Abstract
Dark-grown (etiolated) branches of many recalcitrant plant species root better than their green counterparts. Here it was hypothesized that changes in cell-wall properties and hormones occurring during etiolation contribute to rooting efficiency. Measurements of chlorophyll, carbohydrate and auxin contents, as well as tissue compression, histological analysis and gene-expression profiles were determined in etiolated and de-etiolated branches of the avocado rootstock VC801. Differences in chlorophyll content and tissue rigidity, and changes in xyloglucan and pectin in cambium and parenchyma cells were found. Interestingly, lignin and sugar contents were similar, suggesting that de-etiolated branches resemble the etiolated ones in this respect. Surprisingly, the branches that underwent short de-etiolation rooted better than the etiolated ones, and only a slight difference in IAA content between the two was observed. Gene-expression profiles revealed an increase in ethylene-responsive transcripts in the etiolated branches, which correlated with enrichment in xyloglucan hydrolases. In contrast, transcripts encoding pectin methylesterase and pectolyases were enriched in the de-etiolated branches. Taken together, it seems that the short de-etiolation period led to fine tuning of the conditions favoring adventitious root formation in terms of auxin–ethylene balance and cell-wall properties.
Highlights
The avocado (Persea americana Mill.) is an evergreen tree in the family Lauraceae, grown worldwide for its oil-rich, nutritious and popular fruit
It is shown that short de-etiolation provides an advantage for adventitious root formation in shoots of the clonal avocado rootstock VC801 which were removed from the nurse seedlings on which they were grafted
The results presented here suggest that the sum of changes leads to activation of the cambium, as detected here by the increase in cambial cells per column, and fine-tuned optimization of the conditions for adventitious root formation in these branches
Summary
The avocado (Persea americana Mill.) is an evergreen tree in the family Lauraceae, grown worldwide for its oil-rich, nutritious and popular fruit. Avocado is very sensitive to various types of soil stress, including salinity, lime, and nonaerated soil on the one hand, and dryness on the other, as well as to several soil pathogens, such as Phytophthora cinnamomi [3]. These sensitivities can be mitigated by grafting desired cultivars on selections of elite resistant rootstocks, a common concept used worldwide [4,5], which drove the wide-ranging rootstock survey carried out by Avraham
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