Re: Why do cells need leucoplasts?

That is very interesting question. Some plant species do not store much starch 
but use sucrose or polymers of fructose called fructans as their main storage 
carbohydrate. Both sucrose and fructans are stored in the vacuole. Examples of 
sucrose storers are sugar cane and barley. Sugar cane stores large amounts of 
sucrose in its stems. Barley stores sucrose overnight in its leaves. However, 
barley seeds do contain starch. The Jerusalem artichoke store fructans in its 
edible tubers.

Many plants often form starch grains in their chloroplasts, a type of plastid. 
Leucoplasts are colorless plastids found mainly in seeds, some fruits (banana) 
and underground storage organs such as tubers, storage roots and corms. 
Leucoplasts that store starch are called amyloplasts. Therefore, it is 
reasonable that leucoplasts are also able to store starch. It is certainly 
very convenient to synthesize starch in the chloroplasts. 

Storing sucrose or fructans in the vacuole is a necessity because they are 
osmotically active. In turgid cells, the vacuole has a low (negative) osmotic 
potential that pushs the cell cytoplasm against the cell wall to create 
turgor. Starch is not osmotically active so there is no reason to store it in 
the vacuole. It is believed that use of osmotically-active fructans gives 
plants an advantage under drought conditions.  

It may be an evolutionary happenstance that starch is stored in plastids 
rather than in vacuoles. Then again there may be specific reasons why plastids 
are the site of starch storage. For example, vacuoles may not have the 
appropriate environment for starch synthesis enzymes. Plastids have their own 
DNA and are considered to have originated as endosymbionts. During evolution, 
some of their DNA was either lost or transferred to the cell nucleus. 
Apparently the starch storage function was retained by the plastid.



References


Re: Where do plants store their food?


Amyloplasts


Endosymbiont Theory