Germination: starting the plant life cycle
Germination is when a seed wakes up and starts to grow, and when this happens is a life and death decision on the part of the plant: start growing in the middle of winter, and the frost could kill you; burst into life when it’s too dry and you’ll never make it either. As a result, seeds are constantly sensing their environments to figure out when it’s time to make their move. Shelley Lumba, from the University of Toronto in Canada, works on how they do this, and spoke with Chris Smith...
Shelley - Yeah, it's a great question Chris. As humans, we don't get to decide when to be born. We're fixed at 40 weeks of gestation, and we have to come out. Plants, on the other hand, have evolved this amazing seed; they're basically freeze dried embryos suspended in gelatin. And that means they can be preserved, or as you said, increase their longevity for a long period of time. And upon sensing the right conditions - like springtime - and their immediate environment, they can make this life and death decision - whether or not to start their life cycle. And the way they do that is they integrate all this information inside the seed, and they form what I think is a 'germination code' to determine - is this really the right time or not? When they start to wake up, the cellular processes start becoming active, and they make the committed step for the embryo or the seedling to come out of the seed coat and start to grow.
Chris - So are they almost like chemical detectors on a seed then, and thermal detectors, that it can use to work out, is it warm enough? Is it warm enough for long enough? Because obviously we have warm days in the middle of winter, don't they, and it would be awful if the seeds shot their bolt too soon. What are they actually doing in order to detect these messages coming in from the environment?
Shelley - Yes, that's right. So they have to take into account long term cues about, for example, seasonality. Plants can sense that winter's occurred, and they do that through the right type of sensors. So they can sense, for example, light through photoreceptors; they can also sense temperature, although that mechanism isn't very well known; and they can figure out through light, proper day length, and temperature, that for example it's spring! It's time to start growing. And within short term cues, they also take into account their immediate environment, such as moisture and nutrients. And again, they have sensors around their seed coat to sense all of those particular environmental cues.
Chris - Because some seeds even 'know' when there's been a forest fire, don't they? Because they know, "now's the time to grow, all of the competition's been blitzed by the fire. If I grow now - lots of sunlight, and I'm going to succeed."
Shelley - That's right. There are these very unusual plant species, and there's definitely an alternative germination behaviour, and they're called 'fire chasing' or 'fire follower' species. And the way they start their life cycle is after a forest fire, and just as you said, because they want to take advantage of this extreme environment and start growing very quickly. And what they use as germination triggers are small molecules called karrikins, and karrikins are actually produced after the forest fire has burnt through the plant matter. So fire-chasing plants can sense the karrikin, and that's what they use to trigger their germination rather than the typical environmental cues that normal plants would generally use.
Chris - Do any seeds eavesdrop on what other seeds are doing? Because we know that plants talk to each other chemically, so they know for instance when their neighbours are being eaten by something, and they send out danger signals and that helps them to weaponise themselves with various repellents and so on. Do the seeds do this at all as well? So if they know, "right, everybody's germinating now, now's a good time to go," and then they follow the crowd?
Shelley - That's right. Unfortunately this is the adverse effect of these small molecules in soil. There's a very devastating parasitic plant which is called witchweed - its formal name is Striga hermonthica - and it's a terrible plant pest in Sub-Saharan Africa. It's a parasite, a plant parasite, that can attack crops like maize, sorghum, millet. And the way that they've evolved their lifestyle, to start their life cycle, is to sense a chemical that's actually being exuded by host crops in the soil. So they eavesdrop, and then that makes them realise that there's a host nearby, and that triggers their germination. And of course that means they start to grow towards the host crop, start to take the nutrients, water, and the assimilates from that host. And obviously that's not good in terms of the yield for the farmer. It's causing huge losses for subsistence farmers in Sub-Saharan Africa, and so it's obviously a huge problem. And the more we understand about the germination of the witchweed, the more we'll be able to develop strategies to decrease their infestations.
Chris - So that parasite physically wires itself into the host crop and steals what it's making?
Shelley - That's correct, Chris. Since they're also a plant, what they can actually do is connect through the vascular tissue to the host crop. And that's how they're able to suck all the water, and the nutrients, and the assimilates that it needs to start growing by itself. It's an obligate parasite, so they actually can't live without a host.
Chris - So can't you just wipe them out by just decimating a field with some really nasty weed killer for a season, and then leave the soil fallow, and then that'll be it?
Shelley - Unfortunately once they've gotten hold of a farmer's field, they've deposited millions of seeds into that field, and they're the size and colour of dust. So once they've actually come out of the soil, it's in some sense way too late because they've already done their damage.
Shelley - And so the way that people have been developing strategies, is to force them to germinate without a host nearby. This strategy is called suicide germination, because we know what compounds they use to trigger their own germination, what we've been working on, and other groups, are to develop compounds that they can sense, that also trigger their germination, but there's no host. So within five days, if they germinate without the host, you're going to die. So that's one of the strategies that we're actually trying to develop to try to mitigate striga infestations.
Chris - And are you using the same learning, not just to deal with witchweed, which sounds like a nightmare, but also with crops we do want to control and optimise germination for? Because presumably if you understand what these triggers are, how seeds integrate these signals and then kickstart the cells in the seed into life, can you take that same learning and use it to control things that we do want to grow, and when they grow?
Shelley - That's a great question, Chris. And that's definitely one of my missions in life. I want to be able to use the knowledge that we know about how seeds decide to make this life and death choice to germinate, and able to positively affect agriculture. And one of the major issues for agriculture is to make sure that the seeds germinate at the same time, and pretty much when you actually want to start your growing season and the knowledge that we use, for example, from the small molecules, I think could have huge applications in making sure that the seeds germinate when we want them to, because obviously they can't germinate too early. That's also bad news and they have to germinate simultaneously so that they all grow uniformly. And it's much easier to grow them and harvest the crop.