Evidence of accelerated and altered pollen development after Imazapyr treatment in resistant sunflower

Abstract

Imazapyr (IM) treatment applied at early reproductive developmental stages have been associated with induced male sterility in sunflower (Helianthus annuus L.). The aim of this work was to determine the effects induced by two different doses of IM-treatment (1X and 2X; 80 and 160 g a.i.•ha -1 ) on pollen grain yield, pollen viability and on the exine surface, in an intermediate resistant (I) and a completely resistant (R) genotype. An anatomical analysis of some stages during the microsporogenesis and microgametogenesis was also achieved by confocal microscopy. Plants of both genotypes survived the herbicide treatments and showed no external injury symptoms. Differential response to different doses of IM was obtained in R and I genotypes.The pollen grains per flower and the viability in seeds were significantly reduced in the I genotype by the 2X treatment; however, the pollen viability of the remaining grains was not altered, and the diameter of the pollen grains was increased by the herbicide treatment. Anatomical and cytological observations of the sporogenous tissue development were made via confocal microscopy. IM treatment induced alterations in sporogenous cells during the initial phases of microsporogenesis in both genotypes, and the 2X dose accelerated the microgametogenesis process, particularly in the I genotype. These findings improved the understanding of the mechanism underlying the gametocide effects of the imidazolinones in sunflower.

Gene expression profiling by cDNA‐AFLP reveals potential candidate genes for imazapyr‐induced male sterility in sunflower

“…However, the biological and molecular mechanisms involved in sunflower CIMS are still under study. Our previous reports have provided the first evidence of the gametocidal effect of imazapyr in resistant sunflower and have shown sporogenous tissue damage in both treated R and I genotypes (HA425 and 1058‐1 genotypes, respectively), but accelerated pollen development has been associated only with treated I plants (Ochogavía et al., 2020). We proposed a hypothetical mechanism in which AHAS inhibition induces amino acid starvation, leading to plastid destruction and autophagic cell death in microsporogenous tissue (Ochogavía et al., 2020).…”

“…Our previous reports have provided the first evidence of the gametocidal effect of imazapyr in resistant sunflower and have shown sporogenous tissue damage in both treated R and I genotypes (HA425 and 1058‐1 genotypes, respectively), but accelerated pollen development has been associated only with treated I plants (Ochogavía et al., 2020). We proposed a hypothetical mechanism in which AHAS inhibition induces amino acid starvation, leading to plastid destruction and autophagic cell death in microsporogenous tissue (Ochogavía et al., 2020). This mechanism has also been related to the cytocidal effects of sublethal doses of AHAS‐inhibiting herbicides in susceptible plants of rapeseed and Arabidopsis (Zhao et al., 2015; Liu et al., 2017).…”

“…Chemical induced male sterility is a valuable tool for timing optimization in hybrid crop breeding. In sunflower, imidazolinones have been proven to be promising AHAS‐inhibitory hybridization agents since the application of these herbicides such as imazethapyr or imazapyr during the early reproductive developmental stages induces male sterility in resistant sunflower lines (Sala & Bulos, 2012; Ochogavía et al., 2018, 2020). However, the biological and molecular mechanisms involved in sunflower CIMS are still under study.…”

“…This method, known as chemically induced male sterility (CIMS), represents a promising alternative that could allow breeders to test the combining ability of sunflower maintainer lines during early stages of development, to promote heterosis and outcrossing, with minimal prebreeding requirements. We have previously found that treatment of Imisun sunflower with the field recommended dose of imazapyr allows obtaining partial CIMS and that treated plants show a significant reduction of pollen yield and seed production, associated with pollen development alterations and sporogenous tissue damage (Ochogavía et al., 2018; 2020).…”

“…Based on all the above, the aim of this work was to characterize the molecular mechanisms of the CIMS system through imazapyr treatment during early reproductive stages in two Imisun sunflower lines: a completely resistant, HA425, and an intermediate resistant line, 1058‐1 (Bruniard & Miller, 2001; Miller & Al‐Khatib, 2002). The comparative transcript profiling analysis between treated and control plants was achieved by complementary DNA amplified fragment‐length polymorphism (cDNA‐AFLP) on the three most informative dates during the development of the sporogenous tissue (Ochogavía et al., 2018; 2020). To confirm the cytocidal effect of the treatment at physiological and anatomical levels, the pollen physiology was evaluated and the development of the sporogenous tissue was analyzed by Differential Interference Contrast (DIC) microscopy.…”