I want to make roots twist more vigorously; instead, I found a way to make twisting almost stop. As I have written about earlier (here and here), root twisting in my trials has been somewhat … flakey. I brought seed of six genotypes of lab weed (aka arabidopsis) that are recognized as twisting mutants. These roots twist as they grow; three of them twist to the left, three to the right. As I have been growing them, I have noticed that the twist is irregular: Some of them twist more as they get older and some twist less. Some don’t twist much at all; all of them switch the direction of twist now and then.
I grow seedlings on an agar gel. The gel is clear, letting the roots be seen easily, and aqueous, keeping the roots hydrated. But the solution in the agar gel also contains nutrients—fertilizer—, so the plants can grow fast. Arabidopsis seeds are tiny, smaller than the period at the end of the this sentence. They travel light and need nutrients right away. The particular recipe I use was developed to feed plants by someone called Dennis Hoagland almost 100 years ago. Works well. But most people grow lab weed on a different mix, one developed 30 years later by Murashige and Skoog. Not sure why MS (no one likes to spell Murashige Skoog) medium is so popular, maybe because you can buy it pre-mixed in a packet, like Kool-Aid; in my hands, roots grow faster on Hoagland’s.
Figure 1. Roots growing on 1/2 MS medium don’t twist much. Top panel shows the sku5 genotype, bottom panel shows tua6. Lefthand panel shows Hoagland’s medium, righthand panel shows 1/2 MS. On Hoaglands, roots of sku5 diagonal to the right, those of tua6 go the left. This is the phenomenon called skewing. On 1/2 MS, roots of both genoetypes grow straight down.
For no particularly good reason, I decided to see how roots would twist when grown on MS medium (strictly speaking, half-strength MS). As I expected, they grew a little more slowly than on Hoagland’s. But as I did not expect, on the MS medium, the roots grew straight (Fig. 1). When roots are grown on the surface of the agar, twisting causes them to roll. Because they twist is (mainly) in one handedness, they roll consistently to the left (or to the right, depending on the genotype). The angled path they take growing on the agar is called skewing. On 1/2 MS, root skewing was more or less gone (Fig. 1). In other words, the medium eliminated (or at least greatly suppressed) their twistiness. Skewing in three of the other four genotypes was also reduced tho not quite as totally as for these two (tua6 and sku5).
Wow! But is it the MS? The Ph.D. student who gave me the plates (thanks Max!) makes them without sucrose. I always include sucrose. Of course, sugar is never a component of fertilizer. But lab weed farmers commonly add sucrose along with the usual nutrients to give the seedlings even more of a boost. I have compared plants grown on 1% sucrose and those grown without any (otherwise on Hoagland’s): with sucrose, the variability in growth rate among seedlings is less than without. I interpret this as reflecting the ability of the added sucrose to reduce the effect of seed weight (lighter seeds with less stored nutrients plausibly grow less fast; seeds from even a single plant vary in weight by more than a factor of 2).
Be that as it may, rather than from the presence of MS, the lost twist could arise from the absence of sucrose. And if so, then could more sucrose increase twist? In fact, my lab once used 3% sucrose. This was standard operating procedure when I learned the lab weed trade. Some time along the line, I realized that there was little difference between 3% and 1% sucrose and cut back. When Alex Cobb was working in my lab, back in the 00s, we probably used 3% sucrose and if so his experiments might have benefited from robusticated twisting. A lot of mights. You guessed it: last week I plated the twisty genotypes on Hoagland’s medium containing 3% sucrose (and of course controls on 1%). Will sucrose be the elixir I need to promote vigorous twist? Hmmmmm….