Soybean (Glycine max (L.) Merr.) Root Suberin. Anatomical Distribution, Chemical Composition and Relationship to Partial Resistance to Phytophthora sojae (Kaufmann and Gerdemann)

Raymond Thomas , Xingxiao Fang , Kosala Ranathunge , Terry R. Anderson , Carol A. Peterson , and Mark A. Bernards ^*

Environmental Stress Biology Group, Department of Biology, The University of Western Ontario, London, ON, Canada, N6A 5B7; Department of Biology, University of Waterloo, Waterloo, ON, Canada, N2L 3G1; Agriculture and Agri-Food Canada, Greenhouse and Processing Crops Research Centre, Harrow ON Canada, N0R 1G0

^* Corresponding author; email: bernards{at}uwo.ca.

Soybean (Glycine max (L.) Merr.) is a versatile and importantagronomic crop, currently grown worldwide. Each year millionsof dollars of potential yield revenues are lost due to a rootrot disease caused by the oomycete Phytophthora sojae [Kaufmann& Gerdemann]. Since the root is the primary site of infectionby this organism, we undertook an examination of the physico-chemicalbarriers in soybean root, namely the suberized walls of theepidermis and endodermis, to establish whether or not preformedsuberin (i.e., naturally present in non-infected plants) couldhave a role in partial resistance to P. sojae. Herein we describethe anatomical distribution and chemical composition of soybeanroot suberin as well as its relationship to partial resistanceto P. sojae. Soybean roots contain a state I endodermis (Casparianbands only) within the first 80 mm of the root tip, and a stateII endodermis (Casparian bands and some cells with suberin lamellae)in more proximal regions. A state III endodermis (with thick,cellulosic, tertiary walls) was not present within the 200 mmlong roots examined. An exodermis was also absent but some wallsof the epidermal and neighboring cortical cells were suberized.Chemically, soybean root suberin resembles a typical suberin,and consists of waxes, fatty acids, ${omega}$ -hydroxy acids, ${alpha}$ , ${omega}$ -diacids,primary alcohols and guaiacyl- and syringyl-substituted phenolics.Total suberin analysis of isolated soybean epidermis and endodermistissues demonstrated (1) significantly higher amounts in theendodermis compared to the epidermis/outer cortex, (2) increasedamounts in the endodermis as the root matured from state I tostate II, (3) increased amounts in the epidermis/outer cortexalong the axis of the root, and (4) significantly higher amountsin tissues isolated from a cultivar (Conrad) with a high degreeof partial resistance to P. sojae, compared with a susceptibleone (OX760-6). This latter correlation was extended by an analysisof nine independent cultivars and 32 recombinant inbred lines(derived from a Conrad x OX760-6 cross) ranging in partial resistanceto P. sojae: strong negative correlations (- 0.89 and - 0.72,respectively) were observed between the amount of the aliphaticcomponent of root suberin and plant mortality in P. sojae-infestedfields.

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