Applications of noninvasive physiological sensing to measure indole acetic acid transport
The primary regulatory phytohormone, indole acetic acid (IAA), plays a crucial role in the growth and development of plants. The polar transport of this hormone is essential to processes such as vascular differentiation, organogenesis, apical dominance, and the tropic effects. Due to the importance of this growth regulator, the development of physiologically relevant methods to measure IAA transport is greatly needed. Basipetal IAA transport moves through root epidermal cells that are highly permeable to the hormone. This indicates that a plant root can be used as a window to make physiologically relevant measurements of IAA transport provided that appropriate real time noninvasive methods are available. In order to examine this phenomenon an IAA selective amperometric self referencing microsensor was fashioned to measure IAA flux from the roots of both brachytic 2 (br2), a mutant characterized with compromised IAA transport, and inbred line B73. Prior to microsensor fabrication the electrode design was optimized by testing different designs with large surface area voltametric electrodes (1.6 mM). Examination of each electrode design’s sensitivity and selectivity was conducted. A hybrid platinized platinum and multiwalled carbon nanotube electrode (Pt/Ptblack/MWNTs) was chosen as the most sensitive and stable design. Next, the optimal surface modification was then translated into a microsensor, calibrated and tested for dynamic efficiency. This microsensor was found to be sensitive for IAA and operates consistently at a dynamic efficiency of 0.98. An IAA microsensor was first used to find the saturation point during induced IAA flux in both B73 and br2. Then, the first real time endogenous IAA flux measurements were taken and mathematical methods were devised to analyze the integrated flux. Both induced and endogenous flux indicated that maximal transport occurs at the region of the root referred to as the distal elongation zone (DEZ). B73 seedlings were found to have two orders of magnitude higher induced and endogenous integrated flux. However, endogenous integrated IAA flux revealed a small net loss of IAA at the DEZ. Furthermore, oscillation patterns within efflux and influx of both B73 and br2 were found using the integrated flux methods. B73 were found to have higher amplitudes and shorter oscillatory periods than br2. Oscillation patterns were fitted to a simple harmonic model. Inhibition of IAA efflux and influx was apparent when conducting pharmacological studies with 1-N-naphthaylphthalamic acid (NPA) and 2-naphthoxyacetic acid (2-NOA) respectively. While NPA decreased integrated efflux it also decreased integrated influx. 2-NOA decreased integrated influx but raised integrated efflux considerably. Methods outlined in this body of work may be used to study essential IAA dependent processes such as gravitropism as well as other tropic effects.
Porterfield, Purdue University.
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