Roots are critical for plants to find water and nutrients. They can also be expensive to grow, so a plant needs to send them in the right direction. How do they do this? A new paper by Daniel von Wangenheim and colleagues in Nature Plants examines how lateral roots grow towards water in the very earliest stages of their formation.
The interior of the young root is almost a series of concentric layers. From the outside you pass through the epidermis, the cortex, and the endodermis until you hit the pericycle. The pericycle is a layer of cells that surrounds the stele, the core of the root that contains the vascular tissue. The vascular tissue is the xylem, that carries water and minerals up from the roots, and the phloem that carries the sugars from the leaves. If you were to cut a cross-section of a root, you’d see the xylem reaching from one side of the stele to another, like a bar, as though it has two ends or ‘poles’.
De Smet has written a minireview, that summed up what was known in 2011. It’s not news that lateral roots grow from the pericycle. Nor is it a surprise that the roots appear to grow from by the xylem poles. The lateral roots will be additions to a plant’s internal plumbing, so the ‘pipes’ have to connect. Therefore there has to be a route through to the xylem in the main root.
A feature von Wangenheim and colleagues find is that if there’s a tendency for water to be on one side of the root, the lateral roots grow towards that side. That should be no surprise too. Building roots costs energy and material, the plant will want to do that where it gets the best return. But there is a surprise on which side of the root the lateral roots start to grow from the pericycle. If there are two xylem poles, then you’re equally likely to find lateral roots starting from the dry side as you are the damp side.
But when von Wangenheim’s team grew roots on agar gel to see how primordial lateral roots developed they didn’t find roots from the two sides behaving in the same way. “The primordia emerging on the airside oriented mainly towards the gel surface. In contrast,” they write, “primordia initiating on the contact-side oriented mainly parallel to the gel surface. These results indicate that the lateral root outgrowth angle is highly plastic and steers organ development preferentially towards externally available water sources.”
The team set out to find out how early the lateral root starts growing at an angle. Previously, it was thought that it was the cells over the xylem pole that were most important in developing the new root. However, through asymmetric growth, the cells surrounding the central cell file also direct the root.
Looking at previous work on the subject for writing this post, I found plenty on molecular contributions to lateral root growth, but little at the cellular level. It would seem that it’s not just due to my poor search skills. Von Wangenheim and colleagues write, “In contrast to these earlier studies, which focused on the underlying molecular mechanisms that control this behaviour, this study focused on cell-to-organ-scale mechanisms that contribute to the outgrowth of the LRP in response to water availability. Our study reveals that unlike the xylem pole axis, the selection of pericycle cell files that initiate a new lateral root primordium is linked to the external hydrological landscape.”
The disconnect between the two elements is particularly interesting. The directionality of the root from the pericycle shows that the availability of water is critically important in directing the root. The initiation from either xylem pole, in contrast, shows somewhere the signal to start the process is independent of that hydrological sense. I’m left wondering if there’s something obvious I’m overlooking that explains why growth doesn’t always start at the xylem pole on the dampest side.
It’s through some very clever and skillful work that von Wangenheim’s team have managed to show there’s a problem in connecting the cellular and organ development to the molecular machinery that drives lateral root growth. The paper also has some striking imagery and video that illustrates their work. When the shareable link becomes available, I’ll come back to edit this and direct you to that. In the meantime, the authors have also produced an excellent explanatory video of their work.