Identifying Chloris Species from Cuban Citrus Orchards and Determining Their Glyphosate-Resistance Status

Abstract

The Chloris genus is a C4 photosynthetic species mainly distributed in tropical and subtropical regions. Populations of three Chloris species occurring in citrus orchards from central Cuba, under long history glyphosate-based weed management, were studied for glyphosate-resistant status by characterizing their herbicide resistance/tolerance mechanisms. Morphological and molecular analyses allowed these species to be identified as C. ciliata Sw., Chloris elata Desv., and Chloris barbata Sw. Based on the glyphosate rate that causes 50% mortality of the treated plants, glyphosate resistance (R) was confirmed only in C. elata, The R population was 6.1-fold more resistant compared to the susceptible (S) population. In addition, R plants of C. elata accumulated 4.6-fold less shikimate after glyphosate application than S plants. Meanwhile, populations of C. barbata and C. ciliata with or without glyphosate application histories showed similar LD50 values and shikimic acid accumulation rates, demonstrating that resistance to glyphosate have not evolved in these species. Plants of R and S populations of C. elata differed in 14C-glyphosate absorption and translocation. The R population exhibited 27.3-fold greater 5-enolpyruvyl shikimate-3-phosphate synthase (EPSPS) activity than the S population due to a target site mutation corresponding to a Pro-106-Ser substitution found in the EPSPS gene. These reports show the innate tolerance to glyphosate of C. barbata and C. ciliata, and confirm the resistance of C. elata to this herbicide, showing that both non-target site and target-site mechanisms are involved in its resistance to glyphosate. This is the first case of herbicide resistance in Cuba.

From tolerance to resistance: mechanisms governing the differential response to glyphosate in Chloris barbata

“…In addition, the S C. barbata population showed LD 50 value (545 g ae ha −1 ) close to the standard dose, evidencing a certain innate tolerance level in this species. Sensitive populations of C. elata , C. ciliata , C. truncata and C. virgata also recorded high degrees of tolerance to glyphosate with LD 50 ranging from 515 to 1385 g ae ha −1 of glyphosate . Taking into account that controlling a weed population requires at least twice its estimated LD 50 , the standard dose of 720 g ae ha −1 used is not enough to control the S C. barbata population (Fig.…”

“…However, the contribution of glyphosate metabolism as a resistance mechanism is not clear, and so far occurs at significantly lower rates in glyphosate than glyphosate‐resistant transgenic crops carrying the glyphosate oxidoreductase (GOX) gene . In this respect, these studies demonstrated that treated weed plants, be they susceptible, tolerant or resistant, can metabolize minimal amounts of glyphosate into non‐toxic substances, which could be a biological characteristic . However, metabolism does not appear to be related to resistance to glyphosate of the R1 and R2 C. barbata populations.…”

“…The resistance levels associated with mutations in this position (from Pro to Ser, Ala, Leu or Thr) are well established in weeds, being two to three times higher than the recommended doses. The mutations Pro106‐Ser and ‐Leu/‐Ser were identified in glyphosate‐resistant populations of C. elata from Cuba and C. virgata from Australia, respectively, contributing totally or partially to their resistance.…”

“…Sensitive populations of C. elata, C. ciliata, C. truncata and C. virgata also recorded high degrees of tolerance to glyphosate with LD 50 ranging from 515 to 1385 g ae ha −1 of glyphosate. [16][17][18] Taking into account that controlling a weed population requires at least twice its estimated LD 50 , 3 the standard dose of 720 g ae ha −1 used is not enough to control the S C. barbata population ( Fig. 1).…”

“…14 Chloris barbata (L.) Sw. is widely distributed in the coastal states of Mexico, 14 coinciding with the main citrus-producing regions. It is worth noting that species of the genus Chloris have a certain level of natural tolerance to glyphosate in comparison with species of other genera, [15][16][17][18] which allows these species to survive and reproduce after field herbicide applications lethal to wild plants of other species. 19 The survival of weed individuals after repeated application of the same herbicide or action mechanism is due to evolutionary adaptations resulting in herbicide resistance.…”