What is Plant Growth Optimisation?

The Earth as a Land Resource

Earth has a total land area of 13,056 million hectares. World population experts estimate that there are around 7.6 billion people on the planet. If this land resource was divided up equally, there would be around 1.7 global hectares of land for every person on Earth. However, the world is not divided up evenly. New Zealand has a land mass around the same as the United Kingdom. We have close to 4.8 million people while in the United Kingdom there are almost 67 million people living in the same land area. Australia has a huge land mass of over seven million square kilometres of land, but the land is not nearly as usable as New Zealand’s land. The biocapacity of land is a measure of biological productivity and is measured in global hectares (Gha). A global hectare is one hectare of biologically productive space using world‐average productivity levels. New Zealand has 218 Gha per sq km compared to Australia’s 29 Gha per sq km (FOA, 2008).

Figure 1 shows current population distribution. Figure 2 shows projected population data for 2050. A comparison of these maps paints a picture of the real need for science and agriculture to work together to find better ways of maximising plant growth in order to feed the world.

"The world has no alternative to pursuing Sustainable Crop Production Intensification to meet the growing food and feed demand, to alleviate poverty and to protect its natural resources.” - Shivaji Pandey, Director of FAO's Plant Production and Protection Division, 2009.

Dr Karine David is group leader within the Plant Molecular Sciences Laboratory at The University of Auckland. Dr David and her team at the University of Auckland are conducting research that helps to understand how plants grow, and how this understanding can be applied to maximising plant growth, contributing to a global solution.

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Feeding Nine Billion People

Worldwide, over one billion people are underweight and undernourished. In 2019, up to 22 million people living in these countries will need food assistance. People living in developing countries like the East African countries of Somalia, Ethiopia, Djibouti, Kenya and Uganda are most at risk. Malnutrition caused by undernutrition and insecurity of food supply (not having enough to eat) can weaken the immune system, meaning people are more susceptible or more likely to contract infections and disease. They are also more likely to die from complications caused by usually treatable and preventable communicable diseases such as measles, malaria, pneumonia, skin infections, and diarrhoea. The prevalence and spread of these diseases is also closely associated with living environments which accompany severe food shortage situations like overcrowding, limited access to clean water, or poor sanitation. Over 8 million preventable deaths are caused by a combination of malnutrition and disease.

So what will it take to feed the expected nine billion people who will inhabit the Earth by 2050? This is a complex issue that requires significant scientific knowledge about how to grow food. The issue is centred on energy and maximising plant growth is critical. All food chains start with plants that convert light energy into chemical energy via photosynthesis. No matter what humans are eating, plants are the starting point for ensuring that there is adequate food for the world’s populations. Plant growth is influenced by genetic factors, environmental factors (water, carbon dioxide, light, nutrients) and hormones within the plant.

The Plant Molecular Sciences Lab at the University of Auckland has a large team of scientists who work on understanding of plant growth and collaborate with plant and food research to produce healthier fruit faster with less environmental impact.

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Raging Hormones

Just like teenagers, plants have hormones. These chemical messengers can travel to different parts of the plant. Once they reach their “target” site, the hormone will act to change the way the cells behave, controlling the way in which the plant grows. This control is achieved by the turning on or off of genes that affect growth. Knowing about what plant hormones do and how they do this is vital information that can be used to enhance the way plants are grown for food production. By adding plant hormones to crops, the quality and quantity of food that is produced can be enhanced. Much of the food we eat today is grown with the help of plant hormones. Table 1 below shows the five major groups of plant hormones, their actions and their commercial application. Auxin is the best known of these and has a number of highly significant commercial applications.

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The History of the Mystery

Auxins were the first plant hormones to be discovered and, yet over 100 years on, despite extensive knowledge of how important auxins are and their commercial application, scientists still do not understand exactly how auxins work.

What is known is that auxins can be synthesised by all plants (Woodward and Bartel 2005) and have the ability to affect cell growth, cell division and cell differentiation. They are known to have different effects in different parts of the plant. In the stem, auxins allow the plant to grow towards light by elongating the cells on one side of the stem, making it bend towards the light. In the root, auxins induce new root formation, either by encouraging elongation of the root or, at the right concentrations, encouraging the root to grow branches. What is not known is how this happens at a molecular level.

Charles Darwin

In 1880 Charles Darwin published his work “The Movement of Plants”, in which he reported on extensive observations he had made on the ability of plants to move in response to various environmental factors. Darwin did not know what was causing this movement, but he could identify that there were consistent patterns being shown and that a substance produced in the tip was being transmitted to other parts of the plant where it was having an effect on growth.

“In the case of the radicles of several, probably of all seedling plants, sensitiveness to gravitation is confined to the tip, which transmits an influence to the adjoining upper part, causing it to bend towards the centre of the earth.”

(Darwin 1880)

Darwin also knew that the ability of the stem to bend was caused by a change in the growth patterns of the cells on each side of the stem (Fig. 6). He reports on some of the known ideas around this concept, demonstrating that understanding of how auxins change cell growth has been around for over 100 years. It is clear from the quote below that in 1880 the concept that the turgidty of the cell changed to allow the cell to elongate was established.

“Until recently the cause of all such bending movements was believed to be due to the increased growth of the side which becomes for a time convex; that this side does temporarily grow more quickly than the concave side has been well established; but De Vries has lately shown that such increased growth follows a previously increased state of turgescence on the convex side... On the whole we may at present conclude that increased growth, first on one side and then on another, is a secondary effect, and that the increased turgescence of the cells, together with the extensibility of their walls, is the primary cause of the movement.”