Host-Plant Resistance Mechanisms (HPRs) of Commercial Sugarcane (Saccharum officinarum L.) Varieties in Response to Yellow Sugarcane Aphid (YSA) (Sipha flava Forbes) Herbivory under Irrigated Cropping Systems

Abstract

Worldwide, the well-known and problematic Yellow Sugarcane Aphid (YSA) (Sipha flava) plagues sugarcane (Saccharum officinarum (L.). The study's objective was to classify and document commercial sugarcane accessions host-plant resistance mechanisms according to their morphology, physiology, phytochemistry, and biochemistry in response to YSA herbivory. First, a multidisciplinary systematic literature review (SLR) was conducted to comprehend the host plant resistance mechanisms (HPRs) of different sugarcane accessions in response to YSA herbivory. Second, seven sugarcane cultivars were subjected to factorial experiments utilizing a natural aphid infestation, with the following four objectives in mind; explore leaf pubescence that mediates resistance to YSA, assess physiological response as a tolerance mechanism to YSA herbivory, evaluate biochemical resistance in retort to YSA feeding, carryout phytochemical profiling of metabolites that confer resistance to YSA feeding, and reconnoiter plant resistant index (PRI) of tested sugarcane germplasm in response to YSA incursion. The results of the SLR revealed a bias towards S. flava in sugarcane, with most studies being skewed in favor of other crops, insect arthropods, and other aphid species. These results demonstrated the paucity of knowledge regarding the previously specified SLR objectives in sugarcane. The majority of the research focused on comprehending the host plant resistance mechanisms induced by sugarcane aphid (SCA). Therefore, studying YSA was novel and crucial for controlling this pest of sugarcane. The results of leaf pubescence (trichome density) varied significantly (p<0.05) among the sugarcane accessions in terms of leaf position and aphid number. Leaf pubescence resistance was ranked from high to low based on the trichome density: 00-1165 > ZN 8 > ZN 9 > ZN 3L > 96-1107 > N14 > ZN 10. Information on physiological tolerance was gathered using SPAD and CIRAS-3 instruments. The content of chlorophyll and the gas exchange responses of the sugarcane accessions under investigation showed significant (p<0.05) tolerance variations. In addition, the sugarcane accessions; 96-1107, N14, and ZN 10 were the most susceptible to the physiological damage caused by YSA. The findings showed that in susceptible sugarcane accessions, YSA decreased biochemical parameters (total protein, total soluble sugars, and total chlorophyll content). Furthermore, the evaluated biochemical properties of sugarcane accessions were either completely retained or partially decreased on resistant (00-1165) and moderately resistant (ZN 9, ZN 8, and ZN 3L) sugarcane accessions. The synthesis of high phenol and flavonoid content in sugarcane accessions was significantly (p<0.05) stimulated by YSA feeding. iv Additionally, there was a significant (p<0.05) positive correlation between the following: the percentage change in flavonoid content and the number of aphids, and the percentage change in phenol content. Three mechanisms of resistance; antixenosis, tolerance, and antibiosis were used to evaluate PRI. 00-1165 sugarcane accession is only moderately resistant, according to PRI reading of 37.6, when compared to other accessions that exhibit tolerance, antixenosis, and antibiosis traits. Furthermore, accessions such as ZN 8, ZN 3L, and ZN 9 have relatively >5 PRI in comparison to the susceptible check (ZN 10). Leaf pubescence and leaf position reduced YSA number among the sugarcane varieties. Among the sugarcane accessions, leaf pubescence and leaf position decreased the number of YSA. Additionally, in susceptible sugarcane accessions, YSA reduced the biochemical parameters that were examined. Furthermore, sensitive varieties' chlorophyll content and gas exchange responses were reduced by YSA, but tolerant varieties were able to maintain or compensate for these losses. Moreover, YSA increased the production of flavonoids and phenols in sugarcane accessions, resulting in induced resistance. The 00-1165 sugarcane accession exhibited a moderate expression of the three resistance mechanisms that were studied: tolerance, antibiosis, and antixenosis. Furthermore, antixenosis and tolerance resistance mechanisms were demonstrated by ZN 8, ZN 9, and ZN 3L. The susceptible accessions; N14, ZN 10, and 96-1107 PRI were unable to fall within the three resistance mechanisms. ZN 8 and ZN 9 sugarcane accessions showed low resistance to YSA stress, while 00-1165 exhibited medium resistance as shown by its aphid quantity ratio (AQR), which fell between 0.30-0.60. Moreover, low YSA sensitivity was observed in ZN 3L and 96-1107. N14 had a medium sensitivity as well. Finally, cultivar ZN 10 can be regarded as a very susceptible accession due to its AQR which was more than 1.50. For the Zimbabwean sugar sector, the SLR study on the previously described research objectives was required in order to incorporate resistance into the current Integrated YSA management tactics. Sugarcane growers should update their YSA management programs to include pubescent (high-density trichome) accessions. Growers of sugarcane should use biochemically tolerant YSA accessions in their crop management plans to maintain or compensate for total soluble sugars, according to the study's findings. The physiologically compensating and sustaining behavior of YSA tolerant accessions with respect to gas exchange responses and chlorophyll content should also be taken into consideration by farmers. Since seven sugarcane accessions were used, there is a need for ongoing screening of the industry's sugarcane germplasm for PRI based on antixenosis, v tolerance, and antibiosis. Molecular breeding approaches must be used to isolate the genes of high phenol and flavonoid tolerant varieties for YSA integrated management. As this study indicates, host plant resistance is a promising sustainable management option for controlling S. flava; thus, it needs to be incorporated into the current integrated pest management strategies (IPM) because there is an overreliance on chemical method.