The role of ethylene and auxin in responses of roots to phosphate supply in white clover (Trifolium repens L.) : a thesis presentation in partial fulfilment of the requirements for the degree of Master of Science in Plant Molecular Biology at Massey University, Palmerston North, New Zealand
Phosphate (P) supply is one of the major determining factors to plant productivity,
since the element affects the growth and the development of plants. In response to Pdeficiency
treatment, plants display alterations in root system architecture caused by
changes in primary root (PR) and lateral root (LR) length and LR density. In this
thesis, the root growth of the agronomically important legume, white clover (Trifolium
repens L.) was found to be slightly stimulated in terms of PR length, LR number and
total LR length when plants were grown in a P-deficient media (0.01 mM
orthophosphate; Pi) when compared with plants grown in a P-sufficient media (1.00
mM Pi) when using a hydroponic growth system.
When plants are grown in a P-sufficient media, treatment with 100 nM exogenous 1-
aminocyclopropane-1-carboxylic acid (ACC) and exogenous auxin (5 nM 1-
naphthylacetic acid, NAA) resulted in significant increases in white clover PR length,
LR number and LR length. However, when ethylene action or auxin transport were
inhibited using 300 ppm 1-methylcyclopropene (1-MCP) and 100 nM 1-Nnaphthylphthalamic
acid (NPA), respectively, root growth was significantly reduced
which suggests roles for ethylene and auxin in mediating white clover root growth.
To examine the effects of these hormones on plants grown in P-deficient media, 100
nM ACC treatment significantly enhanced the stimulatory effects of growth on Pdeficient
media only, while exposure of plants to P-deficiency alone was sufficient to
significantly neutralise the inhibitory effects of 1-MCP on root growth. Hence,
exposure to P-deficiency is proposed to increase either ethylene biosynthesis or
ethylene sensitivity in white clover roots. In contrast, for plants grown in P-deficient
media, treatment with 5 nM NAA significantly abolished the stimulation of white
clover root growth observed with P-deficiency so it is proposed that exposure to Pdeficiency
increases either auxin biosynthesis or auxin sensitivity, but the 5nM NAA
concentration used was too high to stimulate root growth. Using DR5p::GUS
transgenic white clover, auxin activity was found in the root tips and root primordia.
Using these plants, it is suggested that P-deficient treatment and ACC treatment
influenced white clover root growth through an increase in auxin sensitivity.
Overall, ethylene and auxin are found to be essential in mediating white clover root
growth in P-sufficiency, and also in mediating root responses to P-deficiency through
changes in terms of the biosynthesis and the sensitivity of these two hormones.