Rôles des brassinostéroïdes dans la résistance des plantes aux maladies : cas du riz (Oryza sativa L.) et du maïs (Zea mays L.)

Abstract

Among the most widely grown cereals, rice and maize are of major economic importance in the world. In Cameroon, their cultivation is hampered by several abiotic and biotic factors, the most widespread pathogens being Helminthosporium turcicum (responsible for heminthosporiosis in maize), and Bipolaris oryzae, (responsible for heminthosporiosis in rice). They can cause yield losses of between 50 and 100% under favourable conditions if no protective measures are taken. These pathogens are being controlled by use of synthetic chemicals, which are both expensive and a source of environmental pollution and food contamination. To overcome this problem, several alternative control methods are commonly used. One of these alternative methods or strategies is based on the optimization of natural defense mechanisms of plants during plant-parasite interaction through the use of natural substances such as brassinosteroids. The main objective of this work was to study the growth stimulating properties and resistance inducing potential of 24-epibrassinolide on a few rice and maize varieties with different sensitivities to cryptogamic diseases. Toxicity tests of 24-epibrassinolide were performed in vitro on isolates of Bipolaris oryzae and Helminthosporium turcicum. Four doses were used (C1: 0.0625; C2: 0.0125; C3: 0.025 and C4: 0.05 ml/ml). The experimental design was a completely randomized bifactorial split-plot in field, comprising of two varieties per species (primary factor), five treatments (secondary factor) and three replicate (blocks). The five treatments (T0: control; T1: 24-epibrassinolide (EBR); T2: Ridomil for maize and BANKO PLUS for rice; T3: NPK fertilizer; T4: mixture: NPK+Ridomil or BANKO PLUS+EBR) were repeated three times each. The in field work took place during two successive campaigns (2015 and 2016) in the 5th agroecological zone of Cameroon, and for each speculation. Agro-morphological, epidemiological and biochemical parameters of the rice and maize plants were assessed. The results of the in vitro tests show a significant effect on the radial growth of the fungi at dose C4 (0.05 ml/ml) with an inhibition percentage of 87.56% for Bipolaris oryzae and from dose C3 (0.025 ml/ml) with an inhibition percentage of around 94.64% for Helminthosporium turcicum against 100% for fungicides. From the regression equation obtained after studying the correlations and inhibition percentages, the MIC50 and MIC90 of 24-epibrassinolide determined are 0.013 ml/ml and 0.86 ml/ml for Bipolaris oryzae respectively. The MIC90 of H. turcicum is 0.66 ml/ml. The MIC50 was not determined. The main field results show the significant action of 24- epibrassinolide on plant growth, yield, disease resistance, synthesis of secondary metabolites and defense proteins. 24-epibrassinolide significantly reduced dry grain losses, promoting a xxiv gain of about 1.5 t/ha compared to the control for the two maize varieties studied (mean EBR gain: 3.93 t/ha and control: 2.47 t/ha). In addition, CMS 90-15 was more productive than CMS 85-01. For the two rice varieties tested, the average gain in paddy grain yield was 1.55 t/ha compared to the control (mean EBR gain: 3.9 t/ha and control: 2.33 t/ha). Although it provided a better plant development, the EBR had no effect on disease incidence. However, the severity was sufficiently reduced on maize by its action with an average technical efficacy of 42.3% for CMS 90-15 and 37.3% for CMS 85-01. On the other hand, the technical efficacy on rice varies from 46.2% for NERICA 3 and 45.3% for KAMKOU. Concerning resistance to helminthosporiosis, EBR acted indirectly on the disease by stimulating the natural defense of plants. Biochemical parameters (total phenols, total proteins, polyphenoloxidases and pathogenicity related PR proteins) showed a high accumulation of the latter in the leaves of EBR-treated plants. For total phenolic compounds, the mean levels in the EBR treatment of CMS 90-15 and CMS 85-01 varieties were 6.91 ± 0.62 and 9.88 ± 0.80 GAeq/mg FW respectively, in contrast to the control plants for which the respective mean levels were 5.58 ± 0.04 and 6.48 ± 1.05 GAeq/mg FW. Total phenol contents were also high in the leaves of rice treated with EBR, with 5.37 ± 1.03 versus 3.58 ± 1.1 for the control; 13.73 ± 1.45 and 6.10 ± 0.80 GAeq/mg FW for the EBR for NERICA 3 and KAMKOU varieties respectively. The soluble and parietal protein contents were high in plants having received 24-epibrassinolide compared to the negative control, irrespective of the variety (CMS 90-15 control: 61.69 ± 5.91 eqBSA/mg FW and CMS 90-15 EBR: 73.39 ± 2.03 eqBSA/mg FW; KAMKOU control 128.18 ± 7.38 eqBSA/mg FW and EBR 148.53 ± 0.44 eqBSA/mg FW). Analyses for the determination of the enzymatic activities of PPOs (for KAMKOU, Control: 0.092 ΔA330/min/g FW, EBR: 0.302 ΔDO330/min/g FW), POX and PR2 followed the same logic as total phenols and total proteins. The activities of these three compounds are elevated by the EBR input. There are positive or negative correlations between certain agro-morphological, epidemiological and biochemical parameters. Given the importance of rice and maize in the diet of populations throughout the world and particularly in Cameroon, there is an urgent need to boost local production of these foodstuffs in order to strengthen food security. These results show that 24-epibrassinolide can be used to control helminthosporiosis in maize and rice and to increase yields of these two crops