Aluminum Stress-Induced Alteration In Cytosine DNA Methylation And Its Possible Association With The Genotypic Difference In Tolerance To Aluminum Toxicity In Maize

Heterosis in crops has been one of the major mechanisms harnessed by breeders to increase grain yield by 15-60% over the last century, yet the molecular basis of heterosis still remains poorly understood. The combination of Aluminium toxicity with soil acidity is a major crop growth limiting factor in over 40% of the arable land in the world. Aluminium (Al) toxicity is particularly one of the most critical abiotic stresses which limit crop production in acid soils reducing maize yield by about 30-40% in developing countries. The Al ions dramatically damage the ability of roots to take water and nutrients therefore impairing crop growth and functions. However, maize and other plants exhibit considerable genotypic and phenotypic natural variation in Al tolerance both within and between species. The epigenome is particularly vulnerable to environmental factors and may become dysregulated during early seedling growth when DNA cytosine methylation patterning and chromatin structure are required for normal development.We investigated genetic and epigenetic variations in 12 maize inbred lines from which 2 Al tolerant and 2 Al sensitive inbreds were selected and used to make 12 reciprocal hybrids for Al screening, DNA cytosine methylation and heterotic studies. We further explored the possibility of heterotic links of such Al screening differences from an epigenetic perspective using Amplified Fragment Length Polymorphism (AFLP) and Methylation Sensitive Amplified Polymorphism (MSAP) analyses. These also included comparative evaluations involving Al toxicity screening using a standardized CIMMYT Al sensitive maize line and reciprocal hybrids. Finally, we used Al toxicity and low pH screening to study field traits on maize morphology and yield as related to their physiology, heterosis and DNA cytosine methylation. This approach was utilized to study the heterotic effect of toxicity of aluminum (Al) in maize in order to elucidate the molecular basis of heterosis using DNA cytosine methylation which is a key epigenetic mechanism. Heterotic morphological growth responses were measured after 4, 24 and 48 hrs exposure to 150μMAl and at pH 4.5 in inorganic half-strength MS nutrient solution.Root tolerance indexes (RTi) and physiological analysis via hematoxylin staining showed a high Al stress response and a rapid adjustment response to in tolerant lines. MSAP analysis of the CCGG hyper and hypo methylation patterns showed a range of 1.5-15% alteration in the total methylation patterns; the %CG level alteration (4.75-5.30) and % CNG (6.76-7.31) were as positively correlated to Al3+ toxicity. This dynamism of the CG and CNG epigenome hyper- and hypomethylation could be reflected in the Net Seminal Root Lengths (NSRL) % analyses and root growth responses. The selective screening of Al for tolerance even at embryonic stage increased the yield by 15.07% in Al tolerant inbred lines and also increased it by 8.17% in Al sensitive inbred lines; but, the screening for low pH was 3.03% in the Al sensitive inbred lines but 3.63% amongst the Al tolerant inbred lines. There results demonstrate and confirm past results that there are differences in Al sensitivity between cultivars but also show that low pH could be also influencing plants independently and at lower level. The influence of heterosis due to parent of origin was shown to be a significant factor to be considered in heterosis breeding, for example, the yield increase for N5xN1 in Al3+ was 23.90% while N1xN5 had 3.12% but, very little difference (2.62% vs 1.75 respectively) was observed due to pH 4.0. Polyacrylamide gel (PAGE) band variants from MSAP analysis found that cell wall and membrane related genes like CSLD3 (CELLULOSE SYNTHASE-LIKE), stress signals like KINASES, phytohormones such as auxins and protein transporters like the ABC transporters are involved in Al tolerance or sensitivity in maize. Al stress was effective in screening for heterosis and could be imprinted in maize at embryonic stage.Hence, these studies provide possible explanations on the complexity of Al tolerance mechanism from an epigenetic aspect in maize. Especially, this study proposes that the dynamisms in epigenetic regulation mechanisms like cytosine DNA methylation and parent of origin phenomenon in the intergenic regions in maize genome, plus the accumulation of retro-transposon insertions and DNA transposons which catalyze a rapid reshuffling of gene orders and gene contents are possible key components which determine the molecular basis of maize heterosis. It also uses the tropical maize Al tolerance level and"Al stress recall memory"to gauge the temperate maize ability to cope with increasing soil acidity and Al toxicity. Furthermore, the findings are profitable in the improvement and breeding of other tropical crops like sorghum, rice and wheat so as to produce genotypes which express Al tolerant genes heterotically and foster an epigenetic dimension in molecular Al tolerance breeding programs in acidic soils.