During the past week, I enjoyed a holiday. Laura and I were visited by our son, brin, and husband, Poe. Along with generally lolling around, we visited, among other places, Winterbourne Gardens, the Pen Museum, and Coventry cathedral. Streamed Dr Who. Good times all around. 

But yesterday (Saturday), I cycled into the lab to check on some plates I had set up at the end of the week before the kids arrived. I set this up because of something I learned while talking with Anna Amtmann at the University of Glasgow during my seminar junket the other week. In her studies on how mineral nutrition influences root development, she noticed that when the growth medium contains almost no potassium, the angle of root skew is reduced, by about 10 to 15 degrees. 

As I posted about a few times awhile back (e.g., here and here), a root sometimes grows down at an angle on the surface of the agar plate (Fig. 1). This kind of growth habit is called skewing; the habit reflects the balance an intrinsic twist, causing the root to roll sideways on the surface, and gravitropism, redirecting growth downwards. The greater the intrinsic twist (or the weaker the gravitropism), the larger the angle of skew becomes (that is, the root is farther from being vertical). With lefthanded twist, roots skew to the right; and vice versa. For my project, I grow roots inside the agar, thereby removing the surface and getting twisted roots that grow straight down. But for following the twistiness in general, it is helpful to grow roots on the surface of the agar and observe skew. Which is what I did to check out what happens on low potassium. 

Something happened. I plated the three genotypes for my project: Columbia, tua4 (a lefty), and spr1 (a righty). I plated each on plates that had either the regular Hoagland’s medium or the same made up with low potassium. I replaced the potassium nitrate and potassium phosphate with their corresponding sodium salts, leaving 0.1 mM potassium (somewhere around a 50-fold drop). Nine days later, I had a look, scanning the plates as a record. 

For tua4 (Fig. 1) and spr1 (Fig. 2), dropping the potassium appeared to do little if anything. I expected that roots might grow more slowly with little potassium but if anything they might have grown faster. Notably, the skew appeared unaffected. To be sure, with spr1, skew is pretty wild. Note that for a while, roots grow more or less straight down and then start skewing steeply, making almost a right-angle bend. And some roots appear to change direction, skewing the other way. Conceivably, the right-angle turn was sharper on the low potassium plate. 

But Columbia roots noticed (Fig. 3). On the regular medium, the roots skew to the left. I have noticed this before and it is definitely annoying. On the low-potassium medium, the roots skewed less. 

Hmm. The stimulus for the wild type to skew is labile because in some growth cabinets the skew is larger than in others; likewise from one experiment to the next; this experiment had just a single plate. Nevertheless, the lessened skew on low potassium repeats Amtmann’s results. Apparently, twisting caused by the mutations in tua4 and spr1 is robust compared to whatever is triggering the wild type (Columbia) to twist, which can be readily tickled (by the growth medium or phase of the moon). So today, Sunday, I am off to the lab again for a weekend experiment. I will repeat for the wild type this with more plates. Will the suppression of twist abide??