Plants and animals in sexual harmony

Just as no man is an island, no biological entity is completely devoid of interaction with other biota. And some of the most intriguing interactions are between flowering plants and animals. Of particular interest are those that show how plants are truly in charge on this planet. And most revealing in this regard are instances where plants use gullible animals to carry out the sexual biddings of their botanical overlords. What on Earth is Mr C. on about?

Pollination, that’s what. For a sedentary, rooted-to-the-spot plant to ‘hook up’ with another – similarly stationary and maybe some distance away – plant for sexual reproduction and produce the next generation, pollen from the male must be transported to the female organs of the other. That transfer may rely on luck – e.g. wind to move pollen amongst plants (anemophily), or moving water (hydrophily) to effect the transfer. But, it’s likely to be more reliable if a more targeted approach is used; hence the animal connection.

Closer study of two groups of plants – both of which were assumed to rely on abiotic agents for pollination – reveals that they may well use more precise pollinating agents than we’d previously thought.

First, Brigitta van Tussenbroek et al. investigated pollination biology in Thalassia testudinum. As a ‘sea grass‘ – a marine angiosperm (flowering plant) that lives submerged in the ocean –  hydrophily has been its presumed method of pollination. The team present experimental evidence that invertebrates engage in pollen transfer activities between male and female flowers at night (when such animals are active). Importantly, the invertebrate ‘option’ is found where water flow is absent. Presence of pollen tubes formed * on the stigmas of the receiving flowers was inferred to indicate successful pollination. So, experimentation and investigation of what actually happens in nature, overturns years of presumption. That’s science in action, and in recognition of the reality of the situation, the authors propose mixed abiotic–biotic pollination for this marine angiosperm. Hydrophily is the abiotic term; the suggested term for the biotic alternative is zoobenthophily.

Second, grasses are a group of plants where wind-pollination is supposed to hold sway. However, work by Eduardo Ruiz-Sanchez et al. (ResearchGate) suggests that pollination in certain bamboos (giant grasses) is not without an animal dimension.** Studying woody bamboos Guadua paniculata and Guadua inermis they discovered four species of bees, a syrphid fly, and some species of hemipteran visiting the flowers. Whilst no evidence is presented that the insects facilitate transfer of pollen between male and female flowers directly (nor is such a claim made), the authors suggest that insect pollen-foraging activities may contribute to release of pollen into the air. Once released, pollen could be wafted by air currents – anemophily – to receptive stigmas “promoting outcrossing and genetic flow among the individuals of the flowering bamboo populations.” Nevertheless, this study adds an extra – important biotic – dimension to the presumption of a solely anemophilous pollination syndrome for this plant group. And let’s not underplay the difficulty of bamboo flower biology work; these plants famously flower infrequently, with periods typically from 3 to 120 years(!). However, you try to get research funding to match the life span of your organism of interest…

* What would really have put the cherry on top of the icing on the cake for this study would have been use of an underwater microscope to observe pollen tube growth in situ (Andrew Mullen et al.).

** And, just to prove that such revelations are only as good and new as one’s literature search, I am grateful to Dr Gerhard Prenner (Royal Botanic Gardens, Kew, UK) for drawing my attention to the study of De-qiang Pu et al.. Investigating the potential for ‘escape’ of pollen from transgenic rice [Shabir Wani et al.], Pu et al. discovered that many insects visited the flowers of this crop that’s supposedly self-pollinated (at best), or wind-pollinated – at worst, but over relatively short distances. In particular, their work showed that European honeybees (Apis mellifera) forage on rice flowers regularly and carry viable pollen over long distances, thereby increasing the frequency of transgene flow from GM rice to non-GM neighbours. The authors argue that this hitherto unsuspected pollination route in rice should be taken account of in assessing ecological risk of transgene flow; not only in rice specifically, but also for other crops that are assumed to be self-pollinated and/or anemophilous. I.e., Science shouldn’t be based on assumptions, but facts (that are evidence-based). Cheers, Gerhard!