Skewed roots! Development matters. The tests I ran for the time course of skewing (and described here) finished. I scanned the plates and had a look. Starting with the two lefty mutants (Fig. 1), the amount of skewing is not constant. For lefty1, the skew angle seems to get progressively more extreme (so the root forms an arc) and then less so; one root even switched directions and skewed to the right. The pattern for lefty2 was similar; skew reversal was striking.
Figure 1. Lefty mutants. Note to the observant and well-read: I flipped these scans (and the ones in Fig’s 2 and 3) so that they appear as they would if you were looking at the plates from the top. That way, lefty mutants skew to the left, matching how they are shown in publications. In fact the scans view from the bottom.
The developmental pattern for spiral1 was particularly striking, with most roots growing straight down for a while before taking a hard right turn (Fig. 2, left). And then after another while, some roots made a hard left turn, sending the roots back to growing more or less straight down. Spiral2 skewed rather weakly (Fig. 2, right). I can safely eliminate that line, at least for now.
Figure 2. Spiral mutants
Yet a distinct pattern was shown by tua4 (Fig. 3, left). In this line, the roots started off skewing steeply to the left and then with time skewed to lesser extents. These roots formed arcs but in the opposite direction as lefty1. Finally, sku5 skewed fairly steadily, with no clear developmental pattern Fig. 3, right). A couple sku5 roots reversed direction.
Figure 3. Two other mutants: tua4 and sku5.
Lesson learned! A root’s propensity to twist (which is revealed by the angle of skew) varies with age. In hindsight, this should not be surprising. The imbalance in the microtubule system driving the phenomenon is subtle (strong microtubule disturbances make the root swell like a balloon). This presumably explains the reversals. For my planned imaging for kinematics, I will need to choose not only the genotypes but also the age of the root. On Friday, I plated tua4 and spiral1 again, on the agar, but with the seeds staggered to minimize one root bumping into its neighbor. I will tick every day so I can get clearer information about these developmental trajectories. In addition, I put plates not only in rip-roaring cabinet C but also in room S207, in which roots grow far more slowly. Growth rate seems relevant because roots grow faster and faster over the time represented by these figures. I want to see whether the slower overall growth rate impacts the trajectory of skew.