In the world of crops, there exists a remarkable "adversity fighter" that can endure drought, withstand scorching heat, and even thrive in barren saline-alkali soil. This is sorghum, the world's fifth-largest cereal crop. Its close relative, sweet sorghum, possesses all these advantages while also being a high-sugar crop suitable for both feed and energy production, showing tremendous potential for advancing livestock farming and efficient land resource utilization.
However, a persistent challenge has hindered its rise: the stubborn lignocellulose in sweet sorghum stalks acts like an impenetrable barrier, making it difficult for ruminants to digest and absorb nutrients. Breaking through this natural barrier is essential to fully unlock sweet sorghum's potential.
Recently, a research team led by Dr. Xie Qi at the Institute of Genetics and Developmental Biology, Chinese Academy of Sciences, achieved a breakthrough in this field. They performed "genetic modification surgery" on sweet sorghum to make its stalks (primarily composed of lignocellulose) more digestible for herbivorous animals, creating superior feed. The key to this surgery was manipulating the "surgical tool" - endo-1,4-β-xylanase - to modify the xylan in sweet sorghum.
First, let's understand the "opponent" we need to overcome: what exactly is the lignocellulose that genetic modification surgery must combat?
The reason plant stems, branches, and stalks are rigid is primarily due to three supporting components: cellulose (like tough rope), hemicellulose (like viscous glue and mesh framework), and lignin (like hardened cement). The super-structure formed by these three tightly bonded components is lignocellulose.
Lignocellulose is widely distributed: crop residues left after harvest (corn stalks, wheat straw, rice straw), wood processing sawdust, specially cultivated energy grasses (such as sweet sorghum and switchgrass), and even urban waste paper and wood are all lignocellulose! They grow annually, are inexhaustible, and represent renewable resources.
As an important biomass resource, lignocellulose plays multiple roles in our lives. It is the primary raw material in papermaking - newspapers, books, cardboard boxes, and toilet paper all depend on it. Additionally, it serves as an ideal biofuel feedstock, as lignocellulose can be decomposed and converted into alcohol (clean energy). Furthermore, lignocellulose can be made into biodegradable plastics, eco-friendly building materials, and textile fibers.
In animal husbandry, forage (primarily composed of lignocellulose) is the main feed for cattle, sheep, and other ruminants. However, herbivorous animals have low digestive efficiency for lignocellulose. If we could improve livestock's digestibility of lignocellulose, we could feed more animals with less feed.
Encouragingly, researchers discovered a naturally occurring sweet sorghum mutant strain M19, which brought hope for solving the lignocellulose digestibility problem.
The research team found that in mutant M19, a gene called SbXyl responsible for producing "xylan scissors" (endo-1,4-β-xylanase) had mutated and become inactive (nonsense mutation, producing a non-functional truncated protein). This mutation triggered a chain reaction: because the "scissors" were broken, normal xylan metabolism and cell wall development (especially vascular tissue responsible for water transport) were affected, resulting in dwarf plants, curled leaves, poor water transport capacity, and low yield (biomass).
However, this seemingly disadvantageous mutation brought unexpected benefits - the cell wall structure of mutant M19 was disrupted (becoming looser), making the lignocellulose inside more susceptible to digestive enzyme breakdown! Whether fed directly to livestock or processed into silage through sealing and fermentation (more durable than fresh feed and more nutritious than dry feed), its digestibility significantly improved by 7.7%-20.1%.
This created an interesting contrast: for individual plants, this mutation was detrimental to growth and development, but as feed, it was a tremendous advantage!
However, transforming this surprising discovery into an efficient solution faced new challenges: while mutant M19 had good feed quality (easily digestible), its yield was too low (dwarf plants), making it impractical for actual production.
To address this, researchers set a dual objective: ensure plants could grow normally (mainly restoring normal vascular tissue development and water transport capacity) while maintaining the feed's easy digestibility advantage (loose cell wall structure).
To achieve this goal, researchers adopted the following innovative strategy:
Step 1: Find a "switch" (promoter from gene Sobic.007G003000) that works only in vascular tissue (the pipeline system responsible for transporting water and nutrients), ensuring the gene it controls is activated only where needed (vascular tissue).
Step 2: Use the identified "switch" to control the expression of the "xylan scissors" gene (SbXyl). Through this design, SbXyl gene expression was strictly limited to vascular tissue.
Step 3: Insert the assembled genetic construct (pSbNAC::SbXyl) back into the M19 mutant through transgenic technology.
In the schematic model, wild-type E048 shows normal vascular bundle (VB) morphology, resulting in normal biomass and lignocellulose characteristics. The M19 mutant with SbXyl loss of function shows reduced vascular bundle area, thinner fiber cell secondary cell walls (SCW), decreased biomass, but higher lignocellulose digestibility. Vascular tissue-specific complementation (VSC) lines rescued growth defects while retaining M19's improved lignocellulose quality.
After greenhouse cultivation, results showed that vascular tissue-specific complementation (VSC) plants achieved the researchers' dual objectives. On one hand, VSC plants had normal vascular bundle development, restored water transport, increased height, and recovered normal yield levels.
On the other hand, microscopic observation of stem vascular and fiber cell morphology in different plants revealed that VSC lines had vascular areas restored to wild-type size, but with thinner walls, indicating growth recovery while maintaining the loose, easily digestible cell wall structure of M19 variant plants!
The research team successfully cultivated new sweet sorghum material (VSC lines) that was both high-yielding and easily digestible, achieving perfect balance between function (high yield) and characteristics (easy digestion). This is significant for animal husbandry, meaning more meat, milk, and other livestock products can be produced using less land and feed resources.
Additionally, this "precision-targeted modification" strategy (expressing specific genes only in specific tissues) is highly ingenious, avoiding the complete collapse that might result from overexpressing xylanase throughout the entire plant (like the dwarf M19), with broad potential applications.
Using this approach, not only can the feed value of forage crops like sweet sorghum and corn be improved, but it could also potentially be applied to modify energy crops like switchgrass, making their biomass more easily convertible to biofuels, improving bioenergy production efficiency and economics.
Overall, the research team's innovative strategy provides a highly promising, sustainable solution to the worldwide problem of lignocellulose "stubbornness." As gene editing technology continues advancing, we may witness the birth of more breakthrough crops, truly achieving a win-win situation of "high quality and high yield" with "resource efficiency."
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