No abstract

E. phyllopogon has multiple copies of the ALS and ACCase genes, and different expression patterns were observed among the copies. The existence of three active ACCase genes and the difference in their relative expression levels could influence the occurrence of target-site resistance to ACCase inhibitors in E. phyllopogon.

Echinochloa oryzicola Vasing. (= Echinochloa phyllopogon Stapf ex Kessenko) is an obligate weed with an elaborated survival strategy in the flooded rice of Japan. In this review various adaptive characters of the weed, which comprise the survival strategy, are discussed through the life cycle. The weed is distributed only in flooded rice. Seeds (spikelets) buried in the soil exhibit annual cycles between dormant and non-dormant state, and non-dormant seeds recurrently appear in spring when rice growers start to prepare seedling beds and fields for rice transplanting. The non-dormant seeds have unique characters metabolically adapted to submerged conditions to germinate and grow by the anaerobic respiration through alcohol fermentation. The weed has seemingly perfect mimicry of the rice plants throughout its development from seedling to heading, by which the weed escapes from manual weeding. In a rice paddy, the weed starts heading coincidentally with the rice plants at the period when the growers are reluctant to walk in the rice paddy to weed. Irrespective of plant height of the rice cultivar, the weed develops a few upper leaves above the rice canopy during the heading period of rice.This phenotypic plasticity of E. oryzicola in plant height is one of the characters conferring its competitive aggressiveness in flooded rice. When weeding is begun again after heading, the dormant weed seeds escape weeding by shattering and join the soil seedbank. The dormant seeds express the gene of an enzyme catalyzing ATP synthesis through the mitochondrial oxidative phosphorylation more abundantly, and have larger oxygen absorption and enzyme activity of the aerobic respiration than the non-dormant seeds, suggesting that the dormant seeds maintain viability by the conventional aerobic respiration in the paddy soil drained from rice harvesting in fall to the next early spring. The various adaptive characters comprising the survival strategy of E. oryzicola in flooded rice consist of those inherited from the wild progenitor and those selected by the crop cultivation pressure. It is suggested that both the mimicry of the weed and the heading coincident with the rice plants have been acquired by the large selection pressure of frequent weeding, which has been done over the past hundred years. However, today, the manual weeding is substituted with herbicides, which cannot detect the mimicry and heading photoperiodic sensitivity. As a result, the dominant species of Echinochloa weeds in flooded rice is changing from E. oryzicola to Echinochloa crus-galli var. crus-galli that has neither mimicry nor photoperiodic sensitivity synchronizing to that of rice, but is more competitive against rice.

Late watergrass is a serious weed of California rice that has evolved resistance to molinate, thiobencarb, fenoxaprop-ethyl, and bispyribac-sodium. To obtain an insight into the origin and spread of resistant (R) late watergrass in California rice fields, we evaluated similarities in morphological traits and amplified fragment length polymorphism (AFLP) fingerprints among 15 R strains compared with susceptible (S) strains. All strains were derived by inbreeding from accessions collected in rice fields of the Sacramento Valley, CA. In the field, R plants were shorter than S plants; they also had narrower and shorter flag leaves and thinner culms. Spikelets also appeared smaller and more slender in R plants. There was greater morphological similarity among the 15 R strains than among the eight S strains. The mean coefficients of variation for morphological traits were much smaller among R strains, which in a cluster analysis (Ward's method) were grouped morphologically apart at early clustering stages from the more variable S strains. AFLP electropherograms also showed greater similarity between R strains. R strains were grouped separately from the S strains in a cluster analysis based on calculated Nei and Li coefficients used in an unweighted pair group method using arithmetic means. However, small genetic differences also existed because the R strains were grouped into six clusters, suggesting that R strains were not samples from an identical strain. It was concluded that R strains originated from a preexisting and preadapted mutant late watergrass population in the Sacramento Valley. This study establishes that resistance moved by spikelet dispersal, not independent mutation events, most likely defined the geographical distribution of R late watergrass in California. Prevention and control of this dispersal combined with elimination of seed-producing survivors after herbicide treatment should be relevant components of the integrated management of herbicide-resistant late watergrass in California rice.

To compare the regulation of anaerobic metabolism during germination in anoxia-tolerant and intolerant plants, enzymes associated with anaerobic metabolism such as sucrose synthase, aldolase, enolase, pyruvate decarboxylase (PDC), alcohol dehydrogenase (ADH), and aldehyde dehydrogenase (ALDH) were assayed in two varieties of Echinochloa crus-galli, formosensis (tolerant) and praticola (intolerant). The initial and intervening enzymes of the pathway (sucrose synthase and aldolase) and enzymes in the last part of the pathway (PDC, ADH and ALDH) revealed similar changing patterns in activities during germination. This implies that each group of enzymes may be controlled by an identical regulatory mechanism. During anoxia, activities of all enzymes increased 1.5-30-fold in both varieties compared to their activities under aerobic conditions. Activities of sucrose synthase, enolase and ADH exhibited the same induction patterns under anoxia in formosensis and praticola. However, the activities of aldolase, ALDH and PDC were more strongly induced in formosensis under anoxia (1.2-2-fold) than in praticola. These enzymes were also assayed in F(3) families which varied in their anaerobic germinability. For PDC, activities under anoxia in anoxia-tolerant families were similar to those of an anoxia-intolerant family during the whole period although the family did not exhibit anaerobic germinability. This suggests that there is no correlation between PDC activity and anaerobic germinability. For ALDH, activities were more strongly induced under anoxia in anoxia-tolerant families than in anoxia-intolerant families, a trend also exhibited by the parents. This indicates that ALDH may play a role in detoxifying acetaldehyde formed through alcoholic fermentation during anaerobic germination.