Periodic Reporting for period 4 - LUSH SPIKE (Genetic and Molecular Determinants of Spikelet Survival in Cereal Crops)
Período documentado: 2021-01-01 hasta 2021-06-30
In barley, the number of spikelets per spike at the awn primordium (AP) stage represents the maximum yield potential per spike. After the AP stage, significant spikelet mortality results in fewer grains per spike. Our previous results clearly indicated that spikelet survival in barley is highly genetically controlled (broad-sense heritability >0.80) and that the period from AP to tipping represents the most critical pre-anthesis phase related to spikelet reduction and grain yield per spike. However, the underlying genetic and molecular determinants of spikelet survival remain to be discovered. I therefore conducted an ambitious research program with an emphasis on using available genetic resources.
Our specific aims during the LUSH SPIKE project were to:
(i) discover quantitative trait loci (QTL) for spikelet survival and grain number per spike and validate these QTL in bi-parental doubled-haploid mapping populations,
(ii) isolate and functionally characterize Mendelized QTL using a map-based approach,
(iii) reveal gene regulatory networks determining spikelet survival during the critical spike growth period from AP to heading, and
(iv) elucidate spatio-temporal patterns of metabolite and phytohormone distributions in spike and spikelet sections during the critical growth period, using mass spectrometric imaging.
The results from LUSH SPIKE will advance our understanding of how to improve yields of cereal crops.
Ad iii.) From our previous works related to the generation of a barley spike transcript atlas using laser-aided micro dissections of meristems (Thiel, Koppolu et al. 2021), we discovered that we had already covered the critical period for the initiation of spikelet abortion. Visible spikelet abortion is rather the end point of an occurring mitotic arrest of the apical inflorescence meristem, which starts on the cellular level much earlier. We therefore found transcript signatures for the onset of abortion/mitotic arrest in our available data set. Established GRNs are still under continuous investigations (Huang et al.; in preparation).
Ad iv.) To improve our understanding of hormonal and metabolite regulation of yield-establishing processes, such as spikelet decline, we established within the LUSH SPIKE project a detailed histological and microscopic analysis using the latest mass spectrometric imaging techniques (Peukert et al. 2014; Plant Cell) to integrate metabolite and hormonal analyses with organ growth over the critical period of development. Using such MALDI-MS imaging, we found for the first time the spatio-temporal distribution of important metabolites, such as sugars, amino acids and the phytohormone melatonin in developing barley spikes. By combining metabolomic, transcriptomic, and genetic approaches we show that apical abortion is associated with sugar depletion, amino acid degradation, and ABA biosynthesis and signaling. Senescence and defense-responsive transcription factor families, viz., NACs, HD-ZIPs, bZIPs, and MYBs, are amongst the putative candidate genes responsible for apical abortion. CRISPR/Cas9-mediated knock-out of barley GRASSY TILLERS1 (HvGT1) encoding an HD-ZIP transcription factor delayed apical abortion, thereby increasing the final spikelet number. We thus propose that modifying apical spikelet abortion by exploiting the identified putative regulators may help increase yield potential in barley and other related cereals (Shanmugaraj et al.; in preparation).
2) We will provide the first MALDI-MS images of developing inflorescences of cereals (i.e. barley), in which we show over time, e.g. chlorophyll biogenesis patterning, amino acid or sugar distributions. MALDI-MS in such a context can be considered as a breakthrough technology by providing a lot of spatial information (~10 μm), in particular where metabolites are located during cereal inflorescence growth and development. (paper in revision for the method development; paper in preparation for the biology)
3) We revealed that spikelet abortion can be initiated at variable developmental time points during inflorescence growth and development; it is indicative following the ‘spikelet stop’ approach. (paper: Venkatasubbu, Schnurbusch (2021) ‘Spikelet stop’ determines the maximum yield potential stage in barley. J Exp Bot, https://doi.org/10.1093/jxb/erab342)
4) We showed that different spike row-types of barley follow different strategies for grain number determination. Increasing grain numbers might be possible by augmenting potential spikelet numbers in two-rowed genotypes, while for six-rowed genotypes, spikelet survival needs to be improved. (Venkatasubbu et al. (2021) Strategies of grain number determination differentiate barley row-types. J Exp Bot, https://doi.org/10.1093/jxb/erab395)