The accumulation of conjugates of indole‐3‐acetic acid (IAA) in Arabidopsis thaliana was studied by incubating tissues with high concentrations of exogenous IAA, followed by reverse phase HPLC analysis of the extracts. Using fluorescence detection, indole‐3‐acetyl‐aspartate, indole‐3‐acetyl‐glutamate, and indole‐3‐acetyl‐glucose were observed and quantitated in extracts of tissue after 24 h incubation with 500 μM IAA. In addition, a new metabolite was detected and positively identified as indole‐3‐acetyl‐glutamine by fast atom bombardment mass spectrometry, exact mass measurement, and tandem mass spectrometry in comparison with a synthetic standard. The amounts of individual conjugates formed differed between leaves, shoot axes and roots. In all three tissues, indole‐3‐acetyl‐aspartate was the most abundant conjugate, the highest level being observed in roots. Highest levels of indole‐3‐acetyl‐glutamine were observed in leaves, where it was the second most abundant conjugate and comprised approximately 12% of the fluorescent metabolites. Accumulation of the three amide conjugates was dramatically inhibited by cycloheximide, whereas accumulation of indole‐3‐acetyl‐glucose was little affected. Based on these data, a screen for Arabidopsis mutants altered in the IAA‐inducible system for auxin conjugate formation was initiated. The first mutant to be isolated and characterized produces more indole‐3‐acetyl‐glutamine and less indole‐3‐acetyl‐aspartate than wild‐type, and is allelic to an existing class of photorespiration mutants (gluS) deficient in chloroplastic glutamate synthase.

The objective of this investigation was to examine the response to exogenous auxin (indole-3-acetic acid; 1AA)of stem segments at two developmental stages. The standard auxin response of excised stem segments and intact plants consists of an initial growth response and a prolonged growth response. We found that this biphasic response does not occur in internodes at very early stages. Stem segments of light grown pea of various genotypes were cut when the fourth internode was at 6-l 3% of full expansion (early-expansion) or at 18-25% of full expansion (mid-expansion). Length measurements of excised segments were made after 48 hours of incubation on buffer with or without auxin. An angular position transducer linked to a computerized data collection system provided high-resolution measurement of growth of stacks of segments incubated in buffer over 20 hours. Early-expansion segments of all genotypes deviated from the standard auxin response, while mid-expansion segments responded in a manner consistent with previous reports. Early-expansion segments of tall, light-grown plants were unique in showing an auxin-induced inhibition of growth. The auxin-induced inhibition correlated with high endogenous auxin content, as determined by HPLC and GUMS, across genotypes and between early-expansion and midexpansion segments of tall plants. Measurement of ethylene evolved from stem segments in response to auxin, and treatment of segments with the ethylene action inhibitor, norbomadiene, showed the inhibition to be mediated in part by heightened ethylene sensitivity. Growth of early-expansion segments of dwarf and severe dwarf plants was stimulated by exogenous auxin, but the growth rate increase was delayed compared to that in mid-expansion segments. This is the first time that such a growth response, termed the delayed growth response has been demonstrated. It is concluded that developmental stage and endogenous hormone content affect tissue response to exogenous auxin.

No abstract

Would you rather solve a forensics puzzle or take a lab practical? An alternative to the traditional lab practical can be used to assess students' skills and knowledge in plant cell biology and anatomy. This forensics project challenges students to analyze evidence from crime scenarios. The instructor supplies the scenarios, evidence collected at the crime scene, and type specimens. The students prepare the evidence and type specimens for analysis by light and polarized light microscopy and then document and report their findings. Students enjoy this project and are able to demonstrate their skills as well as their knowledge.

No abstract