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Altering Petunia Development Rate to Improve Cutting Yield and Crop Production Efficiency

Researchers at Michigan State University, funded by AFE, are exploring how to breed new varieties and improve petunia crop to make production more efficient. This project is working to understand the genetic materials of different petunia varieties, and evaluate their development and performance to maximize efficiency. Read more below, and explore our other currently funded projects here.

The rate at which plants produce new nodes is a fundamental trait with numerous implications for ornamental crop production. The benefits of understanding the genetic regulation of development rate and being able to manipulate development rate in the breeding of new cultivars include:

1) Reduction in crop production time: 

Crop production time for annual bedding plants is a function of the rate of development, a temperature-dependent process, and when the transition from vegetative growth to flowering occurs, which is usually either dependent on development of a certain number of nodes, or a response to photoperiod. 

Many growers have been tempted to reduce greenhouse temperature as a means to reduce heating costs. While this strategy does reduce the daily cost to heat a greenhouse, lowering temperature decreases plant development rate, thereby increasing crop production time. In fact, modeling studies suggest that reducing greenhouse temperature can actually increase the total amount of fuel required for greenhouse heating to produce a spring bedding plant crop. However, not all varieties of a species are similarly sensitive to temperature. For example, reducing greenhouse temperature from 68 ºF to 57 ºF increased time to flower by 16 days of petunia ‘Damask Purple’, but 26 days for petunia ‘Wave Purple’. Breeding varieties with faster development rates and reduced temperature sensitivity (i.e. less increase in production time as temperature decreases) would improve the energy-efficiency of bedding plant production and potentially allow growers to produce more turns per season.

2) Increase efficiency of vegetative cutting production:

Many herbaceous ornamental annual crops are propagated by vegetative unrooted cuttings (URCs). Increasing development rate would benefit the industry by increasing the number of cuttings harvested per stock plant over the same period of time, improving the efficiency of URC production.

The focus for the first year of this project has been developing the genetic materials that will be evaluated in the second year of the project. Specifically, we utilized a transient gene silencing system (virus-induced gene silencing) to screen through a group of candidate genes that we had previously identified as potentially being involved in the genetic control of plant development rate. From this work, we identified that silencing a gene, hereafter referred to as PhM2L. We are now utilizing the CRISPR-associated endonuclease 9 (Cas9) genome editing system to generate petunia plants lacking expression of PhM2L to determine how this influences development rate and rooting performance of URCs.

Plant transformation of Petunia ‘Mitchell Diploid’ with CRISPR plasmids harboring both the Cas9 endonuclease and the PhM2L gRNA sequences is currently ongoing. Empty vector-transformed plants will be used as controls. We anticipate having regenerated transformed plants available to evaluate PhM2L mutation efficiency/genotype by the end of project year 1.

Project objectives to be completed in the upcoming year include:

1) Validating PhM2L edited plants

After transformed plants are regenerated through tissue culture, DNA sequencing will be used to confirm whether PhM2L was mutated. Plantlets with confirmed edits in PhM2L will be transferred out of tissue culture conditions and grown as stock plants for subsequent experiments.

2) Evaluating the development rate of the petunia lines with altered PhM2L expression in diverse environments

PhM2L-silenced and wild-type plants will be grown in greenhouse compartments under different temperatures to determine how temperature influences development rate. Plants will be grown from rooted cuttings and placed in the different temperature zones for 6 weeks, after which the number of new nodes to form on the primary stem will be counted and used to calculate development rate. The experiment will employ at least four independent PhM2L-edited lines. 

3) Evaluating the rooting performance of cuttings from the altered PhM2L expression lines and other petunia germplasm varying in development rate

A series of trials will be conducted to evaluate rooting performance of unrooted cuttings taken from stock plants of petunia lines varying in development rate. Lines will include PhM2L-silenced lines, individuals from our Petunia axillaris x P. integrifolia recombinant inbred lines that have previously been identified to vary in development rate, and petunia cultivars that we have previously determined to vary in development rate. Cuttings will be rooted under intermittent mist at 73 ºF. After three weeks, cuttings will be harvested and assessed for the number of roots greater than 3 cm in length, the length of the longest three roots, and total root dry biomass.

In conclusion, the ability to breed new varieties with faster development rates offers the opportunity to reduce crop production time/cost and to increase the efficiency of unrooted cutting production. The CRISPR/Cas9 genome editing system is an exciting development in the ability to manipulate crop plants to improve specific traits and requires further exploration within the floriculture industry. The current project is on track and we look forward to completing all project objectives and sharing the results this year.

By Ryan Warner, Michigan State University