Who's Responsive to His Environment? You!

{Hormone and tropism growth, explained}

Just like his parents, our little guy is extremely responsive!

Whether it be to light, gravity, wind, touch, or a variety of other factors, our baby boy will respond in a variety of ways. On of these responses is the gravitropic response, moving in response to gravity. 

Root caps are the regions responsible for gravitropism, for cells in the center of each root cap regulate the gravitropic response by respond to gravity, and ultimately initiating gravitropism. 

We still aren't quite sure how our big guy senses gravity, but we think it is probably explained via the statolith hypothesis. This hypothesis claims that amyloplasts -  dense, starch-storing molecules - are pulled to the bottom of root cap cells in response to gravity. As these amyloplasts move, tug, and tumble, the weight of where they are pulled activates sensory protein that are located in the plasma membrane. These sensory proteins are responsible for initiation of the gravitropic response. 

This process happens in our baby's root cap cells. Why, just the other day Paul was playing with our little guy, and he got so excited that he tried to jump up, and just tipped right over. It was the cutest thing! When this happened, the amyloplasts in our little guy fell and settled onto the cell walls of the sensory cells. This weight activated a whole new set of receptors, signal the roots of our baby that he wasn't facing upright anymore.

Now of course Paul picked him up right away, and they both went back to playing! But if our little man were to stay lying on the ground, a change in the distribution of auxin would have occurred in the root. Auxin is typically responsible for acting as a signal to bend in the phototropic response. Auxin promotes cell elongation in the shoot, and in the case of gravitropism auxin [normally] flows down the middle of the root and then toward the outside. But when our little guy fell, his sensory receptors caused a change in multiple transport proteins, redistributing auxin. When auxin is redistributed, the lower part of the afflicted root gets more auxin than the other parts of the root, particularly the upper region. In response to these differences, cells in the lower part of the root grow slower than cells in the upper part of the root, which grow rather quickly. High auxin concentrations inhibit growth, and thus bending occurs. But we would never want our sweetheart's beautiful roots to bend! 

But auxin isn't the only major hormone that is crucial to our baby's development; Cytokinins are also very important to our little guy!

Cytokinins are a group of hormones that, unlike auxin, promote cell division. These cytokinins are synthesized in root tips, growing buds, and developing organs in our baby. Active cytokinin is synthesized in the apical meristems of roots and transported all the way up to the shoot system via the xylem. 

But as our baby boy gets so big, we - the loving, doting, slightly paranoid parents - wanted to know all we could about how cytokinins actually stimulate cellular division in our baby. 

It turns out that receptors in the plasma membranes of target cells, made up of a group of closely related proteins, acts as the binding sight for cytokinin. When cytokinin binds, the receptors activate genes that regulate cellular division. More specifically, however, cytokinins affect molecules that regulate the cell cycle. Cytokinins regulate growth in our baby by activating genes that keep the cell cycle going. If the cell cycle is perpetuated, cells will continue to divide. 

Our little man needs both auxin and cytokinins to make sure he can respond to his environment, grow in a way that is most beneficial to his development, and continually promote cellular division for growth in general.