Assimilation of exogenous 2′-14C-indole-3-acetic acid and 3′-14C-tryptophan exposed to the roots of three wheat varieties

Abstract

This study was conducted to determine if plants can assimilate indole-3-acetic acid (IAA) from rooting media and if exogenous L-tryptophan (L-TRP) can be assimilated and converted by plants into auxins. The addition of 2'-laC-IAA (3.7 kBq plant -s) to wheat (Triticum aestivum L.) seedlings of three varieties grown in nutrient solution resulted in the uptake (avg. = 7.6%) of labelled IAA. Most of the label IAA was recovered in the shoot (avg. = 7.2%) with little accumulation in the root (avg. = 0.43%). A portion of the assimilated IAA-label in the plant was identified by co-chromatography and UV spectral confirmation as IAA-glycine and IAA-aspartic acid conjugates. Little of the assimilated IAA label was found as free IAA in the wheat plants. These same assimilation patterns were observed when 2'-14C-IAA was added to wheat plants grown in sterile and nonsterile soil. In contrast, the wheat varieties assimilated considerably less (avg. = 1.3 %) of the added microbial IAA precursor, 3'-IaC-L-TRP (3.7 kBq plant-5) and thus much lower amounts of IAA conjugates were detected. Glasshouse soil experiments revealed that 2 out of 3 wheat varieties had increased growth rates and increased yields when L-TRP (10 -5 and 10 -7 M) was added to the root zone. It is surmised that this positive response is a result of microbial auxin production within the rhizosphere upon the addition of the precursor, L-TRP. The amino acid composition of the root exudates plays a critical role in microbial production of auxins in the rhizosphere. This study showed that wheat roots can assimilate IAA from their rooting media, which will supplement the endogenous IAA levels in the shoot tissue and may positively influence plant growth and subsequent yield.

Azotobacter and L-tryptophan Application for Improving Wheat Yield

“…It is now well established that various Azotobacter species are capable of synthesizing plant hormones and is considered the most plausible mechanism of action. Significant yield increase in response to L-TRP application in present study is in agreement with the findings of many other workers (Martens and Frankenberger, 1994;Arshad et al, 1995). The increased N concentration and uptake in grain and straw in our study is also in accordance with the findings of many other workers (Arshad et al, 1994a,b;Zahir et al, 1997).…”

“…The effect of L-TRP on growth and yield of wheat could be attributed to either auxin metabolites produced by the rhizosphere microflora which were subsequently taken up by plant roots or direct uptake of L-TRP by the plant roots with subsequent catabolism into auxins within the plant tissue and/or alteration in the balance of the rhizosphere microbial community in response to L-TRP addition which may affect growth and yield of wheat. However, Martens and Frankenberger (1994) reported very poor uptake of labelled L-TRP compared with labelled IAA by wheat seedling roots. They also demonstrated poor endogenous conversion of exogenously applied labelled tryptophan into auxins by wheat seedlings grown under axenic environments.…”

Role of Pseudomonas putida Indoleacetic Acid in Development of the Host Plant Root System

“…In this research we showed that bacterial IAA stimulates the development of the host plant root system. The advantage for root-associated bacteria is a rich supply of nutrients, as much of the metabolic products of the carbon fixed by plants is lost from roots and moves into the rhizosphere as exudates, lysates, and mucilage (32).…”

Pathogenicity of indole-3-acetic acid producing fungus Fusarium delphinoides strain GPK towards chickpea and pigeon pea

“…Tryptophan (Trp) serves as a physiological precursor for the biosynthesis of auxin in higher plants and in microbes (Frankenberger and Arshad 1995). Root exudates are natural sources of Trp for the rhizosphere microflora, which may enhance auxin biosynthesis in the rhizosphere (Martens and Frankenberger 1994). The most efficient auxin producers are found among fungal inhabitants of the plant rhizosphere and phyllosphere (Tsavkelova et al 2005).…”