A collection of papers on Extrafloral Nectaries has recently moved into Free Access at Annals of Botany. One of the papers raises the question, can a plant that never flowers have extrafloral nectaries?
Photo: Koptur et al.
Nectar secretion on fern fronds associated with lower levels of herbivore damage: field experiments with a widespread epiphyte of Mexican cloud forest remnants by Koptur et al. examines why ferns produce nectar. The paper starts with a brief review which includes a few facts that startled me. One is that extrafloral nectaries evolved before floral nectaries. This surprises me because I so deeply associate nectar with flowers. Another shock was that nectaries appear on ferns well before ants appear in the fossil record.
This shouldn’t be a surprise, but we’re so used to evolutionary stories being teleological, like plants evolved nectaries to reward insects, that it’s easy to forget that it’s a huge oversimplification that gets things very wrong. Nectaries didn’t evolve in order to do something with a purpose. Instead that plants with nectaries have a better chance of passing their traits to their offspring because they can reward insects. And what if there are no insects? Koptur et al. say that the early appearance of nectaries supports the ‘leaky phloem’ hypothesis, that sugars are forced out of the plant in weak developing tissues to ease hydrostatic pressure in the plant. This might explain how they formed, but once ants arrived did they help select ferns with better nectaries. Do the nectaries in ferns given them an evolutionary advantage?
The nectaries are on the leaves or fronds of the plant. Developing fronds are a prime target for herbivores, so if the ants were drawn into the leaves they could act as a defence. But do they. The experiment, like many of the best ones, sounds quite simple.
At its simplest, you find a plant with a suitable pair of young fronds. On one you paint over the nectaries with nail polish to prevent access to the nectar. You then see how the plants develop and compare the damage on the untreated leaf with the test leaf. Reality is messy, so they actually did a lot more than that to account for other factors – but the basic experiment was does access to the nectaries matter?
The results were clear. The fronds with blocked nectaries had four times the damage of the untreated fronds. The ferns benefited from hosting plants, and the ones that could attract them best got the best defence. The defence works best against invasive species that haven’t co-evolved with the fern and developed counter-defences against the ants.
It’s easy to see nectar as part of the plant’s reproductive strategy, or maybe as part of the reproductive system that’s been repurposed for something else. I think this paper neatly shows that there’s no need to assume any connection at all. There’s a lot more to nectar than bait for pollination.
Sun/shade conditions and seedling recruitment in a cactus
Early life-history stages of cacti can benefit from the facilitative effects of nurse plants that reduce solar radiation and water stress. Miranda-Jácome et al. conduct a reciprocal transplant experiment, coupled with the artificial manipulation of sun/shade conditions, to test for the effects of local adaptation on germination, seedling survival and growth of the columnar cactus Pilosocereus leucocephalus. They find that significant local adaptation is mainly detected under full sunlight conditions, indicating that sun/shade acts as a selective agent in water-limited environments. Facilitation provided by nurse plants in these environments can attenuate the patterns of local adaptation among plants benefiting from nurse plant effects.
Image: Tennessee Valley Authority, 1942/ Franklin D. Roosevelt Presidential Library and Museum.
This month’s winner in the ‘so simple it’s positively brilliant (but why did nobody think of it before?)’ category is Damar López-Arredondo and Luis Herrera-Estrella’s paper entitled, ‘Engineering phosphorous [sic.] metabolism in plants to produce a dual fertilization and weed control system’.
Apart from the unusual spelling of phosphorus in the title (it is correct in the body of the article – and this is important since the study deals with two similarly worded phosphorus compounds: phosphate and phosphite!), this is a most interesting piece of research. I can do no better than reproduce the paper’s own rather elegant summary of the work (from Laboratorio Nacional de Genómica para la Biodiversidad, Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional, Irapuato, Guanajuato, México) here: ‘High crop yields depend on the continuous input of orthophosphate (PO43–)-based fertilizers and herbicides. Two major challenges for agriculture are that phosphorus is a nonrenewable resource and that weeds have developed broad herbicide resistance. One strategy to overcome both problems is to engineer plants to outcompete weeds and microorganisms for limiting resources, thereby reducing the requirement for both fertilizers and herbicides. Plants and most microorganisms are unable to metabolize phosphite (PO33–), so we developed a dual fertilization and weed control system by generating transgenic plants [arabidopsis and tobacco] that can use phosphite as a sole phosphorus source. Under greenhouse conditions, these transgenic plants require 30–50% less phosphorus input when fertilized with phosphite to achieve similar productivity to that obtained by the same plants using orthophosphate fertilizer and, when in competition with weeds, accumulate 2–10 times greater biomass than when fertilized with orthophosphate’. Or, and in summary, ‘the production of transgenic crop plants able to utilize phosphite, together with the application of phosphite as a source of phosphorus, might potentially become an effective phosphorus-fertilization and weed control scheme in the almost 67% of cultivated land with low orthophosphate availability’.
Whilst the authors are appropriately – and understandably – cautious about the significance of the results and how well they will scale-up to field-sized trials, this work – from the country whose CIMMYT (The International Maize and Wheat Improvement Center) was a major player in the Green Revolution of the last century – sounds like another agronomic development with tremendous potential. ¡Muchas gracias!