UoN The Hidden Half: Wild Pea
From Brian Atkinson
Pisum sativum is more commonly known as the pea plant. These plants are well known for growing peas in pods which are the plant’s fruit. This is an important crop now grown in many parts of the world. It is thought that peas were first domesticated in the area of modern day Greece, Turkey and Syria. Despite now being a commonly eaten vegetable, when peas were first introduced to France in the 1600s they were described as a luxurious delicacy. Peas were also vital to the discovery of modern genetics as Gregor Mendel used simple characteristics, such as shape and colour of the pea seeds, to understand inheritance.
The roots of the pea plant have developed from one primary root, which grows downwards more or less straight. From this primary root, a large number of secondary (or lateral) roots have branched to fill the pot.
How do roots know in which direction to grow? Roots need to grow downwards into the soil (a process called positive gravitropism), and a special set of cells at the very tip of every root tells the root which way is up and down. These cells, called columella cells, contain small starch grains, which are quite heavy compared to the rest of the cell content. Due to their weight, these starch grains will always move to the bottom of the cell. When the primary root grows down straight into the soil, the position of the starch grains is not disturbed. However, if the root is not growing straight down but sideways, for example because a stone is blocking its path, the position of the columella cells changes. What used to be one of the side walls of the cell becomes the bottom, and the starch grains will move towards this point. This movement of the starch grains sets a signalling process in motion that changes root growth. The lower side of the root grows less fast than the upper side of the root, which causes the root to bend until the root tip is pointing straight down again. Lateral roots are programmed not to grow down straight like the primary root, but at an angle. This helps the plant explore the soil for anchorage, nutrients, and water.