The cell wall as a target for improving agronomic and biotechnological traits in maize ("Zea mays L.")

Resum

Maize (Zea mays L.) is the most widely cultivated cereal in the world, followed by wheat and rice. Although it is a very productive and multiple-purpose crop, its yield is compromised by several abiotic and biotic factors. Since cell wall characteristics affect many physiological aspects of the plants, its study is relevant to determine the possible causes related to important issues of maize crop. In this framework, the general objective of this Thesis was to deepen into the understanding of the putative association between cell wall composition and structure and their impact on various aspects affecting maize crop. Lodging leads to maize yield losses worldwide and, at least, stalk strength has been related to lodging resistance. Cell walls of twelve inbred lines with different stalk strength (tested as rind penetration strength) and lodging resistance were analyzed to unravel the suspected interrelation among cell wall, stalk strength and lodging resistance. After analyzing cell wall compositional and structural data, stepwise multiple regression revealed that H lignin subunits confer higher resistance to rind penetration strength. In case of lodging predictive model, ferulic acid content enhances lodging resistance, while total content of diferulates reduce it. These findings suggest that lodging susceptibility and stalk strength may be influenced by structural traits of cell walls rather than the net amount of the major components, such as cellulose, hemicelluloses and lignin. Plants undergo morphological and physiological adaptations in response to stresses, which usually involves cell wall modifications. In this respect, the cell wall constitutes the first barrier to both biotic and abiotic factors, playing an essential role in defense mechanisms. Maize crop faces challenges that affect its production, such as the above-mentioned lodging as well as drought stress. Basing on the results from the lodging trial, two maize inbred lines with different resistance to this trait were selected for a drought experiment. Both inbreds, B73 and EA2024, showed different tolerance to drought, being EA2024 more resistant to this abiotic stress. Well-watered plants of EA2024 showed an increased content of cellulose, uronic acid and p-coumaric acid than B73. Upon water deficit, they follow different strategies to overcome the stress: EA2024 had higher levels of arabinose-enriched polymers, such as arabinogalactan proteins and a lower lignin content, while B73 alters lignin composition and increases uronic acid content. Basing on these results, we propose that a cell wall composition useful for resisting lodging, such as that exhibited by B73, could affect the plasticity of its cell wall and thus increase its susceptibility to drought. Regarding biotic factors, fungal diseases and particularly Fusarium graminearum, which causes stalk rot disease, impact on maize production as well as on animal and human health. Resistance to Fusarium infection has been associated with the cell wall, however, remains somewhat unexplored in maize where most hybrids and inbreds are susceptible. The two genotypes that have shown differences in lodging and drought resistance, B73 and EA2024, were used in an infection experiment with F. graminearum. Differences in susceptibility to fungal disease were found, with B73 being more susceptible than EA2024. To investigate genotypic differences in cell wall composition, which could provide information on differences in resistance to Fusarium between B73 and EA2024, and to search for a putative systemic remodeling response upon infection, cell wall composition and structure of pith tissue from an internode other than the inoculated one (second internode below the main ear) were compared between inoculated and non-inoculated plants. Cell wall analysis of non-inoculated plants showed that there were no quantitative differences in the major components between B73 and EA2024. However, variations in p-coumaric and the degree of acetylation of arabinoxylans may contribute to differences in Fusarium susceptibility between genotypes. Detailed structural analyses of polysaccharides extracted by subcritical water extraction indicated that EA2024 presented a pectin-enriched population extracted at early-time (10 minutes) by subcritical water extraction that could result in damage-associated molecular patterns related to Fusarium resistance. The cell wall characteristics of B73, the susceptible inbred, may promote degradation by the fungus since arabinoxylans extracted at 30 minutes by subcritical water extraction showed higher disubstituted xyloses. In addition, upon infection B73 possesses higher complex substitutions of arabinoxylans making the cell wall more easily degradable enhancing fungal development. These results suggest that genotype-dependent cell wall composition may be relevant in this resistance. The use of maize lignocellulose as feedstock or for biofuel production is determined by efficient degradation of this material. However, plant biomass possesses an intrinsic resistance to enzymatic hydrolysis attributable to the recalcitrance of lignified cell walls. Untreated cell walls from selected internodes of six different maize inbreds -including B73 and EA2024- were exposed to an enzymatic degradation assay followed by compositional analyses to gain insight into the relationship between these two traits. Basing on the quantitative differences in enzymatic degradability among inbreds, two distinct groups (high and low degradability) were generated. The subsequent cell wall analysis of the groups revealed a considerable variation, however, it was found that lower cellulose crystallinity, S-enriched and H-impoverished lignin, high content of esterified ferulic acid and reduced p-coumaric acid content seem to be associated with higher enzymatic degradability. In contrast, lignin H-subunits, hemicelluloses (total and highly cross-linked population) and xylose in loosely cross-linked hemicelluloses are negatively related to cell wall degradability. Demographic growth and economic development will contribute to an increase in food demand of maize. However, the high maize yields are threatened by both biotic and abiotic factors, which are currently being negatively affected by accelerating climate change. The findings obtained in this PhD Thesis contribute to better understand relevant aspects related to maize crop such as lodging, drought, F. graminearum infection and enzymatic degradability from a cell wall perspective. Therefore, these outcomes may be useful in breeding programs aimed at improving maize crops for the studied characters.