, all of which exhibit right-handed twining; and another category where twining stem members are few, but all known members exhibit right-handed twining.

There are very few families known to have exclusively left-handed FCTS. The most certain examples are the Cannabaceae (Humulus genus), Caprifoliaceae, and Schisandraceae (blue fonts).

Some families have members that exhibit both left-handed and right-handed twining, without a clear dominance. For example, in the Rubiaceae family, both left-handed and right-handed twining, as well as VCTS, can be found. The Dioscoreaceae family (known for Dioscorea genus) is also interesting, as it has both left-handed and right-handed members (green fonts). For instance, D. polystachya, D. alata, and D. cirrhosa exhibit right-handed twining, while D. nipponica, D. bulbifera, and D. esculenta are left-handed, with no predominance shown in either of the chiralities.

There are also some families where the majority of twiners are right-handed, but a few are left-handed (pink fonts). For example, in the Papilionoideae subfamily of Fabaceae family, the genera Glycine, Vigna, Phaseolus, Lablab, and Mucuna exhibit right-handed twining, while Wisteriopsis exhibits left-handed twining (Supp. Figure 1). Additionally, the genus Westeria is predominantly right-handed, but W. floribunda, a member of this family, exhibits left-handed twining. In the Apocynaceae family, most twining stems are right-handed, but the twining stem of Strophanthus divaricatus is left-handed (Supp. Figure 2).

From the phylogenetic tree, it can be observed that known VCTS (purple fonts) are all found in eudicots (labeled with brown color). Furthermore, within the eudicots, both left-handed and VCTS are slightly more abundant in the Superasterids (labeled with cyan color), suggesting a higher degree of twining direction uncertainty within this clade, although the trend is not strongly pronounced.

Twining stems are rare in monocots, with the Dioscoreaceae and Stemonaceae being the most common examples. Among gymnosperms, the only known genus with twining stems is Gnetum, and all known species within it exhibit right-handed twining (Supp. Figure 3).